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      <a href="http://www.w3.org/"><img height="48" width="72"
      alt="W3C" src="http://www.w3.org/Icons/w3c_home" /></a> 

      <h1 id="title">RDF Primer</h1>

      <h2 id="hstatus">W3C Recommendation 10 February 2004</h2>




      <dl>
        <dt>This version:</dt>

        <dd><a
        href="http://www.w3.org/TR/2004/REC-rdf-primer-20040210/">http://www.w3.org/TR/2004/REC-rdf-primer-20040210/</a></dd>

        <dt>Latest version:</dt>

        <dd><a
        href="http://www.w3.org/TR/rdf-primer/">http://www.w3.org/TR/rdf-primer/</a></dd>

        <dt>Previous version:</dt>

        <dd><a
        href="http://www.w3.org/TR/2003/PR-rdf-primer-20031215/">http://www.w3.org/TR/2003/PR-rdf-primer-20031215/</a></dd>

        <dt>Editors:</dt>

        <dd>Frank Manola, <a
        href="mailto:fmanola@acm.org">fmanola@acm.org</a></dd>

        <dd>Eric Miller, W3C, <a
        href="mailto:em@w3.org">em@w3.org</a></dd>

        <dt>Series Editor:</dt>

        <dd>Brian McBride, Hewlett-Packard Laboratories, <a
        href="mailto:bwm@hplb.hpl.hp.com">bwm@hplb.hpl.hp.com</a></dd>
      </dl>


<p>Please refer to the <a href="http://www.w3.org/2001/sw/RDFCore/errata#rdf-primer"><strong>errata</strong></a> for this document, which may include some normative corrections.</p>

<p>See also <a href="http://www.w3.org/2001/sw/RDFCore/translation/rdf-primer">translations</a>.</p>

<p class="copyright"><a
      href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>
      &copy; 2004 <a href="http://www.w3.org/"><acronym
      title="World Wide Web Consortium">W3C</acronym></a><sup>&reg;</sup>
      (<a href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute of Technology">MIT</acronym></a>,
      <a href="http://www.ercim.org/"><acronym
      title="European Research Consortium for Informatics and Mathematics">
      ERCIM</acronym></a>, <a href="http://www.keio.ac.jp/">Keio</a>),
      All Rights Reserved. W3C <a
      href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">
      liability</a>, <a
      href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">
      trademark</a>, <a
      href="http://www.w3.org/Consortium/Legal/copyright-documents">document
      use</a> and <a
      href="http://www.w3.org/Consortium/Legal/copyright-software">software
      licensing</a> rules apply.</p>
      <hr title="Separator for header" />
    </div>

    <h2><a id="abstract" name="abstract">Abstract</a></h2>

    <p>The Resource Description Framework (RDF) is a language for
    representing information about resources in the World Wide Web.
    This Primer is designed to provide the reader with the basic
    knowledge required to effectively use RDF. It introduces the basic
    concepts of RDF and describes its XML syntax. It describes how to
    define RDF vocabularies using the RDF Vocabulary Description
    Language, and gives an overview of some deployed RDF applications.
    It also describes the content and purpose of other RDF
    specification documents.</p>

<div class="status">

<h2 class="nonum">
<a id="status" name="status">Status of this Document</a>
</h2>

<!-- Start Status-Of-This-Document Text -->

<p>This document has been reviewed by W3C Members and other interested
parties, and it has been endorsed by the Director as a <a
href="http://www.w3.org/2003/06/Process-20030618/tr.html#RecsW3C">W3C
Recommendation</a>.  W3C's role in making the Recommendation is to
draw attention to the specification and to promote its widespread
deployment. This enhances the functionality and interoperability of
the Web.</p>

<p>This is one document in a <a
href="http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/#section-Introduction">set
of six</a> (<a
href="http://www.w3.org/TR/2004/REC-rdf-primer-20040210/">Primer</a>,
<a
href="http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/">Concepts</a>,
<a
href="http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/">Syntax</a>,
<a
href="http://www.w3.org/TR/2004/REC-rdf-mt-20040210/">Semantics</a>,
<a
href="http://www.w3.org/TR/2004/REC-rdf-schema-20040210/">Vocabulary</a>,
and <a
href="http://www.w3.org/TR/2004/REC-rdf-testcases-20040210/">Test
Cases</a>) intended to jointly replace the original Resource
Description Framework specifications, <a
href="http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/">RDF Model and Syntax (1999
Recommendation)</a> and <a
href="http://www.w3.org/TR/2000/CR-rdf-schema-20000327/">RDF Schema
(2000 Candidate Recommendation)</a>.  It has been developed by the <a
href="http://www.w3.org/2001/sw/RDFCore/">RDF Core Working Group</a>
as part of the <a href="http://www.w3.org/2001/sw/">W3C Semantic Web
Activity</a> (<a href="http://www.w3.org/2001/sw/Activity">Activity
Statement</a>, <a
href="http://www.w3.org/2002/11/swv2/charters/RDFCoreWGCharter">Group
Charter</a>) for publication on 10 February 2004.
</p>

<p>Changes to this document since the <a
href="http://www.w3.org/TR/2003/PR-rdf-primer-20031215/"
shape="rect">Proposed Recommendation Working Draft</a> are detailed in
the <a href="#changes" shape="rect">change log</a>.  </p>

<p> The public is invited to send comments to <a
href="mailto:www-rdf-comments@w3.org">www-rdf-comments@w3.org</a> (<a
href="http://lists.w3.org/Archives/Public/www-rdf-comments/">archive</a>)
and to participate in general discussion of related technology on <a
href="mailto:www-rdf-interest@w3.org"
shape="rect">www-rdf-interest@w3.org</a> (<a
href="http://lists.w3.org/Archives/Public/www-rdf-interest/"
shape="rect">archive</a>).  </p>

<p>A list of <a href="http://www.w3.org/2001/sw/RDFCore/impls">
implementations</a> is available.</p>

<p>The W3C maintains a list of <a href="http://www.w3.org/2001/sw/RDFCore/ipr-statements" 
rel="disclosure">any patent disclosures related to this work</a>.</p>

<p><em>This section describes the status of this document at the time of its 
publication. Other documents may supersede this document. A list of current W3C 
publications and the latest revision of this technical report can be found in 
the <a href="http://www.w3.org/TR/">W3C technical reports index</a> at 
http://www.w3.org/TR/.</em></p> 

<!-- End Status-Of-This-Document Text -->

</div>


    <h2><a id="toc" name="toc">Table of Contents</a></h2>

    <p class="toc">&nbsp;&nbsp;1. <a
    href="#intro">Introduction</a><br />
     &nbsp;&nbsp;2. <a href="#statements">Making Statements About
    Resources</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;2.1 <a
    href="#basicconcepts">Basic Concepts</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;2.2 <a href="#rdfmodel">The
    RDF Model</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;2.3 <a
    href="#structuredproperties">Structured Property Values and Blank
    Nodes</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;2.4 <a
    href="#typedliterals">Typed Literals</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;2.5 <a
    href="#conceptsummary">Concepts Summary</a><br />
     &nbsp;&nbsp;3. <a href="#rdfxml">An XML Syntax for RDF:
    RDF/XML</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;3.1 <a
    href="#basicprinciples">Basic Principles</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;3.2 <a
    href="#newresources">Abbreviating and Organizing RDF
    URIrefs</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;3.3 <a
    href="#rdfxmlsummary">RDF/XML Summary</a><br />
     &nbsp;&nbsp;4. <a href="#othercapabilities">Other RDF
    Capabilities</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;4.1 <a href="#containers">RDF
    Containers</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;4.2 <a href="#collections">RDF
    Collections</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;4.3 <a href="#reification">RDF
    Reification</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;4.4 <a href="#rdfvalue">More
    on Structured Values: rdf:value</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;4.5 <a href="#xmlliterals">XML
    Literals</a><br />
     &nbsp;&nbsp;5. <a href="#rdfschema">Defining RDF Vocabularies: RDF
    Schema</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.1 <a
    href="#schemaclasses">Describing Classes</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.2 <a
    href="#properties">Describing Properties</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.3 <a
    href="#interpretingschema">Interpreting RDF Schema
    Declarations</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.4 <a
    href="#otherschema">Other Schema Information</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.5 <a
    href="#richerschemas">Richer Schema Languages</a><br />
     &nbsp;&nbsp;6. <a href="#applications">Some RDF Applications: RDF
    in the Field</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;6.1 <a
    href="#dublincore">Dublin Core Metadata Initiative</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;6.2 <a
    href="#prism">PRISM</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;6.3 <a
    href="#xpackage">XPackage</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;6.4 <a href="#rss">RSS 1.0:
    RDF Site Summary</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;6.5 <a
    href="#cimxml">CIM/XML</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;6.6 <a href="#geneont">Gene
    Ontology Consortium</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;6.7 <a
    href="#devcap">Describing Device Capabilities and User
    Preferences</a><br />
     &nbsp;&nbsp;7. <a href="#otherparts">Other Parts of the RDF
    Specification</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;7.1 <a href="#semantics">RDF
    Semantics</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;7.2 <a href="#testcases">Test
    Cases</a><br />
     &nbsp;&nbsp;8. <a href="#references">References</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;8.1 <a
    href="#normative-references">Normative References</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;8.2 <a
    href="#informational-references">Informational References</a><br />
     &nbsp;&nbsp;9. <a href="#acknowledgements">Acknowledgments</a></p>

    <h3><a id="appendices" name="appendices">Appendices</a></h3>

    <p class="appendix">&nbsp;&nbsp;A. <a href="#identifiers">More on
    Uniform Resource Identifiers (URIs)</a><br />
     &nbsp;&nbsp;B. <a href="#documents">More on the Extensible Markup
    Language (XML)</a><br />
     &nbsp;&nbsp;C. <a href="#changes">Changes</a><br />
    </p>
    <hr />


    <div class="section">
      <h2 id="introduction"><a id="intro" name="intro"></a>1.
      Introduction</h2>

      <p>The Resource Description Framework (RDF) is a language for
      representing information about resources in the World Wide Web.
      It is particularly intended for representing metadata about Web
      resources, such as the title, author, and modification date of a
      Web page, copyright and licensing information about a Web
      document, or the availability schedule for some shared resource.
      However, by generalizing the concept of a "Web resource", RDF can
      also be used to represent information about things that can be
      <em>identified</em> on the Web, even when they cannot be directly
      <em>retrieved</em> on the Web. Examples include information about
      items available from on-line shopping facilities (e.g.,
      information about specifications, prices, and availability), or
      the description of a Web user's preferences for information
      delivery.</p>
    
      <p>RDF is intended for situations in which this information 
       needs to be processed by applications, rather than being 
       only displayed to people.  

      RDF provides a common framework for expressing this
      information so it can be exchanged between applications without
      loss of meaning. Since it is a common framework, application
      designers can leverage the availability of common RDF parsers and
      processing tools. The ability to exchange information between
      different applications means that the information may be made
      available to applications other than those for which it was
      originally created.</p>

      <p>RDF is based on the idea of identifying things using Web
      identifiers (called <em>Uniform Resource Identifiers</em>, 
      or <em>URIs</em>), and describing resources in terms of simple
      properties and property values. This enables RDF to represent
      simple statements about resources as a <em>graph</em> of nodes
      and arcs representing the resources, and their properties and
      values. To make this discussion somewhat more concrete as soon as
      possible, the group of statements "there is <span class="newstuff">
      <a name="LCC-007" id="LCC-007">a Person 
      identified by <code>http://www.w3.org/People/EM/contact#me</code></a>,</span> whose name is
      Eric Miller, whose email address is em@w3.org, and whose title is
      Dr." could be represented as the RDF graph in <a
      href="#figure1">Figure 1</a>:</p>

      <div class="figure">
        <img src="fig1dec16.png"
        alt="An RDF Graph Describing Eric Miller" /><br />
         <a id="figure1" name="figure1">Figure 1: An RDF Graph
        Describing Eric Miller</a>
      </div>

      <p><a href="#figure1">Figure 1</a> illustrates that RDF uses URIs
      to identify:</p>

      <ul>
        <li>individuals, e.g., Eric Miller, identified by
        <code>http://www.w3.org/People/EM/contact#me</code></li>

        <li>kinds of things, e.g., Person, identified by
        <code>http://www.w3.org/2000/10/swap/pim/contact#Person</code></li>

        <li>properties of those things, e.g., mailbox, identified by
        <code>http://www.w3.org/2000/10/swap/pim/contact#mailbox</code></li>

        <li>values of those properties, e.g. <code>mailto:em@w3.org</code>
        as the value of the mailbox property (RDF also uses character
        strings such as "Eric Miller", and values from other datatypes
        such as integers and dates, as the values of properties)</li>
      </ul>

      <p>RDF also provides an XML-based syntax (called <em>RDF/XML</em>) for
      recording and exchanging these graphs. <a
      href="#example1">Example 1</a> is a small chunk of RDF in RDF/XML
      corresponding to the graph in <a href="#figure1">Figure
      1</a>:</p>

      <div class="exampleOuter">
        <div class="c1">
          <a id="example1" name="example1">Example 1: RDF/XML
          Describing Eric Miller</a>
        </div>

        <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
             xmlns:contact="http://www.w3.org/2000/10/swap/pim/contact#"&gt;

  &lt;contact:Person rdf:about="http://www.w3.org/People/EM/contact#me"&gt;
    &lt;contact:fullName&gt;Eric Miller&lt;/contact:fullName&gt;
    &lt;contact:mailbox rdf:resource="mailto:em@w3.org"/&gt;
    &lt;contact:personalTitle&gt;Dr.&lt;/contact:personalTitle&gt; 
  &lt;/contact:Person&gt;

&lt;/rdf:RDF&gt;
</pre>
        </div>
      </div>

      <p>Note that this RDF/XML also contains URIs, as well as
      properties like <code>mailbox</code> and <code>fullName</code> (in an
      abbreviated form), and their respective values
      <code>em@w3.org</code>, and <code>Eric Miller</code>.</p>

      <p>Like HTML, this RDF/XML is machine processable and, using
      URIs, can link pieces of information across the Web. However,
      unlike conventional hypertext, RDF URIs can refer to any
      identifiable thing, including things that may not be directly
      retrievable on the Web (such as the person Eric Miller). The
      result is that in addition to describing such things as Web
      pages, RDF can also describe cars, businesses, people, news
      events, etc. In addition, RDF properties themselves have URIs, to
      precisely identify the relationships that exist between
      the linked items.</p>

      <p>The following documents contribute to the specification of
      RDF:</p>

      <ul>
        <li><a href="http://www.w3.org/TR/rdf-concepts/">RDF Concepts
        and Abstract Syntax</a> <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a></li>

        <li><a href="http://www.w3.org/TR/rdf-syntax-grammar/">RDF/XML
        Syntax Specification</a> <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a></li>

        <li><a href="http://www.w3.org/TR/rdf-schema/">RDF Vocabulary
        Description Language 1.0: RDF Schema</a> <a
        href="#ref-rdf-vocabulary">[RDF-VOCABULARY]</a></li>

        <li><a href="http://www.w3.org/TR/rdf-mt/">RDF Semantics</a> <a
        href="#ref-rdf-semantics">[RDF-SEMANTICS]</a></li>

        <li><a href="http://www.w3.org/TR/rdf-testcases/">RDF Test
        Cases</a> <a href="#ref-rdf-tests">[RDF-TESTS]</a></li>

        <li><a href="http://www.w3.org/TR/rdf-primer/">RDF Primer</a>
        (this document)</li>
      </ul>

      <p>This Primer is intended to provide an introduction to RDF and
      describe some existing RDF applications, to help information
      system designers and application developers understand the
      features of RDF and how to use them. In particular, the Primer is
      intended to answer such questions as:</p>

      <ul>
        <li>What does RDF look like?</li>

        <li>What information can RDF represent?</li>

        <li>How is RDF information created, accessed, and
        processed?</li>

        <li>How can existing information be combined with RDF?</li>
      </ul>

      <p>The Primer is a <em>non-normative</em> document, which means
      that it does not provide a definitive specification of RDF. The
      examples and other explanatory material in the Primer are
      provided to help readers understand RDF, but they may not always
      provide definitive or fully-complete answers. In such cases, the 
      relevant normative parts of the RDF specification should be consulted. 
      To help in doing this, the Primer describes the roles these other 
      documents play in the complete specification of RDF, and provides 
      links pointing to the relevant parts of the normative specifications,
      at appropriate places in the discussion.</p>

      <p class="newstuff">It should also be noted that these RDF documents update and clarify 
      <a name="LCC-022" id="LCC-022">previously-published RDF specifications</a>, the <a
      href="http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/">Resource
      Description Framework (RDF) Model and Syntax Specification</a> <a
      href="#ref-rdfms">[RDF-MS]</a> and the

      <a href="http://www.w3.org/TR/2000/CR-rdf-schema-20000327/">
          Resource Description Framework (RDF) Schema Specification 1.0</a>
      <a href="#ref-rdf-s">[RDF-S]</a>.

      As a result, there have been some changes in terminology, syntax, and concepts. 
      This Primer reflects the newer set of RDF specifications given in the 
      bulleted list of RDF documents cited above.  Hence, readers   
      familiar with the older specifications, and with earlier tutorial and 
      introductory articles based on them, should be aware that there may be 
      differences between the current specifications and those previous documents.  
      The <a href="http://www.w3.org/2000/03/rdf-tracking/">RDF Issue
      Tracking</a> document <a href="#ref-rdf-issue">[RDFISSUE]</a>
      can be consulted for a list of issues 
      raised concerning the previous RDF specifications, and their resolution in
      the current specifications.</p>

    </div>

    <div class="section">
      <h2><a id="statements" name="statements"></a>2. Making Statements
      About Resources</h2>

      <p>RDF is intended to provide a simple way to make statements
      about Web resources, e.g., Web pages. This section 
      describes the basic ideas behind the way RDF provides these
      capabilities (the normative specification describing these
      concepts is <a href="http://www.w3.org/TR/rdf-concepts/">RDF
      Concepts and Abstract Syntax</a> <a
      href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>).</p>

      <div class="section">
        <h3><a id="basicconcepts" name="basicconcepts"></a>2.1 Basic
        Concepts</h3>

        <p>Imagine trying to state that someone named
        John Smith created a particular Web page. A straightforward way
        to state this in a natural language such as English would be in the form of a simple
        statement such as:</p>

        <p class="ptriple"><code><strong>http://www.example.org/index.html</strong>
        has a <strong>creator</strong> whose value is <strong>John Smith</strong></code></p>

        <p>Parts of this statement are emphasized to illustrate that,
        in order to describe the properties of something, there need to be ways
        to name, or identify, a number of things:</p>

        <ul>
          <li>the thing the statement describes
          (the Web page, in this case)</li>

          <li>a specific property (creator,
          in this case) of the thing the statement describes</li>

          <li>the thing the statement says is
          the value of this property (who the creator is), for the
          thing the statement describes</li>
        </ul>

        <p>In this statement, the Web page's URL (Uniform
        Resource Locator) is used to identify it. In addition, the
        word "creator" is used to identify the property,
        and the two words "John Smith" to identify the thing (a person)
        that is the value of this property.</p>

        <p>Other properties of this Web page could be described by writing
        additional English statements of the same general form, using
        the URL to identify the page, and words (or other expressions)
        to identify the properties and their values. For example, the
        date the page was created, and the language in
        which the page is written, could be described using the additional
        statements:</p>

        <p class="ptriple"><code><strong>http://www.example.org/index.html</strong>
        has a <strong>creation-date</strong> whose value is <strong>August 16,
        1999</strong></code><br />
         <code><strong>http://www.example.org/index.html</strong> has a
        <strong>language</strong> whose value is <strong>English</strong></code></p>

        <p>RDF is based on the idea that the things being described
        have <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-property">properties</a>
        which have values, and that resources can be described by
        making statements, similar to those above, that specify those
        properties and values. RDF uses a particular terminology for
        talking about the various parts of statements. Specifically,
        the part that identifies the thing the statement is about (the
        Web page in this example) is called the <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-subject">subject</a>.
        The part that identifies the property or characteristic of the
        subject that the statement specifies (creator, creation-date,
        or language in these examples) is called the <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-predicate">predicate</a>,
        and the part that identifies the value of that property is
        called the <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-object">object</a>.
        So, taking the English statement</p>

        <p class="ptriple"><code><strong>http://www.example.org/index.html</strong>
        has a <strong>creator</strong> whose value is <strong>John Smith</strong></code></p>

        <p>the RDF terms for the various parts of the statement
        are:</p>

        <ul>
          <li>the <dfn>subject</dfn> is the URL
          <code>http://www.example.org/index.html</code></li>

          <li>the <dfn>predicate</dfn> is the word "creator"</li>

          <li>the <dfn>object</dfn> is the phrase "John Smith"</li>
        </ul>

        <p>However, while English is good for communicating between
        (English-speaking) humans, RDF is about making
        <em>machine-processable</em> statements. To make these kinds of
        statements suitable for processing by machines, two
        things are needed:</p>

        <ul>
          <li>a system of machine-processable identifiers 
          for identifying a subject, predicate, or object in a statement
          without any possibility of confusion with a similar-looking
          identifier that might be used by someone else on the
          Web.</li>

          <li>a machine-processable language for representing these
          statements and exchanging them between machines.</li>
        </ul>

        <p>Fortunately, the existing Web architecture provides both
        these necessary facilities.</p>

        <p>As illustrated earlier, the Web already provides one form of
        identifier, the <dfn>Uniform Resource Locator</dfn> (URL). 
        A URL was used in the original example to identify the Web page
        that John Smith created. A URL is a character string that
        identifies a Web resource by representing its primary access
        mechanism (essentially, its network "location"). However, it
        is also important to be able to record information about many
        things that, unlike Web pages, do not have network locations or
        URLs.</p>

        <p>The Web provides a more general form of identifier for these
        purposes, called the <a
        href="http://www.isi.edu/in-notes/rfc2396.txt">Uniform Resource
        Identifier</a> (URI). URLs are a particular kind of URI. All
        URIs share the property that different persons or organizations
        can independently create them, and use them to identify things.
        However, URIs are not limited to identifying things that have
        network locations, or use other computer access mechanisms. In
        fact, a URI can be created to refer to anything that needs 
        to be referred to in a statement, including</p>

        <ul>
          <li>network-accessible things, such as an electronic
          document, an image, a service (e.g., "today's weather report
          for Los Angeles"), or a group of other resources.</li>

          <li>things that are not network-accessible, such as human
          beings, corporations, and bound books in a library.</li>

          <li>abstract concepts that do not physically exist, such as the
          concept of a "creator".</li>
        </ul>

        <p>Because of this generality, RDF uses URIs as the basis of
        its mechanism for identifying the subjects, predicates, and
        objects in statements. To be more precise, RDF uses <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-URI-reference">URI
        references</a> <a href="#ref-uri">[URIS]</a>. A URI reference
        (or <em>URIref</em>) is a URI, together with an optional
        <em>fragment identifier</em> at the end. For example, the URI
        reference <code>http://www.example.org/index.html#section2</code>
        consists of the URI <code>http://www.example.org/index.html</code>
        and (separated by the "#" character) the fragment identifier
        <code>Section2</code>. 

RDF URIrefs can contain Unicode <a href="#ref-unicode">[UNICODE]</a> characters (see <a href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>), allowing many languages to be reflected in URIrefs.

        RDF defines a <em>resource</em> as anything
        that is identifiable by a URI reference, so using URIrefs
        allows RDF to describe practically anything, and to state
        relationships between such things as well. URIrefs and fragment
        identifiers are discussed further in <a
        href="#identifiers">Appendix A</a>, and in <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>.</p>

        <p>To represent RDF statements in a machine-processable way,
        RDF uses the <a
        href="http://www.w3.org/TR/2000/REC-xml-20001006">Extensible
        Markup Language</a> <a href="#ref-xml">[XML]</a>. XML was
        designed to allow anyone to design their own document format
        and then write a document in that format. RDF defines a
        specific XML markup language, referred to as <em>RDF/XML</em>,
        for use in representing RDF information, and for exchanging it
        between machines. An example of RDF/XML was given in <a
        href="#intro">Section 1</a>. That example (<a
        href="#example1">Example 1</a>) used tags such as
        <code>&lt;contact:fullName&gt;</code> and
        <code>&lt;contact:personalTitle&gt;</code> to delimit the text
        content <code>Eric Miller</code> and <code>Dr.</code>, respectively.
        Such tags allow programs written with an understanding of what
        the tags mean to properly interpret that content. 

Both XML content and (with certain exceptions) tags can contain Unicode <a href="#ref-unicode">[UNICODE]</a> characters, allowing information from many languages to be directly represented.

        <a href="#documents">Appendix B</a> provides further background on
        XML in general. The specific RDF/XML syntax used for RDF is
        described in more detail in <a href="#rdfxml">Section 3</a>,
        and is normatively defined in <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a></p>
      </div>

      <div class="section">
        <h3><a id="rdfmodel" name="rdfmodel"></a>2.2 The RDF Model</h3>

        <p><a href="#basicconcepts">Section 2.1</a> has introduced 
        RDF's basic statement concepts, the idea of using
        URI references to identify the things referred to
        in RDF statements, and RDF/XML as a machine-processable way to
        represent RDF statements.  With that background, this section
        describes how RDF uses URIs to make statements about resources. The
        introduction said that RDF was based on the idea of
        expressing simple statements about resources, where each
        statement consists of a subject, a predicate, and an object.
        In RDF, the English statement:</p>

        <p class="ptriple"><code><strong>http://www.example.org/index.html</strong>
        has a <strong>creator</strong> whose value is <strong>John Smith</strong></code></p>

        <p>could be represented by an RDF statement having:</p>

        <ul>
          <li>a subject <code>http://www.example.org/index.html</code></li>

          <li>a predicate
          <code>http://purl.org/dc/elements/1.1/creator</code></li>

          <li>and an object
          <code>http://www.example.org/staffid/85740</code></li>
        </ul>

        <p>Note how URIrefs are used to identify not only the
        subject of the original statement, but also the predicate and
        object, instead of using the words "creator" and "John Smith",
        respectively (some of the effects of using URIrefs in this
        way will be discussed later in this section).</p>

        <p>RDF models statements as nodes and arcs in a graph. RDF's <a
        href="http://www.w3.org/TR/rdf-concepts/#section-data-model">graph
        model</a> is defined in <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>. In this notation,
        a statement is represented by:</p>

        <ul>
          <li>a node for the subject</li>

          <li>a node for the object</li>

          <li>an arc for the predicate, 
          directed from the subject node to the object node.</li>
        </ul>

        <p>So the RDF statement above would be represented by the graph
        shown in <a href="#figure2">Figure 2</a>:</p>

        <div class="figure">
          <img src="fig2dec16.png"
          alt="A Simple RDF Statement" /><br />
           <a id="figure2" name="figure2">Figure 2: A Simple RDF
          Statement</a>
        </div>

        <p>Groups of statements are represented by corresponding groups
        of nodes and arcs. So, to reflect the additional English statements</p>

        <p class="ptriple"><code><strong>http://www.example.org/index.html</strong>
        has a <strong>creation-date</strong> whose value is <strong>August 16,
        1999</strong></code><br />
         <code><strong>http://www.example.org/index.html</strong> has a
        <strong>language</strong> whose value is <strong>English</strong></code></p>

        <p>in the RDF graph, the graph shown in <a
        href="#figure3">Figure 3</a> could be used
        (using suitable URIrefs to name the properties
        "creation-date" and "language"):</p>

        <div class="figure">
          <img src="fig3nov19.png"
          alt="Several Statements About the Same Resource" /><br />
           <a id="figure3" name="figure3">Figure 3: Several Statements
          About the Same Resource</a>
        </div>

        <p><a href="#figure3">Figure 3</a> illustrates that objects
        in RDF statements may be either URIrefs, or constant values (called <a
        href="http://www.w3.org/TR/rdf-concepts/#section-Literals">literals</a>)
        represented by character strings, in order to represent certain
        kinds of property values. 
        (In the case of the predicate <code>http://purl.org/dc/elements/1.1/language</code>
        the literal is an international standard two-letter code for English.)
        Literals may not be used as subjects or
        predicates in RDF statements. In drawing RDF
        graphs, nodes that are URIrefs are shown as ellipses, 
        while nodes that are literals are shown as boxes.
        (The simple character string
        literals used in these examples are called <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-plain-literal">plain
        literals</a>, to distinguish them from the <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-typed-literal">typed
        literals</a> to be introduced in <a
        href="#typedliterals">Section 2.4</a>. The various kinds of
        literals that can be used in RDF statements are defined in <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>.

Both plain and typed literals can contain Unicode <a href="#ref-unicode">[UNICODE]</a> characters, allowing information from many languages to be directly represented.) </p>

        <p>Sometimes it is not convenient to draw graphs when
        discussing them, so an alternative way of writing down the
        statements, called <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-rdf-triple">triples</a>,
        is also used. In the triples notation, each statement in the
        graph is written as a simple triple of subject, predicate, and
        object, in that order.  For example, the three statements shown in <a
        href="#figure3">Figure 3</a> would be written in the triples notation as:</p>

        <div class="exampleOuter exampleInner">
<pre>
&lt;http://www.example.org/index.html&gt; &lt;http://purl.org/dc/elements/1.1/creator&gt; &lt;http://www.example.org/staffid/85740&gt; .

&lt;http://www.example.org/index.html&gt; &lt;http://www.example.org/terms/creation-date&gt; "August 16, 1999" .

&lt;http://www.example.org/index.html&gt; &lt;http://purl.org/dc/elements/1.1/language&gt; "en" .
</pre>
        </div>

        <p>Each triple corresponds to a single arc in the graph,
        complete with the arc's beginning and ending nodes (the subject
        and object of the statement). Unlike the drawn graph (but like
        the original statements), the triples notation requires that a
        node be separately identified for each statement it appears in.
        So, for example, <code>http://www.example.org/index.html</code>
        appears three times (once in each triple) in the triples
        representation of the graph, but only once in the drawn graph.
        However, the triples represent exactly the same information as
        the drawn graph, and this is a key point: what is fundamental
        to RDF is the <em>graph model</em> of the statements. The
        notation used to represent or depict the graph is
        secondary.</p>

        <p>The full triples notation requires that URI references be
        written out completely, in angle brackets, which, as the
        example above illustrates, can result in very long lines on a page. For
        convenience, the Primer uses a shorthand way of writing triples 
        (the same shorthand is also used in other RDF specifications).
        This shorthand substitutes an XML <em>qualified name</em>
        (or <em>QName</em>) without angle brackets as an abbreviation
        for a full URI reference (QNames are discussed further in <a
        href="#documents">Appendix B</a>). 
        A QName contains a <em>prefix</em> that has
        been assigned to a namespace URI, followed by a colon, and then
        a <em>local name</em>. The full URIref is formed
        from the QName by appending the local name to the
        namespace URI assigned to the prefix.  So, for example, if the
        QName prefix <code>foo</code> is assigned to the namespace URI
        <code>http://example.org/somewhere/</code>, then the QName
        <code>foo:bar</code> is shorthand for the URIref
        <code>http://example.org/somewhere/bar</code>. 
        Primer examples will also use several "well-known" QName
        prefixes (without explicitly specifying them
        each time), defined as follows:<br />
        <br />
         prefix <code>rdf:</code>, namespace URI:
        <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#</code><br />
         prefix <code>rdfs:</code>, namespace URI:
        <code>http://www.w3.org/2000/01/rdf-schema#</code><br />
         prefix <code>dc:</code>, namespace URI:
        <code>http://purl.org/dc/elements/1.1/</code><br />
         prefix <code>owl:</code>, namespace URI:
        <code>http://www.w3.org/2002/07/owl#</code><br />
         prefix <code>ex:</code>, namespace URI:
        <code>http://www.example.org/</code> (or
        <code>http://www.example.com/</code>)<br />
         prefix <code>xsd:</code>, namespace URI:
        <code>http://www.w3.org/2001/XMLSchema#</code></p>

        <p>Obvious variations on the "example" prefix
        <code>ex:</code> will also be used as needed in the examples, for instance,<br />
        <br />
         prefix <code>exterms:</code>, namespace URI:
        <code>http://www.example.org/terms/</code> (for terms used by an
        example organization),<br />
         prefix <code>exstaff:</code>, namespace URI:
        <code>http://www.example.org/staffid/</code> (for the example
        organization's staff identifiers),<br />
         prefix <code>ex2:</code>, namespace URI:
        <code>http://www.domain2.example.org/</code> (for a second example
        organization), and so on.</p>

        <p>Using this new shorthand, the previous set of
        triples can be written as:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:index.html  dc:creator             exstaff:85740 .

ex:index.html  exterms:creation-date  "August 16, 1999" .

ex:index.html  dc:language            "en" .
</pre>
        </div>

<div class="newstuff">

<p><a name="LCC-012b" id="LCC-012b">Since </a>RDF uses URIrefs instead of words to name things in statements, RDF refers to a set of URIrefs (particularly a set intended for a specific purpose) as a <em>vocabulary</em>.  Often, the URIrefs in such vocabularies are organized so that they can be represented as a set of QNames using a common prefix.  That is, a common namespace URIref will be chosen for all terms in a vocabulary, typically a URIref under the control of whoever is defining the vocabulary.  URIrefs that are contained in the vocabulary are formed by appending individual local names to the end of the common URIref.  This forms a set of URIrefs with a common prefix. For instance, as illustrated by the previous examples, an organization such as example.org might define a vocabulary consisting of URIrefs starting with the prefix <code>http://www.example.org/terms/</code> for terms it uses in its business, such as "creation-date" or "product", and another vocabulary of URIrefs starting with <code>http://www.example.org/staffid/</code> to identify its employees.  RDF uses this same approach to define its own vocabulary of terms with special meanings in RDF.  The URIrefs in this RDF vocabulary all begin with <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#</code>, conventionally associated with the QName prefix <code>rdf:</code>. 
The RDF Vocabulary Description Language (described in <a href="#rdfschema">Section 5</a>) defines an additional set of terms having URIrefs that begin with <code>http://www.w3.org/2000/01/rdf-schema#</code>, conventionally associated with the QName prefix <code>rdfs:</code>.  (Where a specific QName prefix is commonly used in connection with a given set of terms in this way, the QName prefix itself is sometimes used as the name of the vocabulary.  For example, someone might refer to "the <code>rdfs:</code> vocabulary".)</p> 

<p>Using common URI prefixes provides a convenient way to organize the URIrefs for a related set of terms.  However, this is just a convention.  The RDF model only recognizes full URIrefs;  it does not "look
inside" URIrefs or use any knowledge about their structure.  In particular, RDF does not assume there is any relationship between URIrefs just because they have a common leading prefix (see <a href="#identifiers">Appendix A</a> for further discussion).  Moreover, there is nothing that says that URIrefs with different leading prefixes cannot be considered part of the same vocabulary.  A particular organization, process, tool, etc. can define a vocabulary that is significant for it, using URIrefs from any number of other vocabularies as part of its vocabulary. </p>

<p>In addition, sometimes an organization will use a vocabulary's namespace URIref as the URL of a Web resource that provides further information about that vocabulary.  For example, as noted earlier, the QName prefix <code>dc:</code> will be used in Primer examples, associated with the namespace URIref <code>http://purl.org/dc/elements/1.1/</code>.  In fact, this refers to the Dublin Core vocabulary described in <a href="#dublincore">Section 6.1</a>.  Accessing this namespace URIref in a Web browser will retrieve additional information about the Dublin Core vocabulary (specifically, an RDF schema).  However, this is also just a convention.  RDF does not assume that a namespace URI identifies a retrievable Web resource (see <a href="#documents">Appendix B</a> for further discussion).  </p>

<p>In the rest of the Primer, the term <em>vocabulary</em> will be used when referring to a set of URIrefs defined for some specific purpose, such as the set of URIrefs defined by RDF for its own use, or the set of URIrefs defined by example.org to identify its employees. The term <em>namespace</em> will be used only when referring specifically to the syntactic concept of an XML namespace (or in describing the URI assigned to a prefix in a QName).</p>

<p>URIrefs from different vocabularies can be freely mixed in RDF graphs.  For example, the graph in <a href="#figure3">Figure 3</a> uses URIrefs from the <code>exterms:</code>, <code>exstaff:</code>, and <code>dc:</code> vocabularies.  Also, RDF imposes no restrictions on <a name="LCC-030" id="LCC-030">how many statements</a> using a given URIref as predicate can appear in a graph to describe the same resource.  For example, if the resource <code>ex:index.html</code> had been created by the cooperative efforts of several staff members in addition to John Smith, example.org might have written the statements:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:index.html  dc:creator  exstaff:85740 .

ex:index.html  dc:creator  exstaff:27354 .

ex:index.html  dc:creator  exstaff:00816 .
</pre>
        </div>

</div>

        <p>These examples of RDF statements begin to
        illustrate some of the advantages of using URIrefs as RDF's
        basic way of identifying things. For instance, in the first
        statement, instead of
        identifying the creator of the Web page by
        the character string "John Smith", he has been assigned a URIref,
        in this case (using a URIref based on his employee number)
        <code>http://www.example.org/staffid/85740</code> . An advantage of
        using a URIref in this case is that the identification
        of the statement's subject can be more precise. 
        That is, the creator of the page is not the
        character string "John Smith", or any one of the thousands of
        people named John Smith, but the particular John Smith
        associated with that URIref (whoever created the URIref defines
        the association). Moreover, since there is a URIref to refer to
        John Smith, he is a full-fledged resource, and 
        additional information can be recorded about him, simply by adding
        additional RDF statements with John's URIref as the subject.  
        For example, <a href="#figure4">Figure 4</a> shows some additional
        statements giving John's name and age.</p>


        <div class="figure">
          <img src="fig4dec16.png"
          alt="More Information About John Smith" /><br />
           <a id="figure4" name="figure4">Figure 4: More Information
          About John Smith</a>
        </div>

        <p>These examples also illustrate that RDF uses URIrefs as
        <em>predicates</em> in RDF statements. That is, rather than
        using character strings (or words) such as "creator" or "name"
        to identify properties, RDF uses URIrefs. Using URIrefs to
        identify properties is important for a number of reasons.
        First, it distinguishes the properties one person may use from
        different properties someone else may use that would otherwise be
        identified by the same character string. For instance, in the
        example in <a href="#figure4">Figure
        4</a>, example.org uses "name" to mean someone's full name
        written out as a character string literal (e.g., "John Smith"),
        but someone else may intend "name" to mean something different
        (e.g., the name of a variable in a piece of program text). A
        program encountering "name" as a property identifier on the Web
        (or merging data from multiple sources) would not necessarily be 
        able to distinguish these uses. However, if example.org writes
        <code>http://www.example.org/terms/name</code> for its "name"
        property, and the other person writes
        <code>http://www.domain2.example.org/genealogy/terms/name</code>
        for hers, it is clear that there are distinct
        properties involved (even if a program cannot automatically
        determine the distinct meanings). Also, using URIrefs to identify properties 
        enables the properties to be treated as resources themselves.
        Since properties are resources, additional
        information can be recorded about them (e.g., the English description of what
        example.org means by "name"), simply by adding additional RDF
        statements with the property's URIref as the subject.</p>

        <p>Using URIrefs as subjects, predicates, and objects in RDF
        statements supports the development and use of shared
        vocabularies on the Web, since people can discover and begin using 
        vocabularies already used by others to describe things, reflecting  
        a shared understanding of those concepts. For example, in
        the triple</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:index.html   dc:creator   exstaff:85740 .
</pre>
        </div>

        <p>the predicate <code>dc:creator</code>, when fully expanded as a
        URIref, is an unambiguous reference to the "creator" attribute
        in the Dublin Core metadata attribute set (discussed further in
        <a href="#dublincore">Section 6.1</a>), a widely-used set of
        attributes (properties) for describing information of all
        kinds. The writer of this triple is effectively saying that the
        relationship between the Web page (identified by
        <code>http://www.example.org/index.html</code> ) and the creator of
        the page (a distinct person, identified by
        <code>http://www.example.org/staffid/85740</code> ) is exactly the
        concept identified by
        <code>http://purl.org/dc/elements/1.1/creator</code>. 
        <span class="newstuff"> <a name="LCC-010" id="LCC-010">Another</a>
        person familiar with the Dublin Core vocabulary, 
        or who finds out what <code>dc:creator</code> 
        means (say by looking up its definition on the Web) 
        will know what is meant by this relationship. 
        In addition, based on this understanding, people can 
        write programs to behave in accordance with
        that meaning when processing triples containing the predicate
        <code>dc:creator</code>.</span></p>

<p><span class="newstuff"><a name="LCC-009" id="LCC-009">Of course,</a> this depends on increasing the general use of URIrefs to refer to things instead of using literals; e.g., using URIrefs like <code>exstaff:85740</code> and <code>dc:creator</code> instead of character string literals like <code>John Smith</code> and <code>creator</code>.</span>

        Even then, RDF's use of URIrefs does not solve all identification
        problems because, for example, people can still use different
        URIrefs to refer to the same thing. 

For this reason, it is a good idea to try to use terms from existing vocabularies (such as the
Dublin Core) where possible, rather than making up new terms that might overlap with those of 
some other vocabulary.  Appropriate vocabularies for use in specific application areas are
being developed all the time, as illustrated by the applications described in <a href="#applications">Section 6</a>.

        However, even when synonyms are created, the fact that
        these different URIrefs are used in the commonly-accessible
        "Web space" provides the opportunity both to identify
        equivalences among these different references, and to migrate
        toward the use of common references.</p>

<div class="newstuff">
<p>In addition, it is important to distinguish between any meaning that <em>RDF itself</em> associates with terms (such as <code>dc:creator</code> in the previous example) used in RDF statements and additional, <em>externally-defined</em> meaning that people (or programs written by those people) might associate with those terms.  

As a language, RDF directly defines only the graph syntax of subject, predicate, and object triples, certain meanings associated with URIrefs in the <code>rdf:</code> vocabulary, and certain other concepts to be described later.  These things are normatively defined in <a href="#ref-rdf-concepts">[RDF-CONCEPTS]</a> and <a href="#ref-rdf-semantics">[RDF-SEMANTICS]</a>. However, RDF does not define the meanings of terms from other vocabularies, such as <code>dc:creator</code>, that might be used in RDF statements.  Specific vocabularies will be created, with specific meanings assigned to the URIrefs defined in them, externally to RDF.  RDF statements using URIrefs from these vocabularies may convey the specific meanings associated with those terms to people familiar with these vocabularies, or to RDF applications written to process these vocabularies, without conveying any of these meanings to an arbitrary RDF application <em>not</em> specifically written to process these vocabularies.</p> 

        <p>For example, people can associate meaning with 
        a triple such as</p>
        <div class="exampleOuter exampleInner">
<pre>
ex:index.html  dc:creator  exstaff:85740 .
</pre>
        </div>
        <p>based on the meaning they associate with the appearance of the
        word "creator" as part of the URIref <code>dc:creator</code>, or based on
        their understanding of the specific definition of  <code>dc:creator</code>
        in the Dublin Core vocabulary.

         However, as far as an arbitrary RDF application is concerned the triple might as 
         well be something like</p>

        <div class="exampleOuter exampleInner">
<pre>
fy:joefy.iunm  ed:dsfbups  fytubgg:85740 .
</pre>
        </div>
         <p>as far as any built-in meaning is concerned.  Similarly, any 
         natural language text describing the meaning of <code>dc:creator</code> 
         that might be found on the Web provides no 
         additional meaning that an arbitrary RDF application can directly use.</p>

<p>Of course, URIrefs from a particular vocabulary can be used in RDF statements even though a given application may not be able to associate any special meanings with them.  
For example, 

         generic RDF software would recognize that
         the above expression is an RDF statement, that <code>ed:dsfbups</code> is the
         predicate, and so on.  It will simply not associate with the triple any special meaning that the vocabulary developer might have associated with a URIref like <code>ed:dsfbups</code>.  Moreover, based on their understanding of a given vocabulary, people can write RDF applications to behave in accordance with the special meanings assigned to URIrefs from that vocabulary, even though that meaning will not be accessible to RDF applications not written in that way.</p> 

</div>

        <p>The result of all this is that RDF provides a way to make
        statements that applications can more easily process. An
        application cannot actually "understand" such statements, as noted
        already, <span class="newstuff"> any more than a database system "understands" terms like "employee" or "salary" in processing a query like <code>SELECT NAME FROM EMPLOYEE WHERE SALARY > 35000</code>.  
</span>  

However, if an application is appropriately written,  
        it can deal with RDF statements in a way that makes it seem
        like it does understand them, <span class="newstuff">just as a database system and its applications can do useful work in processing employee and payroll information without understanding "employee" and "payroll".</span>

For example, a user could search the Web for all
        book reviews and create an average rating for each book. Then,
        the user could put that information back on the Web. Another
        Web site could take that list of book rating averages and
        create a "Top Ten Highest Rated Books" page. Here, the
        availability and use of a shared vocabulary about ratings, and
        a shared group of URIrefs identifying the books they apply to,
        allows individuals to build a mutually-understood and
        increasingly-powerful (as additional contributions are made)
        "information base" about books on the Web. The same principle
        applies to the vast amounts of information that people create
        about thousands of subjects every day on the Web.</p>

        <p>RDF statements are similar to a number of other formats for
        recording information, such as:</p>

        <ul>
          <li>entries in a simple record or catalog listing describing
          the resource in a data processing system.</li>

          <li>rows in a simple relational database.</li>

          <li>simple assertions in formal logic</li>
        </ul>

        <p>and information in these formats can be treated as RDF
        statements, allowing RDF to be used to integrate data from many
        sources.</p>

      </div>

      <div class="section">
        <h3><a id="structuredproperties"
        name="structuredproperties"></a>2.3 Structured Property Values
        and Blank Nodes</h3>

        <p>Things would be very simple if the only types of information
        to be recorded about things were obviously in the form of the
        simple RDF statements illustrated so far. However, most
        real-world data involves structures that are more complicated
        than that, at least on the surface. For instance, in the
        original example, the date the Web page was created is recorded
        as a single <code>exterms:creation-date</code> property, with a
        plain literal as its value. However, suppose 
        the value of the <code>exterms:creation-date</code> property
        needed to record
        the month, day, and year as separate pieces of information? Or,
        in the case of John Smith's personal information, suppose 
        John's address was being described. The whole address could be
        written out as a plain literal, as in the triple</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:85740   exterms:address   "1501 Grant Avenue, Bedford, Massachusetts 01730" .
</pre>
        </div>

        <p>However, suppose John's address needed to be recorded as a
        <em>structure</em> consisting of separate street, city, state,
        and postal code values? How would this be done in RDF?</p>

        <p>Structured information like this is represented in RDF by
        considering the aggregate thing to be described (like
        John Smith's address) as a resource, and then making statements
        about that new resource. So, in the RDF graph, in order to
        break up John Smith's address into its component parts, 
        a new node is created to represent the concept of John Smith's
        address, with a new URIref to identify it,
        say <code>http://www.example.org/addressid/85740</code> 
        (abbreviated as <code>exaddressid:85740</code>). 
        RDF statements (additional arcs and nodes) can then be
        written with that
        node as the subject, to represent the additional information,
        producing the graph shown in <a href="#figure5">Figure
        5</a>:</p>

        <div class="figure">
          <img src="fig5may19.png"
          alt="Breaking Up John's Address" /><br />
           <a id="figure5" name="figure5">Figure 5: Breaking Up John's
          Address</a>
        </div>

        <p>or the triples:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:85740       exterms:address        exaddressid:85740 .
exaddressid:85740   exterms:street         "1501 Grant Avenue" .
exaddressid:85740   exterms:city           "Bedford" .
exaddressid:85740   exterms:state          "Massachusetts" .
exaddressid:85740   exterms:postalCode     "01730" .
</pre>
        </div>

        <p>This way of representing structured information in RDF can 
        involve generating numerous "intermediate" URIrefs 
        such as <code>exaddressid:85740</code> to represent aggregate concepts such as
        John's address. Such concepts may never need to be referred to
        directly from outside a particular graph, and hence may not
        require "universal" identifiers. In addition, in the
        <em>drawing</em> of the graph representing the group of
        statements shown in <a href="#figure5">Figure 5</a>, 
        the URIref assigned to identify "John Smith's
        address" is not really needed, since the graph could just as easily 
        have been drawn as in <a href="#figure6">Figure 6</a>:</p>

        <div class="figure">
          <img src="fig6may19.png" alt="Using a Blank Node" /><br />
           <a id="figure6" name="figure6">Figure 6: Using a Blank
          Node</a>
        </div>

        <p><a href="#figure6">Figure 6</a>, which is a perfectly
        good RDF graph, uses a node without a URIref to stand for
        the concept of "John Smith's address". This <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-blank-node">blank
        node</a> serves its purpose in the drawing without needing a
        URIref, since the node itself provides the necessary
        connectivity between the various other parts of the graph.
        (Blank nodes were called <em>anonymous resources</em> in <a
        href="#ref-rdfms">[RDF-MS]</a>.) However, some
        form of explicit identifier for that node is needed in order to
        represent this graph as triples. To see this, trying to
        write the triples corresponding to what is shown in <a
        href="#figure6">Figure 6</a> would produce something like:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:85740   exterms:address         ??? .
???             exterms:street          "1501 Grant Avenue" .
???             exterms:city            "Bedford" .
???             exterms:state           "Massachusetts" .
???             exterms:postalCode      "01730" .
</pre>
        </div>

        <p>where ??? stands for something that indicates the presence
        of the blank node. Since a complex graph might contain more
        than one blank node, there also needs to be a way to differentiate
        between these different blank nodes in a triples representation
        of the graph. As a result, triples use <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-blank-node-id">blank
        node identifiers</a>, having the form <code>_:name</code>, to
        indicate the presence of blank nodes. For instance,
        in this example a blank node identifier <code>_:johnaddress</code> 
        might be used to refer to the blank node, in which
        case the resulting triples might be:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:85740   exterms:address         _:johnaddress .
_:johnaddress   exterms:street          "1501 Grant Avenue" .
_:johnaddress   exterms:city            "Bedford" .
_:johnaddress   exterms:state           "Massachusetts" .
_:johnaddress   exterms:postalCode      "01730" .
</pre>
        </div>

        <p>In a triples representation of a graph, each distinct blank
        node in the graph is given a different blank node identifier.
        Unlike URIrefs and literals, blank node identifiers are not
        considered to be actual parts of the RDF graph (this can be
        seen by looking at the drawn graph in <a href="#figure6">Figure
        6</a> and noting that the blank node has no blank node
        identifier). Blank node identifiers are just a way of
        representing the blank nodes in a graph (and distinguishing one
        blank node from another) when the graph is written in triple
        form. Blank node identifiers also have significance only within
        the triples representing a <em>single</em> graph (two different
        graphs with the same number of blank nodes might independently
        use the same blank node identifiers to distinguish them, and it
        would be incorrect to assume that blank nodes from different
        graphs having the same blank node identifiers are the same). If
        it is expected that a node in a graph will need to be
        referenced from outside the graph, a URIref should be assigned
        to identify it.  Finally, because blank node identifiers represent
        (blank) <em>nodes</em>, rather than arcs, in the triple form of
        an RDF graph, blank node identifiers may only appear as subjects or
        objects in triples; blank node identifiers may not be used as
        predicates in triples.</p>

        <p>The beginning of this section noted that aggregate structures, 
        like John Smith's address, can be represented by
        considering the aggregate thing to be described as a separate
        resource, and then making statements about that new resource.
        This example illustrates an important aspect of RDF: RDF
        directly represents only <em>binary</em> relationships, e.g.
        the relationship between John Smith and the literal
        representing his address. Representing the
        relationship between John and the group of separate
        <em>components</em> of this address involves dealing with an
        <em>n-ary</em> (n-way) relationship (in this case, n=5) between
        John and the street, city, state, and postal code components. In order
        to represent such structures directly in RDF (e.g., considering
        the address as a group of street, city, state, and postal code
        components), this n-way relationship must be broken up
        into a group of separate binary relationships. Blank nodes provide
        one way to do this. For each n-ary relationship,
        one of the participants is chosen as the subject of the
        relationship (John in this case), and a blank node is created to
        represent the rest of the relationship (John's address in this
        case). The remaining participants in the
        relationship (such as the city in this example) are then
        represented as separate
        properties of the new resource represented by the blank
        node.</p>

        <p>Blank nodes also provide a way to more accurately make
        statements about resources that may not have URIs, but that are
        described in terms of relationships with other resources that
        <em>do</em> have URIs. For example, when making statements
        about a person, say Jane Smith, it may seem natural to use a
        URI based on that person's email address as her URI, e.g.,
        <code>mailto:jane@example.org</code>. However, this approach can
        cause problems. For example, it may be necessary to record information
        both about <em>Jane's mailbox</em> (e.g., the server it is on) as well as
        about <em>Jane herself</em> (e.g., her current physical address), and using a
        URIref for Jane based on her email address makes it difficult
        to know whether it is Jane or her mailbox that is being described. 
        The same problem exists when a company's Web page URL, say
        <code>http://www.example.com/</code>, is used as the URI of the
        company itself. Once again, it may be necessary to record information
        about the Web page itself (e.g., who created it and when) as well as
        about the company, and using <code>http://www.example.com/</code>
        as an identifier for both makes it difficult to know which
        of these is the actual subject.</p>

        <p>The fundamental problem is that using Jane's
        <em>mailbox</em> as a stand-in for <em>Jane</em> is not really
        accurate: Jane and her mailbox are not the same thing, and
        hence they should be identified differently. When Jane herself
        does not have a URI, a blank node provides a more accurate way
        of modeling this situation. Jane can be represented by a blank
        node, and that blank node used as the subject of a statement
        with <code>exterms:mailbox</code> as the property
        and the URIref <code>mailto:jane@example.org</code> as
        its value. The blank node could also be described with an
        <code>rdf:type</code> property having a value of
        <code>exterms:Person</code> (types are discussed in more detail
        in the following sections), an <code>exterms:name</code> property
        having a value of <code>"Jane Smith"</code>, and any other
        descriptive information that might be useful, as shown in
        the following triples:</p>

        <div class="exampleOuter exampleInner">
<pre>
_:jane   exterms:mailbox   &lt;mailto:jane@example.org&gt; .
_:jane   rdf:type          exterms:Person .
_:jane   exterms:name      "Jane Smith" .
_:jane   exterms:empID     "23748"  .
_:jane   exterms:age       "26" .
</pre>
        </div>

        <p>(Note that <code>mailto:jane@example.org</code>
        is written within angle brackets in the first triple.  This is because 
        <code>mailto:jane@example.org</code> is a full URIref in the
        <code>mailto</code> URI scheme, rather than a QName abbreviation,
        and full URIrefs must be enclosed in angle brackets in the triples
        notation.)</p>

        <p>This says, accurately, that "there is a resource of type
        <code>exterms:Person</code>, whose electronic mailbox is identified
        by <code>mailto:jane@example.org</code>, whose name is <code>Jane
        Smith</code>, etc." That is, the blank node can be read as "there
        is a resource". Statements with that blank node as subject then
        provide information about the characteristics of that
        resource.</p>

        <p>In practice, using blank nodes instead of URIrefs in these
        cases does not change the way this kind of
        information is handled very much. For example, if it is known
        that an email address uniquely identifies someone at
        example.org (particularly if the address is unlikely to be
        reused), that fact can still be used to associate information
        about that person from multiple sources, even though the email
        address is not the person's URI. In this case, if some
        RDF is found on the Web that describes a book, and
        gives the author's contact information as
        <code>mailto:jane@example.org</code>, it might be reasonable,
        combining this new information with the previous set of
        triples, to conclude
        that the author's name is Jane Smith. The point is that saying
        something like "the author of the book is
        <code>mailto:jane@example.org</code>" is typically a shorthand for
        "the author of the book is someone whose mailbox is
        <code>mailto:jane@example.org</code>". Using a blank node to
        represent this "someone" is just a more accurate way to
        represent the real world situation. (Incidentally, some
        RDF-based schema languages allow specifying that certain
        properties are <em>unique identifiers</em> of the resources they
        describe. This is discussed further in
        <a href="#richerschemas">Section 5.5</a>.)</p>

        <div class="newstuff">
        <p>Using blank nodes in this way can also help avoid the use of
        literals in what might be inappropriate situations.  For example, 
        in describing Jane's book, lacking a URIref to identify the author,
        the publisher might have written (using the 
        publisher's own <code>ex2terms:</code> vocabulary):</p>  

        <div class="exampleOuter exampleInner">
<pre>
ex2terms:book78354   rdf:type          ex2terms:Book .
ex2terms:book78354   ex2terms:author   "Jane Smith" .
</pre>
        </div>

        <p>However, the author of the book is not really the character
        string "Jane Smith", but a person whose <em>name</em> is 
        Jane Smith.  The same information might be more accurately
        given by the publisher using a blank node, as:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex2terms:book78354   rdf:type          ex2terms:Book .
ex2terms:book78354   ex2terms:author   _:author78354 .
_:author78354        rdf:type          ex2terms:Person .
_:author78354        ex2terms:name     "Jane Smith" .
</pre>
        </div>

        <p>This essentially says "resource <code>ex2terms:book78354</code>
        is of type <code>ex2terms:Book</code>, and its author is a resource of type
        <code>ex2terms:Person</code>, whose name is <code>Jane
        Smith</code>."  Of course, in this particular case the publisher might instead 
        have assigned its own URIrefs to its authors instead of using blank nodes to 
        identify them, in order to encourage external 
        references to its authors.</p>

<p>Finally, the example above giving Jane's age as 26 illustrates the fact that sometimes the value of a property may appear to be simple, but actually may be more complex.  In this case, Jane's age is actually 26 <em>years</em>, but the units information (years) is not explicitly given.  Such information is often omitted in contexts where it can be safely assumed that anyone accessing the property value will understand the units being used.  However, in the wider context of the Web, it is generally <em>not</em> safe to make this assumption.  For example, a U.S. site might give a weight value in pounds, but someone accessing that data from outside the U.S. might assume that weights are given in kilograms.  In general, careful consideration should be given to explicitly representing units and similar information.  This issue is discussed further in <a href="#rdfvalue">Section 4.4</a>, which describes an RDF feature for representing such information as structured values, as well as some other techniques for representing such information.</p>

      </div>

      </div>

      <div class="section">
        <h3><a id="typedliterals" name="typedliterals"></a>2.4 Typed
        Literals</h3>

        <p>The last section described how to handle situations
        in which property values represented by plain
        literals had to be broken up into structured values to
        represent the individual parts of those literals. Using
        this approach, instead of, say, recording the date a Web page
        was created as a single <code>exterms:creation-date</code>
        property, with a single plain literal as its value, 
        the value would be represented as a structure consisting of the month,
        day, and year as separate pieces of information, using separate
        plain literals to represent the corresponding values. 
        However, so
        far, all constant values that serve as objects in RDF statements 
        have been represented by these plain
        (untyped) literals, even when the intent is probably for the value
        of the property to be a number (e.g., the value of a
        <code>year</code> or <code>age</code> property) or some other kind of
        more specialized value.</p>

        <p>For example, <a href="#figure4">Figure 4</a> 
        illustrated an RDF graph recording information about John
        Smith. That graph recorded the value of John Smith's
        <code>exterms:age</code> property as the plain literal "27", as
        shown in <a href="#figure7">Figure 7</a>:</p>

        <div class="figure">
          <img src="fig7dec16.png"
          alt="Representing John Smith's Age" /><br />
           <a id="figure7" name="figure7">Figure 7: Representing John
          Smith's Age</a>
        </div>

        <p>In this case, the hypothetical organization example.org
        probably intends for "27" to be interpreted as a number, rather
        than as the string consisting of the character "2" followed by
        the character "7" 
        <span class="newstuff">(since the literal represents the value of an "age" 
        property).  However, there is no information in Figure 7's graph
        that explicitly indicates that "27" should be interpreted as
        a number.  Similarly, example.org also
        probably intends for "27" to be interpreted as a <em>decimal</em>
        number, i.e., the value <em>twenty seven</em>, rather than, say,
        as an <em>octal</em> number, i.e., the value <em>twenty three</em>.
        However, once again there is no information in Figure 7's graph that
        explicitly indicates this.  Specific applications might be written 
        with the understanding that they should 
        interpret values of the <code>exterms:age</code> property as decimal 
        numbers, but this would mean that proper interpretation of this
        RDF would depend on information not explicitly provided
        in the RDF graph, and hence on information that would not necessarily 
        be available to other applications that might need to interpret this RDF.</span></p>  

        <p>The common practice in
        programming languages or database systems is to provide this
        additional information about how to interpret a literal
        by associating a <em>datatype</em> with the
        literal, in this case, a datatype like <code>decimal</code> or
        <code>integer</code>. An application that understands the datatype
        then knows, for example, whether the literal "10" is intended
        to represent the number <em>ten</em>, the number <em>two</em>,
        or the string consisting of the character "1" followed by the
        character "0", depending on whether the specified datatype is
        <code>integer</code>, <code>binary</code>, or <code>string</code>. 

(More specialized datatypes could also be used to include the units information mentioned 
at the end of <a href="#structuredproperties">Section 2.3</a>, e.g., a datatype 
<code>integerYears</code>, although the Primer will not elaborate on this idea.)

        In RDF,
        <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-typed-literal">typed
        literals</a> are used to provide this kind of information.</p>
        
        <p><span class="newstuff">An RDF typed literal is formed by pairing a string
        with a URIref that identifies a particular datatype.  This
        results in a single literal node in the RDF graph with the pair as the literal.
        The value represented by the typed literal is the value that 
        the specified datatype associates with the specified string.</span>

        For example, using a typed literal, John Smith's age could be described as
        being the integer number <em>27</em> using the triple:</p>

        <div class="exampleOuter exampleInner">
<pre>
&lt;http://www.example.org/staffid/85740&gt;  &lt;http://www.example.org/terms/age&gt; "27"^^&lt;http://www.w3.org/2001/XMLSchema#integer&gt; .
</pre>
        </div>

        <p>or, using the QName simplification for writing long
        URIs:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:85740  exterms:age  "27"^^xsd:integer .
</pre>
        </div>

        <p>or as shown in <a href="#figure8">Figure 8</a>:</p>

        <div class="figure">
          <img src="fig8jul23.png"
          alt="A Typed Literal for John Smith's Age" /><br />
           <a id="figure8" name="figure8">Figure 8: A Typed Literal for
          John Smith's Age</a>
        </div>

        <p>Similarly, in the graph shown in <a href="#figure3">Figure
        3</a> describing information about a Web page, the
        value of the page's <code>exterms:creation-date</code> property 
        was written as
        the plain literal "August 16, 1999". However, using a typed
        literal, the creation date of the Web page could be explicitly described as
        being the date <em>August 16, 1999</em>, using the triple:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:index.html  exterms:creation-date  "1999-08-16"^^xsd:date .
</pre>
        </div>

        <p>or as shown in <a href="#figure9">Figure 9</a>:</p>

        <div class="figure">
          <img src="fig9jul23.png"
          alt="A Typed Literal for a Web Page's Creation Date" /><br />
           <a id="figure9" name="figure9">Figure 9: A Typed Literal for
          a Web Page's Creation Date</a>
        </div>

        <p>Unlike typical programming languages and database systems,
        RDF has no built-in set of datatypes of its own, such as
        datatypes for integers, reals, strings, or dates. 
        <span class="newstuff">Instead,
        RDF typed literals simply provide a way to explicitly
        indicate, for a given literal, what datatype should be used to
        interpret it.  The datatypes used in typed literals are defined 
        externally to RDF, and identified by their <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-datatype-URI">datatype
        URIs</a>.
        (There is one exception:  RDF defines a built-in datatype with the
        URIref <code>rdf:XMLLiteral</code> to represent XML content as a literal
        value.  This datatype is defined in 
        <a href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>, and its use is
        described in <a href="#xmlliterals">Section 4.5</a>.)
        </span>
        For instance, the examples in <a href="#figure8">Figure 8</a> 
        and <a href="#figure9">Figure 9</a> use the datatypes <code>integer</code> and
        <code>date</code> from the XML Schema datatypes defined in <a
        href="http://www.w3.org/TR/xmlschema-2/">XML Schema Part 2:
        Datatypes</a> <a href="#ref-xmlschema2">[XML-SCHEMA2]</a>. 

        An advantage of this approach is that it
        gives RDF the flexibility to directly represent information
        coming from different sources without the need to perform type
        conversions between these sources and a native set of RDF
        datatypes. (Type conversions would still be required when
        moving information between systems having different sets of datatypes,
        but RDF would impose no extra conversions into and out
        of a native set of RDF datatypes.)</p>

        <div class="newstuff">
        <p>RDF datatype concepts are based on 
        <a name="LCC-025" id="LCC-025">a conceptual framework</a>
        from XML Schema datatypes <a
        href="#ref-xmlschema2">[XML-SCHEMA2]</a>, as described
        in <a href="http://www.w3.org/TR/rdf-concepts/">RDF
        Concepts and Abstract Syntax</a> <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>.

        This conceptual framework defines a datatype as consisting of:</p>

      <ul>
        <li>A set of values, called the <em>value space</em>, that 
        literals of the datatype are intended
        to represent.  For example, for the XML Schema datatype 
        <code>xsd:date</code>, this set of values is a set of dates.</li>

        <li>A set of character strings, called the <em>lexical space</em>, that
        the datatype uses to represent its values.  This set determines
        which character strings can legally be used to represent 
        literals of this datatype.  For example, the datatype 
        <code>xsd:date</code> defines <code>1999-08-16</code> as being a legal
        way to write a literal of this type 
        (as opposed, say, to <code>August 16, 1999</code>).

As defined in <a href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>, the lexical space of a datatype is a set of Unicode <a href="#ref-unicode">[UNICODE]</a> strings, allowing information from many languages to be directly represented.

</li>

        <li>A <em>lexical-to-value mapping</em> from the lexical 
        space to the value space.  This determines the value that a
        given character string from the lexical space represents 
        for this particular datatype.  For example, 
        the lexical-to-value mapping for datatype <code>xsd:date</code>
        determines that, for this datatype, the string <code>1999-08-16</code>
        represents the date <em>August 16, 1999</em>.  The lexical-to-value
        mapping is a factor because the same character string may represent
        different values for different datatypes.</li>
      </ul>
 
        <p>Not all datatypes are suitable for use in RDF.  For a datatype 
        to be suitable for use in RDF, it
        must conform to the conceptual framework just described.  This basically means
        that, given a character string, the datatype must 
        unambiguously define 
        whether or not the string is in its lexical space, and
        what value in its value space the string represents.

        For example, the basic XML Schema datatypes 
        such as <code>xsd:string</code>, <code>xsd:boolean</code>, <code>xsd:date</code>, 
        etc. are suitable
        for use in RDF.  However, some of the built-in XML Schema datatypes
        are not suitable for use in RDF.  For example, <code>xsd:duration</code> does
        not have a well-defined value space, and <code>xsd:QName</code> requires an
        enclosing XML document context.  Lists of the XML Schema datatypes
        that are currently considered suitable and unsuitable for use in
        RDF are given in <a href="#ref-rdf-semantics">[RDF-SEMANTICS]</a>.</p> 
</div>

        <p> <span class="newstuff">Since the value that a given typed literal denotes is defined
        by the typed literal's datatype, and, with the exception of 
        <code>rdf:XMLLiteral</code>, RDF does not define any datatypes,
        the actual interpretation of a 
        typed literal appearing in an RDF graph (e.g., determining
        the value it denotes) must be performed by software
        that is written to correctly process not only RDF, but the
        typed literal's datatype as well.  Effectively, this software must
        be written to process an extended language that includes not
        only RDF, but also the datatype, as part of its built-in
        vocabulary.
        This raises the issue of which datatypes will be generally available in
        RDF software.

        Generally, the XML Schema datatypes that are listed as suitable 
        for use in RDF in <a href="#ref-rdf-semantics">[RDF-SEMANTICS]</a> 
        </span>
        have a "first among equals" status in RDF. 
        As noted already, the examples in <a href="#figure8">Figure 8</a> and
        <a href="#figure9">Figure 9</a>
        used some of these XML Schema datatypes, and the Primer will be 
        using these datatypes in
        most of its other examples of typed literals as well (for one thing, XML Schema
        datatypes already have assigned URIrefs that can be used to refer to them, specified
        in <a href="#ref-xmlschema2">[XML-SCHEMA2]</a>). These XML Schema datatypes are
        treated no differently than any other datatype, but they are
        expected to be the most widely used, and therefore the most
        likely to be interoperable among different software. As a
        result, it is expected that much RDF software will also be
        written to process these datatypes. However, RDF software
        could be written to process other sets of datatypes as
        well, assuming they were determined to be suitable for use
        with RDF, as described already.</p>

        <p>In general, RDF software may be called on to process RDF
        data that contains references to datatypes that the software 
        has not been written to
        process, in which case there are some things the software
        will not be able to do. 
        <span class="newstuff">
        For one thing, with the exception of <code>rdf:XMLLiteral</code>,
        RDF itself does not define the URIrefs that identify datatypes.
        As a result, RDF software, unless it has been written to recognize specific
        URIrefs, will not be able to determine whether or not
        a URIref written in a typed literal actually identifies a datatype.
        Moreover, even when a URIref does identify a datatype, RDF
        itself does not define the validity of pairing that datatype
        with a particular literal.</span> This validity can only be determined
        by software written to correctly process that particular datatype. </p>

        <p>For example, the typed literal in the triple:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:85740  exterms:age  "pumpkin"^^xsd:integer .
</pre>
        </div>

        <p>or the graph shown in <a href="#figure10">Figure 10</a>:</p>

        <div class="figure">
          <img src="fig10jul23.png"
          alt="An Invalid Typed Literal for John Smith's Age" /><br />
           <a id="figure10" name="figure10">Figure 10: An Invalid Typed
          Literal for John Smith's Age</a>
        </div>

        <p>is valid RDF, but obviously an error as far as the
        <code>xsd:integer</code> datatype is concerned, since <code>"pumpkin"</code> is
        not defined as being in the lexical space of
        <code>xsd:integer</code>.

        RDF software not written to
        process the <code>xsd:integer</code> datatype would not be able
        to recognize <span class="newstuff">this error.</span></p>

        <p class="newstuff">However, proper use of RDF typed literals provides more information about
        the intended interpretation of literal values, and hence makes
        RDF statements a better means of information exchange among applications.</p>

      </div>

      <div class="section">
        <h3><a name="conceptsummary" id="conceptsummary">2.5 Concepts
        Summary</a></h3>

        <p>Taken as a whole, RDF is basically simple: nodes-and-arcs diagrams
        interpreted as statements about things identified by URIrefs.
        This section has presented an introduction to these concepts.
        As noted earlier, the normative (i.e., definitive) RDF
        specification describing these concepts is <a
        href="http://www.w3.org/TR/rdf-concepts/">RDF Concepts and
        Abstract Syntax</a> <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>, which should be
        consulted for further information. The formal semantics (meaning)
        of these concepts is defined in the (normative) <a
        href="http://www.w3.org/TR/rdf-mt/">RDF Semantics</a> <a
        href="#ref-rdf-semantics">[RDF-SEMANTICS]</a> document.</p>

        <p>However, in addition to the basic techniques for
        describing things using RDF statements discussed
        so far, it should be clear that people or organizations also need a way
        to describe the <em>vocabularies</em> (terms) they intend to use in those
        statements, specifically, vocabularies for:</p>

        <ul>
          <li>describing types of things (like <code>exterms:Person</code>)</li>

          <li>describing properties (like <code>exterms:age</code> and
          <code>exterms:creation-date</code>), and</li>

          <li>describing the types of things that can serve as the
          subjects or objects of statements involving those properties
          (such as specifying that the value of an <code>exterms:age</code>
          property should always be an <code>xsd:integer</code>).</li>
        </ul>

        <p>The basis for describing such vocabularies in RDF is the <a
        href="http://www.w3.org/TR/rdf-schema/">RDF Vocabulary
        Description Language 1.0: RDF Schema</a> <a
        href="#ref-rdf-vocabulary">[RDF-VOCABULARY]</a>, which will be
        described in <a href="#rdfschema">Section 5</a>.</p>

        <p>Additional background on the basic ideas underlying RDF, and
        its role in providing a general language for describing Web
        information, can be found in <a
        href="#ref-webdata">[WEBDATA]</a>. RDF draws upon ideas from
        knowledge representation, artificial intelligence, and data
        management, including Conceptual Graphs, logic-based knowledge
        representation, frames, and relational databases. Some possible
        sources of background information on these subjects include <a
        href="#ref-sowa">[SOWA]</a>, <a href="#ref-cg">[CG]</a>, <a
        href="#ref-kif">[KIF]</a>, <a href="#ref-hayes">[HAYES]</a>, <a
        href="#ref-luger">[LUGER]</a>, and <a
        href="#ref-gray">[GRAY]</a>.</p>
      </div>
    </div>

    <div class="section">
      <h2><a id="rdfxml" name="rdfxml"></a>3. An XML Syntax for RDF:
      RDF/XML</h2>

      <p>As described in Section 2, RDF's conceptual model is a
      graph. RDF provides an XML syntax for writing down and exchanging
      RDF graphs, called <em>RDF/XML</em>. Unlike triples, which are
      intended as a shorthand notation, RDF/XML is the normative syntax
      for writing RDF. RDF/XML is defined in the <a
      href="http://www.w3.org/TR/rdf-syntax-grammar/">RDF/XML Syntax
      Specification</a> <a href="#ref-rdf-syntax">[RDF-SYNTAX]</a>.
      This section describes this RDF/XML syntax.</p>

      <div class="section">
        <h3><a name="basicprinciples" id="basicprinciples">3.1 Basic
        Principles</a></h3>

        <p>The basic ideas behind the RDF/XML syntax can be illustrated
        using some of the examples presented already. Take as an example
        the English statement:</p>

        <p class="ptriple"><code><strong>http://www.example.org/index.html</strong>
        has a <strong>creation-date</strong> whose value is <strong>August 16,
        1999</strong></code></p>

        <p>The RDF graph for this single statement, after assigning a
        URIref to the <code>creation-date</code> property, is shown in <a
        href="#figure11">Figure 11</a>:</p>

        <div class="figure">
          <img src="fig11dec16.png"
          alt="Describing a Web Page's Creation Date" /><br />
           <a id="figure11" name="figure11">Figure 11: Describing a Web
          Page's Creation Date</a>
        </div>

        <p>with a triple representation of:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:index.html   exterms:creation-date   "August 16, 1999" .
</pre>
        </div>

        <p class="newstuff">(Note that a typed literal is not used for the date value in 
        this example.  Representing typed literals in RDF/XML will be
        described later in this section.)</p>

        <p><a href="#example2">Example 2</a> shows the RDF/XML syntax
        corresponding to the graph in <a href="#figure11">Figure
        11</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example2" name="example2">Example 2: RDF/XML for the
            Web Page's Creation Date</a>
          </div>

          <div class="exampleInner">
<pre>
1. &lt;?xml version="1.0"?&gt;
2. &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.             xmlns:exterms="http://www.example.org/terms/"&gt;

4.   &lt;rdf:Description rdf:about="http://www.example.org/index.html"&gt;
5.       &lt;exterms:creation-date&gt;August 16, 1999&lt;/exterms:creation-date&gt;
6.   &lt;/rdf:Description&gt;

7. &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>(Line numbers are added to help in explaining the
        example.)</p>

        <p>This seems like a lot of overhead. It is easier to understand 
        what is going on by considering each part of this XML in turn
        (a brief introduction to XML is provided in <a
        href="#documents">Appendix B</a>).</p>

        <p>Line 1, <code>&lt;?xml version="1.0"?&gt;</code>, is the <em>XML
        declaration</em>, which indicates that the following content is XML,
        and what version of XML it is.</p>

        <p>Line 2 begins an <code>rdf:RDF</code> element. This indicates
        that the following XML content (starting here and ending with
        the <code>&lt;/rdf:RDF&gt;</code> in line 7) is intended to
        represent RDF. Following the <code>rdf:RDF</code> on this same line
        is an XML namespace declaration, represented as an
        <code>xmlns</code> attribute of the <code>rdf:RDF</code> start-tag.
        <a name="LCC-012a" id="LCC-012a">This declaration</a>
         specifies that all tags in this content
        prefixed with <code>rdf:</code> are part of the namespace
        identified by the URIref
        <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#</code>. 
        <span class="newstuff">URIrefs beginning with the string 
        <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#</code>
        are used for terms from the RDF vocabulary.</span></p>

        <p>Line 3 specifies another XML namespace declaration, this
        time for the prefix <code>exterms:</code>. This is expressed as
        another <code>xmlns</code> attribute of the <code>rdf:RDF</code>
        element, and specifies that the namespace URIref
        <code>http://www.example.org/terms/</code> is to be associated with
        the <code>exterms:</code> prefix. 

        <span class="newstuff">URIrefs beginning with the string 
        <code>http://www.example.org/terms/</code>
        are used for terms from the vocabulary defined by the example organization,
        example.org.</span> 

        The "&gt;" at the end of line 3 indicates the end
        of the <code>rdf:RDF</code> start-tag. Lines 1-3 are general
        "housekeeping" necessary to indicate that this is
        RDF/XML content, and to identify the namespaces being used 
        within the RDF/XML content.</p>

        <p>Lines 4-6 provide the RDF/XML for the specific statement
        shown in <a href="#figure11">Figure
        11</a>. An obvious way to talk about any RDF
        statement is to say it is a <em>description</em>, and that it is
        <em>about</em> the subject of the statement (in this case,
        about http://www.example.org/index.html), and this is the way
        RDF/XML represents the statement. The <code>rdf:Description</code>
        start-tag in line 4 indicates the start of a
        <em>description</em> of a resource, and goes on to identify the
        resource the statement is <em>about</em> (the subject of the
        statement) using the <code>rdf:about</code> attribute to specify
        the URIref of the subject resource. 
        <span class="newstuff">Line 5 provides a
        <em>property element</em>, with the QName
        <code>exterms:creation-date</code> as its tag, to 
        represent the predicate and object of the statement.  
        The QName <code>exterms:creation-date</code> is chosen
        so that appending
        the local name <code>creation-date</code> to the URIref of the
        <code>exterms:</code> prefix (<code>http://www.example.org/terms/</code>)
        gives the statement's predicate URIref
        <code>http://www.example.org/terms/creation-date</code>. 
        The content of this property element is the object of the
        statement, the plain literal <code>August 19, 1999</code>
        (the value of the creation-date property of the subject resource).
        </span>
        The property element is nested within the containing
        <code>rdf:Description</code> element, indicating that this property
        applies to the resource specified in the <code>rdf:about</code>
        attribute of the <code>rdf:Description</code> element. Line 6
        indicates the end of this particular <code>rdf:Description</code>
        element.</p>

        <p>Finally, Line 7 indicates the end of the <code>rdf:RDF</code>
        element started on line 2.  Using an <code>rdf:RDF</code> element
        to enclose RDF/XML content is optional in situations where 
        the XML can be identified as RDF/XML by context.  This is discussed 
        further in <a href="#ref-rdf-syntax">[RDF-SYNTAX]</a>.  However, it
        does not hurt to provide the <code>rdf:RDF</code> element in any case,
        and Primer examples will generally (but not always) provide one.</p>

        <p><a href="#example2">Example 2</a> illustrates the basic
        ideas used by RDF/XML to encode an RDF graph as XML elements,
        attributes, element content, and attribute values. The URIrefs
        of predicates (as well as some nodes) are written as XML
        <em>QNames</em>, consisting of a short <em>prefix</em> denoting
        a namespace URI, together with a <em>local name</em> denoting a
        namespace-qualified element or attribute, as described in <a
        href="#documents">Appendix B</a>. The (namespace URIref, local
        name) pair is chosen so that concatenating them forms the
        URIref of the original node or predicate. The URIrefs of subject nodes are
        written as XML attribute values (URIrefs of object nodes may sometimes be
        written as attribute values as well). Literal nodes
        (which are always object nodes) become element text content or
        attribute values. (Many of these options are described later in 
        the Primer;  all of these options are described in <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>.)</p>

        <p>An RDF graph consisting of multiple statements can be represented 
        in RDF/XML by using RDF/XML similar to Lines 4-6 in
        <a href="#example2">Example 2</a> to separately represent each
        statement. For example, to write the following two
        statements:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:index.html   exterms:creation-date   "August 16, 1999" .
ex:index.html   dc:language             "en" .
</pre>
        </div>

        <p>the RDF/XML in <a href="#example3">Example
        3</a> could be used:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example3" name="example3">Example 3: RDF/XML for Two
            Statements</a>
          </div>

          <div class="exampleInner">
<pre>
1.  &lt;?xml version="1.0"?&gt;
2.  &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.              xmlns:dc="http://purl.org/dc/elements/1.1/"
4.              xmlns:exterms="http://www.example.org/terms/"&gt;

5.    &lt;rdf:Description rdf:about="http://www.example.org/index.html"&gt;
6.        &lt;exterms:creation-date&gt;August 16, 1999&lt;/exterms:creation-date&gt;
7.    &lt;/rdf:Description&gt;

8.    &lt;rdf:Description rdf:about="http://www.example.org/index.html"&gt;
9.        &lt;dc:language&gt;en&lt;/dc:language&gt;
10.   &lt;/rdf:Description&gt;

11. &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p><a href="#example3">Example 3</a> is the same as <a
        href="#example2">Example 2</a>, with the addition of
        a second <code>rdf:Description</code> element (in lines 8-10)
        to represent the second statement. (An additional namespace declaration 
        is also given in line 3
        to identify the additional namespace used in this statement.) 
        An arbitrary number of
        additional statements could be written in the same way, using a separate
        <code>rdf:Description</code> element for each additional statement.
        As <a href="#example3">Example 3</a> illustrates, once the
        overhead of writing the XML and namespace declarations is dealt
        with, writing each additional RDF statement in RDF/XML is both
        straightforward and not too complicated.</p>

        <p>The RDF/XML syntax provides a number of abbreviations to
        make common uses easier to write. For example, it is typical
        for the same resource to be described with several properties
        and values at the same time, as in <a href="#example3">Example
        3</a>, where the resource <code>ex:index.html</code> is the subject
        of several statements. To handle such cases, RDF/XML allows
        multiple property elements representing those properties to be
        nested within the <code>rdf:Description</code> element that
        identifies the subject resource. For example, to
        represent the following group of statements about
        <code>http://www.example.org/index.html</code>:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:index.html   dc:creator              exstaff:85740 .
ex:index.html   exterms:creation-date   "August 16, 1999" .
ex:index.html   dc:language             "en" .
</pre>
        </div>

        <p>whose graph (the same as <a href="#figure3">Figure 3</a>) is
        shown in <a href="#figure12">Figure 12</a>:</p>

        <div class="figure">
          <img src="fig3nov19.png"
          alt="Several Statements About the Same Resource" /><br />
           <a id="figure12" name="figure12">Figure 12: Several
          Statements About the Same Resource</a>
        </div>

        <p>the RDF/XML shown in <a
        href="#example4">Example 4</a> could be written:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example4" name="example4">Example 4: Abbreviating
            Multiple Properties</a>
          </div>

          <div class="exampleInner">
<pre>
1.  &lt;?xml version="1.0"?&gt;
2.  &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.              xmlns:dc="http://purl.org/dc/elements/1.1/"
4.              xmlns:exterms="http://www.example.org/terms/"&gt;

5.    &lt;rdf:Description rdf:about="http://www.example.org/index.html"&gt;
6.         &lt;exterms:creation-date&gt;August 16, 1999&lt;/exterms:creation-date&gt;
7.         &lt;dc:language&gt;en&lt;/dc:language&gt;
8.         &lt;dc:creator rdf:resource="http://www.example.org/staffid/85740"/&gt;
9.    &lt;/rdf:Description&gt;

10. &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>Compared with the previous two examples, <a
        href="#example4">Example 4</a> adds an additional <code>dc:creator</code>
        property element (in line 8). In addition, the
        property elements for the three properties whose subject is
        <code>http://www.example.org/index.html</code> are nested within a single
        <code>rdf:Description</code> element identifying that subject,
        rather than writing a separate <code>rdf:Description</code> element
        for each statement.</p>

        <p>Line 8 also introduces a new form of property element. The
        <code>dc:language</code> element in line 7 is similar to the
        <code>exterms:creation-date</code> element used in <a
        href="#example2">Example 2</a>. Both these elements represent
        properties with plain literals as property values, and such
        elements are written by enclosing the literal within start-
        and end-tags corresponding to the property name. However, the
        <code>dc:creator</code> element on line 8 represents a property
        whose value is <em>another resource</em>, rather than a
        literal. If the URIref of this resource were written as a
        plain literal within start- and end-tags in the same way as 
        the literal values of the other elements, this would
        say that the value of the <code>dc:creator</code> element was
        the <em>character string</em>
        <code>http://www.example.org/staffid/85740</code>, rather than the
        resource identified by that literal interpreted as a URIref. In
        order to indicate the difference, the
        <code>dc:creator</code> element is written using what XML calls an
        <em>empty-element tag</em> (it has no separate end-tag), and
        the property value is written using an <code>rdf:resource</code>
        attribute within that empty element. The <code>rdf:resource</code>
        attribute indicates that the property element's value is
        another resource, identified by its URIref. Because the URIref
        is being used as an attribute <em>value</em>, RDF/XML requires
        the URIref to be written out (as an absolute or relative URIref), 
        rather than abbreviating it as a
        QName as was done in writing element and attribute
        <em>names</em> (absolute and relative URIrefs are discussed in
        <a href="#identifiers">Appendix A</a>).</p>

        <p>It is important to understand that the RDF/XML in <a
        href="#example4">Example 4</a> is an <em>abbreviation</em>. The
        RDF/XML in <a href="#example5">Example 5</a>, in which each
        statement is written separately, describes exactly the same RDF
        graph (the graph of <a href="#figure12">Figure 12</a>):</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example5" name="example5">Example 5: Writing Example
            4 as Separate Statements</a>
          </div>

          <div class="exampleInner">
<pre>
 &lt;?xml version="1.0"?&gt;
 &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
             xmlns:dc="http://purl.org/dc/elements/1.1/"
             xmlns:exterms="http://www.example.org/terms/"&gt;

   &lt;rdf:Description rdf:about="http://www.example.org/index.html"&gt;
       &lt;exterms:creation-date&gt;August 16, 1999&lt;/exterms:creation-date&gt;
   &lt;/rdf:Description&gt;

   &lt;rdf:Description rdf:about="http://www.example.org/index.html"&gt;
       &lt;dc:language&gt;en&lt;/dc:language&gt;
   &lt;/rdf:Description&gt;

   &lt;rdf:Description rdf:about="http://www.example.org/index.html"&gt;
       &lt;dc:creator rdf:resource="http://www.example.org/staffid/85740"/&gt;
   &lt;/rdf:Description&gt;

 &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>The following sections will describe a few additional 
        RDF/XML abbreviations. <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a> provides a more thorough
        description of the abbreviations that are available.</p>

        <p>RDF/XML can also represent graphs that include
        nodes that have no URIrefs, i.e., the <em>blank nodes</em>
        described in <a href="#structuredproperties">Section 2.3</a>. For
        example, <a href="#figure13">Figure 13</a> (taken from <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>) shows a graph saying
        "the document 'http://www.w3.org/TR/rdf-syntax-grammar' has a
        title 'RDF/XML Syntax Specification (Revised)' and has an
        editor, the editor has a name 'Dave Beckett' and a home page
        'http://purl.org/net/dajobe/' ".</p>

        <div class="figure">
          <img src="fig13dec16.png"
          alt="A Graph Containing a Blank Node" /><br />
           <a id="figure13" name="figure13">Figure 13: A Graph
          Containing a Blank Node</a>
        </div>

        <p>This illustrates an idea discussed in <a
        href="#structuredproperties">Section 2.3</a>: the use of a
        blank node to represent something that does not have a URIref,
        but can be described in terms of other information. In this
        case, the blank node represents a person, the editor of the
        document, and the person is described by his name and home
        page.</p>

        <p>RDF/XML provides several ways to represent graphs
        containing blank nodes. These are all described in <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>. The approach 
        illustrated here, which is the most direct approach, is to
        assign a <em>blank node identifier</em> to each blank node. A
        blank node identifier serves to identify a blank node within a
        particular RDF/XML document but, unlike a URIref, is unknown
        outside the document in which it is assigned. A blank node is
        referred to in RDF/XML using an <code>rdf:nodeID</code> attribute,
        with a blank node identifier as its value, in places where the
        URIref of a resource would otherwise appear. Specifically,
        a statement with a blank node as its <em>subject</em> can be written in
        RDF/XML using an <code>rdf:Description</code> element with
        an <code>rdf:nodeID</code> attribute instead of an
        <code>rdf:about</code> attribute. Similarly, a statement with a
        blank node as its <em>object</em> can be written using a property
        element with an <code>rdf:nodeID</code> attribute instead of an
        <code>rdf:resource</code> attribute. Using <code>rdf:nodeID</code>, <a
        href="#example6">Example 6</a> shows the RDF/XML corresponding
        to <a href="#figure13">Figure 13</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example6" name="example6">Example 6: RDF/XML
            Describing a Blank Node</a>
          </div>

          <div class="exampleInner">
<pre>
1.  &lt;?xml version="1.0"?&gt;
2.  &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.              xmlns:dc="http://purl.org/dc/elements/1.1/"
4.              xmlns:exterms="http://example.org/stuff/1.0/"&gt;

5.     &lt;rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar"&gt;
6.       &lt;dc:title&gt;RDF/XML Syntax Specification (Revised)&lt;/dc:title&gt;
7.       &lt;exterms:editor rdf:nodeID="abc"/&gt;
8.     &lt;/rdf:Description&gt;

9.     &lt;rdf:Description rdf:nodeID="abc"&gt;
10.        &lt;exterms:fullName&gt;Dave Beckett&lt;/exterms:fullName&gt;
11.        &lt;exterms:homePage rdf:resource="http://purl.org/net/dajobe/"/&gt;
12.    &lt;/rdf:Description&gt;

13. &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>In <a href="#example6">Example 6</a>, the blank node
        identifier <code>abc</code> is used in line 9 to identify the blank
        node as the subject of several statements, and is used in line
        7 to indicate that the blank node is the value of a resource's
        <code>exterms:editor</code> property. The advantage of using a
        blank node identifier over some of the other approaches
        described in <a href="#ref-rdf-syntax">[RDF-SYNTAX]</a> is that
        using a blank node identifier allows the same blank node to be
        referred to in more than one place in the same RDF/XML
        document.</p>

        <p>Finally, the <em>typed literals</em> described in <a
        href="#typedliterals">Section 2.4</a> may be used as property
        values instead of the plain literals used in the
        examples so far. A typed literal is represented in RDF/XML by
        adding an <code>rdf:datatype</code> attribute specifying a datatype
        URIref to the property element containing the literal.</p>

        <p>For example, to change the statement in <a
        href="#example2">Example 2</a> to use a typed literal instead
        of a plain literal for the <code>exterms:creation-date</code> property, the
        triple representation would be:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:index.html   exterms:creation-date   "1999-08-16"^^xsd:date .
</pre>
        </div>

        <p>with corresponding RDF/XML syntax shown in <a
        href="#example7">Example 7</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example7" name="example7">Example 7: RDF/XML Using a
            Typed Literal</a>
          </div>

          <div class="exampleInner">
<pre>
1. &lt;?xml version="1.0"?&gt;
2. &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
3.             xmlns:exterms="http://www.example.org/terms/"&gt;

4.   &lt;rdf:Description rdf:about="http://www.example.org/index.html"&gt;
5.     &lt;exterms:creation-date rdf:datatype=
         "http://www.w3.org/2001/XMLSchema#date"&gt;1999-08-16
       &lt;/exterms:creation-date&gt;
6.   &lt;/rdf:Description&gt;

7. &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>In line 5 of <a href="#example7">Example 7</a>, a typed
        literal is given as the value of the <code>exterms:creation-date</code>
        property element by adding an <code>rdf:datatype</code> attribute
        to the element's start-tag to specify the datatype. The value
        of this attribute is the URIref of the datatype, in this case,
        the URIref of the XML Schema <code>date</code> datatype. Since this
        is an attribute value, the URIref must be written out, rather
        than using the QName abbreviation <code>xsd:date</code> 
        used in the triple. A literal appropriate to this datatype is
        then written as the element content, in this case, the literal
        <code>1999-08-16</code>, which is the literal representation for
        August 16, 1999 in the XML Schema <code>date</code> datatype.</p>

<div class="newstuff">
        <p>In the 
        <a name="LCC-013" id="LCC-013">rest of the Primer,</a> the examples will
        use typed literals from appropriate datatypes rather than
        plain (untyped) literals, in order to emphasize the value of
        typed literals in conveying more information about the intended
        interpretation of literal values.  (The exceptions will be that plain
        literals will continue to be used in 
        examples taken from actual applications that do not currently use
        typed literals, in order to accurately reflect the usage in those
        applications.)

In RDF/XML, both plain and typed literals (and, with certain exceptions, tags) can contain Unicode <a href="#ref-unicode">[UNICODE]</a> characters, allowing information from many languages to be directly represented.

</p>

        <p><a href="#example7">Example 7</a> illustrates that using typed literals requires writing an <code>rdf:datatype</code> attribute with
        a URIref identifying the datatype for each element whose value is a typed literal.  As noted earlier, RDF/XML requires that URIrefs used as attribute values
        must be written out, rather than abbreviated as a QName.   
        XML <em>entities</em> can be used in RDF/XML to improve readability 
        in such cases, by providing an additional abbreviation 
        facility for URIrefs.  Essentially, an XML entity declaration 
        associates a name with a string of characters.  When the entity 
        name is referenced elsewhere within an XML document, XML processors replace 
        the reference with the corresponding string.  For example, the 
        <code>ENTITY</code> declaration (specified as part of a <code>DOCTYPE</code>
        declaration at the beginning of the RDF/XML document):</p> 

        <div class="exampleOuter exampleInner">
<pre>
&lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt; 
</pre>
        </div>

        <p>defines the entity <code>xsd</code> to be the string representing the 
        namespace URIref for XML Schema datatypes.  This declaration allows 
        the full namespace URIref to be abbreviated elsewhere in the XML
        document by the <dfn>entity reference</dfn> <code>&amp;xsd;</code>.  
        Using this abbreviation, 
        <a href="#example7">Example 7</a> could also be written as shown in 
        <a href="#example8">Example 8</a>.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example8" name="example8">Example 8: RDF/XML Using a
            Typed Literal and an XML Entity</a>
          </div>

          <div class="exampleInner">
<pre>
1. &lt;?xml version="1.0"?&gt;
2. &lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;

3. &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
4.             xmlns:exterms="http://www.example.org/terms/"&gt;

5.   &lt;rdf:Description rdf:about="http://www.example.org/index.html"&gt;
6.     &lt;exterms:creation-date rdf:datatype="&amp;xsd;date"&gt;1999-08-16
       &lt;/exterms:creation-date&gt;
7.   &lt;/rdf:Description&gt;

8. &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>The <code>DOCTYPE</code> declaration in line 2 defines the entity
        <code>xsd</code>, which is used in line 6.</p>

        <p>The use of XML entities as an abbreviation mechanism is optional 
        in RDF/XML, and hence the use of an XML <code>DOCTYPE</code> declaration 
        is also optional 
        in RDF/XML. (For readers familiar with XML, RDF/XML is only required 
        to be "well-formed" XML. RDF/XML is not designed to be 
        validated against a DTD by a validating XML processor.  This is 
        discussed more fully in <a href="#documents">Appendix B</a>, which 
        provides additional information about XML.)</p> 

        <p>For readability purposes, examples in the rest of the 
        Primer will use the XML entity <code>xsd</code> as just described.
        XML entities are discussed 
        further in <a href="#documents">Appendix B</a>.
        As illustrated in <a href="#documents">Appendix B</a>, 
        other URIrefs (and, more generally, other strings)
        can also be abbreviated using XML entities. 
        However, the URIrefs for XML Schema datatypes are the only ones that will be
        abbreviated in this way in Primer examples. </p> 
</div>
  
        <p>Although additional abbreviated forms for writing RDF/XML
        are available, the facilities illustrated so far
        provide a simple but general way to express graphs in
        RDF/XML. Using these facilities, an RDF graph is written in
        RDF/XML as follows:</p>

        <ul>
          <li>All blank nodes are assigned blank node identifiers.</li>

          <li>Each node is listed in turn as the subject of an
          un-nested <code>rdf:Description</code> element, using an
          <code>rdf:about</code> attribute if the node has a URIref, or
          an <code>rdf:nodeID</code> attribute if the node is
          blank.<br />
           For each triple with this node as subject, an appropriate
          property element is created, with either literal content
          (possibly empty), an <code>rdf:resource</code> attribute
          specifying the object of the triple (if the object node has a
          URIref), or an <code>rdf:nodeID</code> attribute specifying
          the object of the triple (if the object node is blank).</li>
        </ul>

        <p>Compared to some of the more abbreviated 
        approaches described in <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>, this simple
        approach provides the most direct representation
        of the actual graph structure, and is particularly recommended
        for applications in which the output RDF/XML is to be used in
        further RDF processing.</p>
      </div>

      <div class="section">
        <h3><a name="newresources" id="newresources">3.2 Abbreviating
        and Organizing RDF URIrefs</a></h3>

        <p>So far, the examples have assumed that the resources 
        being described have been given URIrefs already. For instance, the initial
        examples provided descriptive information about
        example.org's Web page, whose URIref was
        <code>http://www.example.org/index.html</code>. This resource was identified
        in RDF/XML
        using an <code>rdf:about</code> attribute citing its full URIref.
        Although RDF does not specify or control how URIrefs are
        assigned to resources, sometimes it is desirable to achieve the
        effect of assigning URIrefs to resources that are part
        of an organized group of resources. For example, suppose a
        sporting goods company, example.com, wanted to provide an
        RDF-based catalog of its products, such as tents, hiking boots,
        and so on, as an RDF/XML document, identified by (and located
        at) <code>http://www.example.com/2002/04/products</code>. In that
        resource, each product might be given a separate RDF
        description. This catalog, along with one of these
        descriptions, the catalog entry for a model of tent called the
        "Overnighter", might be written in RDF/XML as shown in <a
        href="#example9">Example 9</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example9" name="example9">Example 9: RDF/XML for
            example.com's Catalog</a>
          </div>

          <div class="exampleInner">
<pre>
1.   &lt;?xml version="1.0"?&gt;
2.   &lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
3.   &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
4.               xmlns:exterms="http://www.example.com/terms/"&gt;

5.     &lt;rdf:Description rdf:ID="item10245"&gt;
6.          &lt;exterms:model rdf:datatype="&amp;xsd;string"&gt;Overnighter&lt;/exterms:model&gt;
7.          &lt;exterms:sleeps rdf:datatype="&amp;xsd;integer"&gt;2&lt;/exterms:sleeps&gt;
8.          &lt;exterms:weight rdf:datatype="&amp;xsd;decimal"&gt;2.4&lt;/exterms:weight&gt;
9.          &lt;exterms:packedSize rdf:datatype="&amp;xsd;integer"&gt;784&lt;/exterms:packedSize&gt;
10.    &lt;/rdf:Description&gt;

  ...other product descriptions...

11.  &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p><a href="#example9">Example 9</a> is similar to previous
        examples in the way it represents the properties (model,
        sleeping capacity, weight) of the resource (the tent) being
        described. 

        (The surrounding xml, DOCTYPE, RDF, and namespace
        information is included in lines 1 through 4, and line 11, but this
        information would only need to be provided once for the whole
        catalog, not repeated for each entry in the catalog.


Note also that although the <em>datatypes</em> associated with the various property values
are given explicitly, the <em>units</em> associated with some of these property values are not, even
though this information should be available to properly interpret the values.  Representing units and similar information that may be associated with property values is discussed in <a href="#rdfvalue">Section 4.4</a>.  In this example, the value of <code>exterms:sleeps</code> is the number of persons the tent can sleep, the value of <code>exterms:weight</code> is given in kilograms, and the value of <code>exterms:packedSize</code> is given in square centimeters, the area the tent occupies on a backpack.)</p>

        <p>An important <em>difference</em> from previous examples
        is that, in line 5, the <code>rdf:Description</code>
        element has an <code>rdf:ID</code> attribute instead of an
        <code>rdf:about</code> attribute. Using <code>rdf:ID</code> specifies
        a <em>fragment identifier</em>, given by the
        value of the <code>rdf:ID</code> attribute (<code>item10245</code> in
        this case, which might be the catalog number assigned by
        example.com), as an abbreviation of the complete URIref of the
        resource being described. The fragment identifier
        <code>item10245</code> will be interpreted relative to a <em>base
        URI</em>, in this case, the URI of the containing catalog
        document. The full URIref for the tent is formed by taking the
        base URI (of the catalog), and appending the character
        "<code>#</code>" (to
        indicate that what follows is a fragment identifier) and then
        <code>item10245</code> to it, giving the absolute URIref
        <code>http://www.example.com/2002/04/products#item10245</code>.</p>

        <p>The <code>rdf:ID</code> attribute is somewhat similar to the <code>ID</code>
        attribute in XML and HTML, in that it defines a name which must
        be unique relative to the current base URI (in this example, that of the catalog). 
        In this case, the <code>rdf:ID</code>
        attribute appears to be assigning a name (<code>item10245</code>)
        to this particular kind of tent. Any other RDF/XML within this
        catalog could refer to the tent by using either the absolute URIref
        <code>http://www.example.com/2002/04/products#item10245</code>, 
        or the <em>relative URIref</em> <code>#item10245</code>. The
        relative URIref would be understood as being a URIref defined relative to the
        base URIref of the catalog. Using a similar abbreviation, 
        the URIref of the tent could also be given by specifying
        <code>rdf:about="#item10245"</code> in the catalog entry (i.e., by
        specifying the relative URIref directly) instead of
        <code>rdf:ID="item10245"</code> . As an abbreviation mechanism, 
        the two forms are essentially
        synonyms: the full URIref formed by RDF/XML is the same in
        either case:
        <code>http://www.example.com/2002/04/products#item10245</code>. 
        However, using <code>rdf:ID</code> provides an additional check
        when assigning a set of distinct names, since a given value of the
        <code>rdf:ID</code> attribute can only appear once relative to the
        same base URI (the catalog document, in this example).  Using
        either form, example.com would be giving the URIref for the
        tent in a two-stage process, first assigning the URIref for the
        whole catalog, and then using a relative URIref in the
        description of the tent in the catalog to indicate the URIref
        that has been assigned to this particular kind of tent.
        Moreover, this use of a relative URIref can be thought of either 
        as being an abbreviation for a full URIref that has been
        assigned to the tent independently of the RDF, or as being the
        assignment of the URIref to the tent within the catalog.</p>

        <p>RDF located <em>outside</em> the catalog could refer to this
        tent by using the full URIref, i.e., by concatenating the
        relative URIref <code>#item10245</code> of the tent to the base URI
        of the catalog, forming the absolute URIref
        <code>http://www.example.com/2002/04/products#item10245</code>. For
        example, an outdoor sports Web site exampleRatings.com might
        use RDF to provide ratings of various tents. The (5-star)
        rating given to the tent described in <a
        href="#example9">Example 9</a> might then be represented on
        exampleRatings.com's Web site as shown in <a
        href="#example10">Example 10</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example10" name="example10">Example 10:
            exampleRatings.com's Rating of the Tent</a>
          </div>

          <div class="exampleInner">
<pre>
1.  &lt;?xml version="1.0"?&gt;
2.  &lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
3.  &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
4.              xmlns:sportex="http://www.exampleRatings.com/terms/"&gt;

5.    &lt;rdf:Description rdf:about="http://www.example.com/2002/04/products#item10245"&gt;
6.         &lt;sportex:ratingBy rdf:datatype="&amp;xsd;string"&gt;Richard Roe&lt;/sportex:ratingBy&gt;
7.         &lt;sportex:numberStars rdf:datatype="&amp;xsd;integer"&gt;5&lt;/sportex:numberStars&gt;
8.    &lt;/rdf:Description&gt;
9.  &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>In <a href="#example10">Example 10</a>, line 5 uses an
        <code>rdf:Description</code> element with an <code>rdf:about</code>
        attribute whose value is the full URIref of the tent. The use
        of this URIref allows the tent being referred to in the rating
        to be precisely identified.</p>

        <p>These examples illustrate several points. First, even though
        RDF does not specify or control how URIrefs are assigned to
        resources (in this case, the various tents and other items in
        the catalog), the <em>effect</em> of assigning URIrefs to
        resources in RDF can be achieved by combining a process
        (external to RDF) that identifies a single document (the
        catalog in this case) as the source for descriptions of those
        resources, with the use of relative URIrefs in descriptions of
        those resources within that document. For instance, example.com
        could use this catalog as the central source where its products
        are described, with the understanding that if a product's item
        number is not in an entry in this catalog, it is not a product
        known to example.com. (Note that RDF does not assume any
        particular relationship exists between two resources just
        because their URIrefs have the same base, or are otherwise
        similar. This relationship may be known to example.com, but it
        is not directly defined by RDF.)</p>

        <p>These examples also illustrate one of the basic
        architectural principles of the Web, <span class="newstuff">which is that 
        <a name="LCC-019" id="LCC-019">anyone</a>
        should be able to freely add information
        about an existing resource, using any vocabulary they
        please</span> <a href="#ref-berners-lee98">[BERNERS-LEE98]</a>. The examples
        further illustrate that the RDF describing a particular
        resource does not need to be located all in one place; instead,
        it may be distributed throughout the Web. This is true not only
        for situations like this one, in which one organization is
        rating or commenting on a resource defined by another, but also
        for situations in which the original definer of a resource (or
        anyone else) wishes to amplify the description of that resource
        by providing additional information about it. This may be done
        by modifying the RDF document in which the resource was
        originally described, to add the properties and values needed
        to describe the additional information. Or, as this example
        illustrates, a separate document could be created, providing the
        additional properties and values in <code>rdf:Description</code>
        elements that refer to the original resource via its URIref
        using <code>rdf:about</code>.</p>

        <p>The discussion above indicated that relative URIrefs
        such as <code>#item10245</code> will be interpreted relative to a
        <em>base URI</em>. By default, this base URI would be the URI
        of the resource in which the relative URIref is used.
        However, in some cases it is desirable to be able to explicitly
        specify this base URI. For instance, suppose that in addition
        to the catalog located at
        <code>http://www.example.com/2002/04/products</code>, example.org
        wanted to provide a duplicate catalog on a mirror site, say at
        <code>http://mirror.example.com/2002/04/products</code>. This could
        create a problem, since if the catalog was accessed from the
        mirror site, the URIref for the example tent would be generated
        from the URI of the containing document, forming
        <code>http://mirror.example.com/2002/04/products#item10245</code>,
        rather than
        <code>http://www.example.com/2002/04/products#item10245</code>, and
        hence would apparently refer to a different resource than the
        one intended. Alternatively, example.org might want to assign a
        base URIref for its set of product URIrefs <em>without</em> publishing a
        single source document whose location defines the base.</p>

        <p>To deal with such cases, RDF/XML supports <a
        href="http://www.w3.org/TR/2001/REC-xmlbase-20010627/">XML
        Base</a> <a href="#ref-xml-base">[XML-BASE]</a>, which allows
        an XML document to specify a base URI other than the URI of the
        document itself. <a href="#example11">Example 11</a> shows how
        the catalog would be described using XML Base:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example11" name="example11">Example 11: Using XML
            Base in example.com's Catalog</a>
          </div>

          <div class="exampleInner">
<pre>
1.   &lt;?xml version="1.0"?&gt;
2.   &lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
3.   &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
4.               xmlns:exterms="http://www.example.com/terms/"
5.               xml:base="http://www.example.com/2002/04/products"&gt;

6.     &lt;rdf:Description rdf:ID="item10245"&gt;
7.          &lt;exterms:model rdf:datatype="&amp;xsd;string"&gt;Overnighter&lt;/exterms:model&gt;
8.          &lt;exterms:sleeps rdf:datatype="&amp;xsd;integer"&gt;2&lt;/exterms:sleeps&gt;
9.          &lt;exterms:weight rdf:datatype="&amp;xsd;decimal"&gt;2.4&lt;/exterms:weight&gt;
10.         &lt;exterms:packedSize rdf:datatype="&amp;xsd;integer"&gt;784&lt;/exterms:packedSize&gt;
11.    &lt;/rdf:Description&gt;

  ...other product descriptions...

12.  &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>In <a href="#example11">Example 11</a>, the
        <code>xml:base</code> declaration in line 5 specifies that the base
        URI for the content within the <code>rdf:RDF</code> element (until
        another <code>xml:base</code> attribute is specified) is
        <code>http://www.example.com/2002/04/products</code>, and all
        relative URIrefs cited within that content will be interpreted
        relative to that base, no matter what the URI of the containing
        document is. As a result, the relative URIref of the tent,
        <code>#item10245</code>, will be interpreted as the same absolute
        URIref,
        <code>http://www.example.com/2002/04/products#item10245</code>, no
        matter what the actual URI of the catalog document is, or
        whether the base URIref actually identifies a particular
        document at all.</p>

        <p>So far, the examples have used a single product
        description, a particular model of tent, from example.com's
        catalog. However, example.com will probably offer several
        different models of tents, as well as multiple instances of
        other categories of products, such as backpacks, hiking boots,
        and so on. This idea of things being classified into different
        <em>kinds</em> or <em>categories</em> is similar to the
        programming language concept of objects having different
        <em>types</em> or <em>classes</em>. RDF supports this concept
        by providing a predefined property, <code>rdf:type</code>. When an
        RDF resource is described with an <code>rdf:type</code> property,
        the value of that property is considered to be a resource that
        represents a category or <em>class</em> of things, and the
        subject of that property is considered to be an
        <em>instance</em> of that category or class. Using
        <code>rdf:type</code>, <a href="#example12">Example 12</a> shows
        how example.com might indicate that the product description is
        that of a tent:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example12" name="example12">Example 12: Describing a
            Tent with <code>rdf:type</code></a>
          </div>

          <div class="exampleInner">
<pre>
1.   &lt;?xml version="1.0"?&gt;
2.   &lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
3.   &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
4.               xmlns:exterms="http://www.example.com/terms/"
5.               xml:base="http://www.example.com/2002/04/products"&gt;

6.     &lt;rdf:Description rdf:ID="item10245"&gt;
7.          &lt;rdf:type rdf:resource="http://www.example.com/terms/Tent"/&gt;
8.          &lt;exterms:model rdf:datatype="&amp;xsd;string"&gt;Overnighter&lt;/exterms:model&gt;
9.          &lt;exterms:sleeps rdf:datatype="&amp;xsd;integer"&gt;2&lt;/exterms:sleeps&gt;
10.         &lt;exterms:weight rdf:datatype="&amp;xsd;decimal"&gt;2.4&lt;/exterms:weight&gt;
11.         &lt;exterms:packedSize rdf:datatype="&amp;xsd;integer"&gt;784&lt;/exterms:packedSize&gt;
12.    &lt;/rdf:Description&gt;

  ...other product descriptions...

13.  &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>In <a href="#example12">Example 12</a>, the
        <code>rdf:type</code> property in line 7 indicates that the
        resource being described is an instance of the class identified by the URIref
        <code>http://www.example.com/terms/Tent</code>. This assumes
        that example.com has described its classes as part of
        the same vocabulary that it uses to describe its other terms
        (such as the property <code>exterms:weight</code>), so the
        absolute URIref of the class is used to refer to it. If example.com had
        described these classes as part of the product catalog itself,
        the relative URIref <code>#Tent</code> could have been used to refer
        to it.</p>

        <p>RDF itself does not provide facilities for defining
        application-specific classes of things, such as <code>Tent</code>
        in this example, or their properties, such as <code>exterms:weight</code>.
        Instead, such classes would be described in an <em>RDF schema</em>,
        using the <em>RDF Schema</em> language 
        discussed in <a href="#rdfschema">Section 5</a>. Other such
        facilities for describing classes can also be defined, such as
        the <em>DAML+OIL</em> and <em>OWL</em> languages described in
        <a href="#richerschemas">Section 5.5</a>.</p>

        <p>It is fairly common in RDF for resources to have <code>rdf:type</code>
        properties that describe the resources as instances of specific types
        or classes.  Such resources are called <em>typed nodes</em> in the
        graph, or <em>typed node elements</em> in the RDF/XML.  
        RDF/XML provides a special abbreviation for describing
        these typed nodes. In this abbreviation, the
        <code>rdf:type</code> property and its value are removed, and the
        <code>rdf:Description</code> element for the node is replaced by an element
        whose name is the QName corresponding to the value of the
        removed <code>rdf:type</code> property (a URIref that names a class).
        Using this abbreviation, example.com's tent from <a
        href="#example12">Example 12</a> could also be described as
        shown in <a href="#example13">Example 13</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example13" name="example13">Example 13: Abbreviating
            the Tent's Type</a>
          </div>

          <div class="exampleInner">
<pre>
1.   &lt;?xml version="1.0"?&gt;
2.   &lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
3.   &lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
4.               xmlns:exterms="http://www.example.com/terms/"
5.               xml:base="http://www.example.com/2002/04/products"&gt;

6.     &lt;exterms:Tent rdf:ID="item10245"&gt;
7.          &lt;exterms:model rdf:datatype="&amp;xsd;string"&gt;Overnighter&lt;/exterms:model&gt;
8.          &lt;exterms:sleeps rdf:datatype="&amp;xsd;integer"&gt;2&lt;/exterms:sleeps&gt;
9.          &lt;exterms:weight rdf:datatype="&amp;xsd;decimal"&gt;2.4&lt;/exterms:weight&gt;
10.         &lt;exterms:packedSize rdf:datatype="&amp;xsd;integer"&gt;784&lt;/exterms:packedSize&gt;
11.    &lt;/exterms:Tent&gt;

  ...other product descriptions...

12.  &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

<div class="newstuff">

        <p>Since a resource may be described as an instance of more 
        than one class, a resource may have more than one 
        <code>rdf:type</code> property.  However, only one of these 
        <code>rdf:type</code> properties can be abbreviated in this way. 
        The others must be written out using <code>rdf:type</code> properties, 
        in the manner illustrated by the
        <code>rdf:type</code> property in <a href="#example12">Example 12</a>.</p>
        
        <p>In addition to its use in describing instances of user-defined
        classes such as <code>exterms:Tent</code>, the typed node abbreviation 
        is also commonly used in RDF/XML when describing instances of 
        the built-in RDF classes (such as <code>rdf:Bag</code>) to be
        described in <a href="#othercapabilities">Section 4</a>, and
        the built-in RDF Schema classes (such as <code>rdfs:Class</code>) to be described in 
        <a href="#rdfschema">Section 5</a>.</p>
</div>

        <p>Both <a href="#example12">Example 12</a> and <a
        href="#example13">Example 13</a> illustrate that RDF statements
        can be written in RDF/XML in a way that closely resembles
        descriptions that might have been written directly in (non-RDF) XML. This
        is an important consideration, given the increasing use of XML
        in all kinds of applications, since it suggests that RDF could
        be used in these applications without requiring major changes
        in the way their information is structured.</p>
      </div>

      <div class="section">
        <h3><a name="rdfxmlsummary" id="rdfxmlsummary">3.3 RDF/XML
        Summary</a></h3>

        <p>The examples above have illustrated some of the basic ideas
        behind the RDF/XML syntax. These examples provide enough
        information to begin writing useful RDF/XML. 
        A more thorough discussion of the principles behind the
        modeling of RDF statements in XML (known as <em>striping</em>),
        together with a presentation of the other RDF/XML abbreviations
        available, and other details and examples about writing RDF in
        XML, is given in the (normative) <a
        href="http://www.w3.org/TR/rdf-syntax-grammar/">RDF/XML Syntax
        Specification</a> <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>.</p>
      </div>
    </div>

    <div class="section">
      <h2><a id="othercapabilities" name="othercapabilities"></a>4.
      Other RDF Capabilities</h2>

      <p>RDF provides a number of additional capabilities, 
      <span class="newstuff">such as
      built-in types and properties for representing groups of
      resources and RDF statements, and capabilities for representing
      XML fragments as property values.</span> These additional capabilities
      are described in the following sections.</p>

      <div class="section">
        <h3><a name="containers" id="containers">4.1 RDF
        Containers</a></h3>

        <p>There is often a need to describe <em>groups</em> of things:
        for example, to say that a book was created by
        several authors, or to list the students in a course, or the
        software modules in a package. RDF provides several predefined
        (built-in) types and properties that can be used to describe such
        groups.</p>

        <p>First, RDF provides a <em>container vocabulary</em>
        consisting of three predefined types (together with some
        associated predefined properties). A <em>container</em> is a
        resource that contains things. The contained things are called
        <em>members</em>. The members of a container may be resources
        (including blank nodes) or literals. RDF defines three types of containers:</p>

        <ul>
          <li><code>rdf:Bag</code></li>

          <li><code>rdf:Seq</code></li>

          <li><code>rdf:Alt</code></li>
        </ul>

        <p>A <em>Bag</em> (a resource having type <code>rdf:Bag</code>)
        <span class="newstuff">represents</span>
        a group of resources or literals, possibly including duplicate
        members, where there is no significance in the order of the
        members. For example, a Bag might be used to describe a group
        of part numbers in which the order of entry or processing of
        the part numbers does not matter.</p>

        <p>A <em>Sequence</em> or <em>Seq</em> (a resource having type
        <code>rdf:Seq</code>) <span class="newstuff">represents</span> 
        a group of resources or literals, possibly
        including duplicate members, where the order of the members is
        significant. For example, a Sequence might be used to describe
        a group that must be maintained in alphabetical order.</p>

        <p>An <em>Alternative</em> or <em>Alt</em> (a resource having
        type <code>rdf:Alt</code>) <span class="newstuff">represents</span>
        a group of resources or literals that
        are <em>alternatives</em> (typically for a single value of a
        property). For example, an Alt might be used to describe
        alternative language translations for the title of a book, or
        to describe a list of alternative Internet sites at which a
        resource might be found. An application using a property whose
        value is an Alt container should be aware that it can choose
        any one of the members of the group as appropriate.</p>

        <p>To describe a resource as being one of these types of
        containers, the resource is given an <code>rdf:type</code> property
        whose value is one of the predefined resources
        <code>rdf:Bag</code>, <code>rdf:Seq</code>, or <code>rdf:Alt</code>
        (whichever is appropriate). The container resource (which may
        either be a blank node or a resource with a URIref) denotes the
        group as a whole. The <em>members</em> of the container can be
        described by defining a <em>container membership property</em>
        for each member with the container resource as its subject and
        the member as its object. These container membership properties
        have names of the form <code>rdf:_<em>n</em></code>, where
        <em>n</em> is a decimal integer greater than zero, with no
        leading zeros, e.g., <code>rdf:_1</code>, <code>rdf:_2</code>,
        <code>rdf:_3</code>, and so on, and are used specifically for
        describing the members of containers. Container resources may
        also have other properties that describe the container, in
        addition to the container membership properties and the
        <code>rdf:type</code> property.</p>

        <p>It is important to understand that while these types of
        containers are described using predefined RDF types and
        properties, any special meanings associated with these
        containers, e.g., that the members of an Alt container are
        alternative values, are only <em>intended</em> meanings. These
        specific container types, and their definitions, are provided
        with the aim of establishing a shared convention among those
        who need to describe groups of things. All RDF does is provide
        the types and properties that can be used to construct the RDF
        graphs to describe each type of container. RDF has no more
        built-in understanding of what a resource of type
        <code>rdf:Bag</code> is than it has of what a resource of type
        <code>ex:Tent</code> (discussed in <a
        href="#newresources">Section 3.2</a>) is. In each case,
        applications must be written to behave according to the
        particular meaning involved for each type. This point will be
        expanded on in the following examples.</p>

        <p>A typical use of a container is to indicate that the value
        of a property is a group of things. For example, to represent
        the sentence "Course 6.001 has the students Amy, Mohamed, Johann,
        Maria, and Phuong", the course could be described by giving it a
        <code>s:students</code> property (from an appropriate vocabulary)
        whose value is a container of type
        <code>rdf:Bag</code> (<span class="newstuff">representing</span>
        the group of students).  Then, using the
        container membership properties, individual
        students could be identified as being members of that group, 
        as in the RDF
        graph shown in <a href="#figure14">Figure 14</a>:</p>

<div class="newstuff">
        <div class="figure">
          <img src="fig14july12.png"
          alt="A Simple Bag Container Description" /><br />
           <a id="figure14" name="figure14">Figure 14: A Simple Bag
          Container Description</a>
        </div>

</div>

        <p>Since the value of the <code>s:students</code> property in this
        example is described as a Bag, there is no intended
        significance in the order given for the URIrefs of the
        students, even though the membership properties in the graph have integers
        in their names. It is up to applications creating and
        processing graphs that include <code>rdf:Bag</code> containers to
        ignore any (apparent) order in the names of the membership
        properties.</p>

        <p>RDF/XML provides some special syntax and abbreviations to
        make it simpler to describe such containers. For example, <a
        href="#example14">Example 14</a> describes the graph shown in
        <a href="#figure14">Figure 14</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example14" name="example14">Example 14: RDF/XML for
            a Bag of Students</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:s="http://example.org/students/vocab#"&gt;

   &lt;rdf:Description rdf:about="http://example.org/courses/6.001"&gt;
      &lt;s:students&gt;
         &lt;rdf:Bag&gt;
            &lt;rdf:li rdf:resource="http://example.org/students/Amy"/&gt;
            &lt;rdf:li rdf:resource="http://example.org/students/Mohamed"/&gt;
            &lt;rdf:li rdf:resource="http://example.org/students/Johann"/&gt;
            &lt;rdf:li rdf:resource="http://example.org/students/Maria"/&gt;
            &lt;rdf:li rdf:resource="http://example.org/students/Phuong"/&gt;
         &lt;/rdf:Bag&gt;
      &lt;/s:students&gt;
   &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p><a href="#example14">Example 14</a> shows that RDF/XML
        provides <code>rdf:li</code> as a convenience element to avoid having
        to explicitly number each membership property. The numbered
        properties <code>rdf:_1</code>, <code>rdf:_2</code>, and so on are
        generated from the <code>rdf:li</code> elements in forming the
        corresponding graph. The element name <code>rdf:li</code> was chosen to
        be mnemonic with the term "list item" from HTML. Note also the
        use of a <code>&lt;rdf:Bag&gt;</code> element nested within the
        <code>&lt;s:students&gt;</code> property element. The
        <code>&lt;rdf:Bag&gt;</code> element is another example of the
        abbreviation used in <a href="#example13">Example 13</a>
        that replaces both an <code>rdf:Description</code> element
        and an <code>rdf:type</code> element with a single element 
        when describing an instance of a type (an instance of
        <code>rdf:Bag</code> in this case).  Since
        no URIref is specified, the Bag is a blank node. Its nesting
        within the <code>&lt;s:students&gt;</code> property element is an
        abbreviated way of indicating that the blank node is the value
        of this property. These abbreviations are described further in
        <a href="#ref-rdf-syntax">[RDF-SYNTAX]</a>.</p>

        <p>The graph structure for an <code>rdf:Seq</code> container, and
        the corresponding RDF/XML, are similar to those for an
        <code>rdf:Bag</code> (the only difference is in the type,
        <code>rdf:Seq</code>). Once again, although an <code>rdf:Seq</code>
        container is intended to describe a sequence, it is up to
        applications creating and processing the graph to appropriately
        interpret the sequence of integer-valued property names.</p>

        <p>To illustrate an Alt container, the sentence "The
        source code for X11 may be found at ftp.example.org,
        ftp1.example.org, or ftp2.example.org" could be expressed in
        the RDF graph shown in <a href="#figure15">Figure 15</a>:</p>

<div class="newstuff">
        <div class="figure">
          <img src="fig15july12.png"
          alt="A Simple Alt Container Description" /><br />
           <a id="figure15" name="figure15">Figure 15: A Simple Alt
          Container Description</a>
        </div>

</div>

        <p><a href="#example15">Example 15</a> shows how the graph in
        <a href="#figure15">Figure 15</a> could be written in
        RDF/XML:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example15" name="example15">Example 15: RDF/XML for
            an Alt Container</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:s="http://example.org/packages/vocab#"&gt;

   &lt;rdf:Description rdf:about="http://example.org/packages/X11"&gt;
      &lt;s:DistributionSite&gt;
         &lt;rdf:Alt&gt;
            &lt;rdf:li rdf:resource="ftp://ftp.example.org"/&gt;
            &lt;rdf:li rdf:resource="ftp://ftp1.example.org"/&gt;
            &lt;rdf:li rdf:resource="ftp://ftp2.example.org"/&gt;
         &lt;/rdf:Alt&gt;
      &lt;/s:DistributionSite&gt;
   &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>An Alt container is intended to have at least one member,
        identified by the property <code>rdf:_1</code>. This member is
        intended to be considered as the default or preferred value.
        Other than the member identified as <code>rdf:_1</code>, the order
        of the remaining elements is not significant.</p>

        <p>The RDF in <a href="#figure15">Figure 15</a> <em>as
        written</em> states simply that the value of the
        <code>s:DistributionSite</code> site property is the Alt container
        resource itself. Any additional meaning that is to be read into
        this graph, e.g., that one of the <em>members</em> of the Alt
        container is to be considered as the value of the
        <code>s:DistributionSite</code> site property, or that
        <code>ftp://ftp.example.org</code> is the default or preferred
        value, must be built into an application's understanding of 
        the intended meaning of  
        an Alt container, and/or into the meaning defined
        for the particular property (<code>s:DistributionSite</code> in
        this case), which also must be understood by the
        application.</p>

        <p>Alt containers are frequently used in conjunction with
        language tagging. (RDF/XML permits the use of the <code>xml:lang</code>
        attribute defined in <a href="#ref-xml">[XML]</a> to indicate
        that the element content is in a specified language.  The use
        of <code>xml:lang</code> is described in <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>, and illustrated later in
        <a href="#prism">Section 6.2</a>.) For example, a work whose title has been
        translated into several languages might have its <code>title</code>
        property pointing to an Alt container holding literals representing
        the titles expressed in each of the language variants.</p>

        <p>The distinction between the intended meanings of a Bag and
        an Alt can be further illustrated by considering the authorship
        of the book "Huckleberry Finn". The book has exactly one
        author, but the author has two names (Mark Twain and Samuel
        Clemens). Either name is sufficient to specify the author. Thus
        using an Alt container for the author's names more accurately
        represents the relationship than using a Bag (which might
        suggest there are two <em>different</em> authors).</p>

        <p>Users are free to choose their own ways to describe groups of
        resources, rather than using the RDF container vocabulary. These RDF
        containers are merely provided as common definitions that, if
        generally used, could help make data involving groups of
        resources more interoperable.</p>

        <p>Sometimes there are clear alternatives to using these RDF
        container types. For example, a relationship between a
        particular resource and a group of other resources could be
        indicated by making the first resource the subject of multiple
        statements using the same property. This is structurally different
        from the resource being the subject of a single
        statement whose object is a container containing multiple
        members. In some cases, these two structures may have
        equivalent meaning, but in other cases they may not. The choice
        of which to use in a given situation should be made with this
        in mind.</p>

        <p>Consider as an example the relationship between a writer and
        her publications, as in the sentence:</p>

        <blockquote>
          <p>Sue has written "Anthology of Time", "Zoological
          Reasoning", and "Gravitational Reflections".</p>
        </blockquote>

        <p>In this case, there are three resources each of which was
        written independently by the same writer. This could be
        expressed using repeated properties as:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:Sue   exterms:publication   ex:AnthologyOfTime .
exstaff:Sue   exterms:publication   ex:ZoologicalReasoning .
exstaff:Sue   exterms:publication   ex:GravitationalReflections .
</pre>
        </div>

        <p>In this example there is no stated relationship between the
        publications other than that they were written by the same
        person. Each of the statements is an independent fact, and so
        using repeated properties would be a reasonable choice.
        However, this could just as reasonably be represented as a
        statement about the group of resources written by Sue:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:Sue   exterms:publication   _:z .
_:z           rdf:type              rdf:Bag .
_:z           rdf:_1                ex:AnthologyOfTime .
_:z           rdf:_2                ex:ZoologicalReasoning .
_:z           rdf:_3                ex:GravitationalReflections .
</pre>
        </div>

        <p>On the other hand, the sentence:</p>

        <blockquote>
          <p>The resolution was approved by the Rules Committee, having
          members Fred, Wilma, and Dino.</p>
        </blockquote>

        <p>says that the committee <em>as a whole</em> approved the resolution;
        it does not necessarily state that each committee member
        <em>individually</em> voted in favor of the resolution. In this case, it
        would be potentially misleading to model this sentence as three
        separate <code>exterms:approvedBy</code> statements, one for each
        committee member, as shown below:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:resolution   exterms:approvedBy   ex:Fred .
ex:resolution   exterms:approvedBy   ex:Wilma .
ex:resolution   exterms:approvedBy   ex:Dino .
</pre>
        </div>

        <p>since these statements say that each member individually
        approved the resolution.</p>

        <p>In this case, it would be better to model the sentence as a
        single <code>exterms:approvedBy</code> statement whose subject is
        the resolution and whose object is the committee itself. The
        committee resource could then be described as a Bag whose
        members are the members of the committee, as in the following
        triples:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:resolution       exterms:approvedBy   ex:rulesCommittee .
ex:rulesCommittee   rdf:type             rdf:Bag .
ex:rulesCommittee   rdf:_1               ex:Fred .
ex:rulesCommittee   rdf:_2               ex:Wilma .
ex:rulesCommittee   rdf:_3               ex:Dino .
</pre>
        </div>

        <p>When using RDF containers, it is important to
        understand that the statements are not <em>constructing</em> containers,
        as in a programming language data structure. Instead,
        the statements are <em>describing</em> containers (groups of things) that
        presumably exist. For instance, in the Rules Committee example
        just given, the Rules Committee is an unordered group of
        people, whether it is described in RDF that way or not. 
        Saying that the resource <code>ex:rulesCommittee</code> has type
        <code>rdf:Bag</code> is not saying that the Rules Committee is a data
        structure, or constructing a particular data structure
        to hold the members of the group (the
        Rules Committee could be described as a Bag without 
        describing any members at
        all). Instead, it is describing the Rules Committee as having
        characteristics corresponding to those associated with a Bag
        container, namely that it has members, and their order of description
        is not significant.

        Similarly, using the container membership
        properties simply describes a container resource as
        having certain things as members. This does not necessarily
        say that the things described as members are the
        <em>only</em> members that exist. For example, the triples
        given above to describe the Rules Committee say only that Fred,
        Wilma, and Dino are members of the committee, not that they are the
        <em>only</em> members of the committee.</p>

<div class="newstuff">
        <p>Also, <a href="#example14">Example 14</a> and 
        <a href="#example15">Example 15</a> illustrated a common "pattern" 
        in describing containers, regardless of the type of container 
        involved (e.g., use of a blank node with
        an appropriate <code>rdf:type</code> property to
        represent the container itself, and use of <code>rdf:li</code> to 
        generate sequentially-numbered container membership properties).  

        However, it is important to understand that RDF does not
        <em>enforce</em> this particular way of using the RDF
        container vocabulary, and so it is possible to use this
        vocabulary in other ways.  For example, in some cases
        it might be appropriate to
        use a container resource having a URIref rather
        than using a blank node.  
        <a name="LCC-026b" id="LCC-026b">Moreover,</a> it is possible
        to use the container vocabulary in ways that may not 
        describe graphs with the "well-formed" structures
        shown in the previous examples.

        For example, <a href="#example16">Example 16</a> shows
        the RDF/XML for a graph similar to the Alt container shown in 
        <a href="#figure15">Figure 15</a>, but which writes the container
        membership properties explicitly, rather than using  
        <code>rdf:li</code> to generate them:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example16" name="example16">Example 16: RDF/XML for
            an "Ill-Formed" Alt Container</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:s="http://example.org/packages/vocab#"&gt; 
 
 &lt;rdf:Description rdf:about="http://example.org/packages/X11"&gt;
    &lt;s:DistributionSite&gt;
       &lt;rdf:Alt&gt;
          &lt;rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Bag"/&gt; 
          &lt;rdf:_2 rdf:resource="ftp://ftp.example.org"/&gt;
          &lt;rdf:_2 rdf:resource="ftp://ftp1.example.org"/&gt;
          &lt;rdf:_5 rdf:resource="ftp://ftp2.example.org"/&gt;
      &lt;/rdf:Alt&gt;
    &lt;/s:DistributionSite&gt;
 &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div> 

        <p>As noted in <a
        href="#ref-rdf-semantics">[RDF-SEMANTICS]</a>, 
        RDF imposes no "well-formedness" conditions on the use of 
        the container vocabulary, so <a href="#example16">Example 16</a>
        is perfectly legal, even though the container
        is described as <em>both</em> a Bag and an Alt, it is
        described as having
        two distinct values of the <code>rdf:_2</code> property, 
        and it does not have <code>rdf:_1</code>, <code>rdf:_3</code>,
        or <code>rdf:_4</code> properties.</p>

        <p>As a result, RDF applications that require containers
        to be "well-formed" should be written to check that the
        container vocabulary is being used appropriately, in order to
        be fully robust.</p>
</div>


      </div>

      <div class="section">
        <h3><a name="collections" id="collections">4.2 RDF
        Collections</a></h3>

        <p>A limitation of the containers described in <a
        href="#containers">Section 4.1</a> is that there is no way to
        <em>close</em> them, i.e., to say "these are all the members of
        the container".  As noted in <a
        href="#containers">Section 4.1</a>, a container 
        only says that certain identified
        resources are members;  it does not say that other members do
        not exist.  Also, while one graph may describe
        some of the members, there is no way to exclude the possibility
        that there is another graph somewhere that describes additional
        members. RDF provides support for describing groups containing
        only the specified members, in the form of RDF
        <em>collections</em>. An RDF collection is a group of things
        represented as a list structure in the RDF graph. This list
        structure is constructed using a predefined <em>collection
        vocabulary</em> consisting of the predefined type
        <code>rdf:List</code>, the predefined properties <code>rdf:first</code>
        and <code>rdf:rest</code>, and the predefined resource
        <code>rdf:nil</code>.</p>

        <p>To illustrate this, the sentence "The
        students in course 6.001 are Amy, Mohamed, and Johann" could be represented
        using the
        graph shown in <a href="#figure16">Figure 16</a>:</p>

<div class="newstuff">
        <div class="figure">
          <img src="fig16bjuly12.png"
          alt="An RDF Collection (list structure)" /><br />
           <a id="figure16" name="figure16">Figure 16: An RDF
          Collection (list structure)</a>
        </div>

        <p>In this graph, each member of the collection, such as 
        <code>s:Amy</code>, is the object of an <code>rdf:first</code> property whose
        subject is a resource (a blank node in this example) that
        represents a list.  
        This list resource is linked to the rest of the list by an
        <code>rdf:rest</code> property. The end of the list is indicated by
        the <code>rdf:rest</code> property having as its object the resource
        <code>rdf:nil</code> (the resource <code>rdf:nil</code> represents the empty
        list, and is defined as being of type <code>rdf:List</code>). 
        This structure will be familiar to those who
        know the Lisp programming language. As in Lisp, the
        <code>rdf:first</code> and <code>rdf:rest</code> properties allow
        applications to traverse the structure.
        Each of the blank nodes forming this list structure is 
        implicitly of type <code>rdf:List</code>
        (that is, each of these nodes implicitly has an <code>rdf:type</code> property whose
        value is the predefined type <code>rdf:List</code>),
        although this is not explicitly shown in the graph.  
        The RDF Schema language <a
        href="#ref-rdf-vocabulary">[RDF-VOCABULARY]</a> 
        defines the properties <code>rdf:first</code> and <code>rdf:rest</code>
        as having subjects of type <code>rdf:List</code>, so the information about
        these nodes being lists can generally be inferred, rather than the corresponding 
        <code>rdf:type</code> triples being written out all the time.</p>
</div>

        <p>RDF/XML provides a special notation to make it easy to
        describe collections using graphs of this form. 
        In RDF/XML, a collection can be described by
        a property element that has the attribute
        <code>rdf:parseType="Collection"</code>, and that contains a group
        of nested elements representing the members of the collection.

        <span class="newstuff">RDF/XML provides the <code>rdf:parseType</code> attribute to 
        indicate that the contents of an element are to be interpreted 
        in a special way.  In this case, the <code>rdf:parseType="Collection"</code> 
        attribute indicates that the enclosed elements are to be used to 
        create the corresponding list structure in the RDF graph 
        (other values of the <code>rdf:parseType</code> attribute will 
        be described in later sections of the Primer).</span></p>

        <p>To illustrate how <code>rdf:parseType="Collection"</code> works, 
        the RDF/XML from <a
        href="#example17">Example 17</a> would result in the RDF graph
        shown in <a href="#figure16">Figure 16</a>:</p>

<div class="newstuff">

        <div class="exampleOuter">
          <div class="c1">
            <a id="example17" name="example17">Example 17: RDF/XML for
            a Collection of Students</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:s="http://example.org/students/vocab#"&gt;

   &lt;rdf:Description rdf:about="http://example.org/courses/6.001"&gt;
      &lt;s:students rdf:parseType="Collection"&gt;
            &lt;rdf:Description rdf:about="http://example.org/students/Amy"/&gt;
            &lt;rdf:Description rdf:about="http://example.org/students/Mohamed"/&gt;
            &lt;rdf:Description rdf:about="http://example.org/students/Johann"/&gt;
      &lt;/s:students&gt;
   &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>The use of <code>rdf:parseType="Collection"</code> in RDF/XML always
        defines a list structure like the one
        shown in <a href="#figure16">Figure 16</a>, i.e., a
        fixed finite list of items with a given length and terminated
        by <code>rdf:nil</code>, and which uses "new" blank nodes that are
        unique to the list structure itself. However, RDF does not
        <em>enforce</em> this particular way of using the RDF
        collection vocabulary, and so it is possible to use this
        vocabulary in other ways, some of which may not describe lists
        or closed collections.

        To see why, note that the graph shown in <a href="#figure16">Figure 16</a> 
        could also be written in RDF/XML by writing out the same triples "in longhand" 
        (without using <code>rdf:parseType="Collection"</code>) using the 
        collection vocabulary, as in 
        <a href="#example18">Example 18</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example18" name="example18">Example 18: RDF/XML for
            a Collection of Students in "Longhand"</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:s="http://example.org/students/vocab#"&gt; 

&lt;rdf:Description rdf:about="http://example.org/courses/6.001"&gt; 
   &lt;s:students rdf:nodeID="sch1"/&gt;
&lt;/rdf:Description&gt;
 
&lt;rdf:Description rdf:nodeID="sch1"&gt; 
   &lt;rdf:first rdf:resource="http://example.org/students/Amy"/&gt;
   &lt;rdf:rest rdf:nodeID="sch2"/&gt; 
&lt;/rdf:Description&gt;
 
&lt;rdf:Description rdf:nodeID="sch2"&gt; 
   &lt;rdf:first rdf:resource="http://example.org/students/Mohamed"/&gt;
   &lt;rdf:rest rdf:nodeID="sch3"/&gt; 
&lt;/rdf:Description&gt;
 
&lt;rdf:Description rdf:nodeID="sch3"&gt; 
   &lt;rdf:first rdf:resource="http://example.org/students/Johann"/&gt;
   &lt;rdf:rest rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#nil"/&gt; 
&lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div> 

        <p>As noted in <a
        href="#ref-rdf-semantics">[RDF-SEMANTICS]</a>
        (and as was the case for the container vocabulary described
        in <a href="#containers">Section 4.1</a>), 
        <a name="LCC-026a" id="LCC-026a">RDF imposes</a> no "well-formedness" 
        conditions on the use of the collection vocabulary so, when 
        writing triples in longhand, it is possible to define RDF 
        graphs with structures other than the well-structured graphs
        that would be automatically
        generated by using <code>rdf:parseType="Collection"</code>. 

        For example, it is not illegal to assert that a given node has
        two distinct values of the <code>rdf:first</code> property, to
        create structures that have forked or non-list tails, or to
        simply omit part of the description of a collection. 

       Also, graphs defined by using the collection vocabulary
       in longhand could use URIrefs to identify the components of the list 
       instead of blank nodes unique to the list structure.  In this case, 
       it would be possible to create triples in other graphs that 
       effectively added elements to the collection, making it non-closed.</p>

        <p>As a result, RDF applications that require collections
        to be well-formed should be written to check that the
        collection vocabulary is being used appropriately, in order to
        be fully robust.  In addition, languages such as 
        <a
        href="http://www.w3.org/TR/owl-ref/">OWL</a> <a
        href="#ref-owl">[OWL]</a>, 
       which can define additional constraints on 
       the structure of RDF graphs, can rule out some of these cases.</p>

</div>

      </div>

      <div class="section">
        <h3><a name="reification" id="reification">4.3 RDF
        Reification</a></h3>

<div class="newstuff">

        <p>RDF applications sometimes need to describe other RDF statements 
        using RDF, for instance, to record information about when
        statements were made, who made them, or other similar
        information (this is sometimes referred to as "provenance" 
        information). For example, <a href="#example9">Example 9</a> in 
        <a href="#newresources">Section 3.2</a>
        described a particular tent with URIref <code>exproducts:item10245</code>, 
        offered for sale by example.com.
        One of the triples from that description, describing the weight
        of the tent, was:</p>

        <div class="exampleOuter exampleInner">
<pre>
exproducts:item10245   exterms:weight   "2.4"^^xsd:decimal .
</pre>
        </div>

<p>and it might be useful for example.com to record who provided that particular
piece of information.  </p>

        <p>RDF provides a built-in vocabulary intended for describing RDF statements. 
        A description of a statement using this vocabulary is called a 
        <em>reification</em> of the statement.

        The RDF reification vocabulary consists of the type
        <code>rdf:Statement</code>, and the properties
        <code>rdf:subject</code>, <code>rdf:predicate</code>, and
        <code>rdf:object</code>. However, while RDF provides this
        reification vocabulary, care is needed in using it, because it
        is easy to imagine that the vocabulary defines some things that are
        not actually defined.  This point will be discussed further later in
        this section.</p>

        <p>Using the reification vocabulary, a <em>reification</em> of the statement 
         about the tent's weight would be given by assigning the statement
         a URIref such as <code>exproducts:triple12345</code> 
        (so statements can be written describing it), and then 
         describing the statement using the statements:</p>

        <div class="exampleOuter exampleInner">
<pre>
exproducts:triple12345   rdf:type        rdf:Statement .
exproducts:triple12345   rdf:subject     exproducts:item10245 .
exproducts:triple12345   rdf:predicate   exterms:weight . 
exproducts:triple12345   rdf:object      "2.4"^^xsd:decimal .
</pre>
        </div>

        <p>These statements say that the resource identified by the URIref 
        <code>exproducts:triple12345</code> is an RDF statement, that the subject
        of the statement refers to the resource identified by <code>exproducts:item10245</code>, 
        the predicate of the statement refers to the resource identified by <code>exterms:weight</code>,
        and the object of the statement refers to the decimal value identified by the typed literal
        <code>"2.4"^^xsd:decimal</code>.

        Assuming that the original statement is actually identified by
        <code>exproducts:triple12345</code>, it should be clear by comparing the original
        statement with the reification that the reification
        actually does describe it.  The conventional use of the RDF reification 
        vocabulary always involves describing a statement using four statements  
        in this pattern;  the four statements are sometimes referred to as a 
        "reification quad" for this reason.</p>

        <p>Using reification according to this convention, example.com could 
        record the 
        fact that John Smith made the original statement about the tent's weight
        by first assigning the original statement a URIref (such as
        <code>exproducts:triple12345</code> as before), describing that statement
        using the reification just described, and then adding an additional 
        statement that <code>exproducts:triple12345</code> was 
        written by John Smith (using a URIref to identify which John
        Smith is being referred to).  The resulting statements would be:</p>

        <div class="exampleOuter exampleInner">
<pre>
exproducts:triple12345   rdf:type        rdf:Statement .
exproducts:triple12345   rdf:subject     exproducts:item10245 . 
exproducts:triple12345   rdf:predicate   exterms:weight . 
exproducts:triple12345   rdf:object      "2.4"^^xsd:decimal .
exproducts:triple12345   dc:creator      exstaff:85740 . 
</pre>
        </div>

       <p>The original statement, together with the reification
       and the attribution of the statement to John Smith, 
       forms the graph shown in <a href="#figure17">Figure 17</a>:</p>

      <div class="figure">
        <img src="reificationFigJul22.png"
        alt="A Statement, Its Reification, and Its Attribution" /><br />
         <a id="figure17" name="figure17">Figure 17: A Statement, Its
        Reification, and Its Attribution</a>
      </div>

       <p>This graph
       could be written in RDF/XML as shown in
       <a href="#example19">Example 19</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example19" name="example19">Example 19: RDF/XML for
            the Reification Example</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
            xmlns:dc="http://purl.org/dc/elements/1.1/"
            xmlns:exterms="http://www.example.com/terms/"
            xml:base="http://www.example.com/2002/04/products"&gt;

  &lt;rdf:Description rdf:ID="item10245"&gt;
     &lt;exterms:weight rdf:datatype="&amp;xsd;decimal"&gt;2.4&lt;/exterms:weight&gt;
  &lt;/rdf:Description&gt;

  &lt;rdf:Statement rdf:about="#triple12345"&gt;
     &lt;rdf:subject rdf:resource="http://www.example.com/2002/04/products#item10245"/&gt;
     &lt;rdf:predicate rdf:resource="http://www.example.com/terms/weight"/&gt;
     &lt;rdf:object rdf:datatype="&amp;xsd;decimal"&gt;2.4&lt;/rdf:object&gt;

     &lt;dc:creator rdf:resource="http://www.example.com/staffid/85740"/&gt;
  &lt;/rdf:Statement&gt;

&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

       <p><a href="#newresources">Section 3.2</a> introduced the use of the 
       <code>rdf:ID</code> attribute in RDF/XML in an <code>rdf:Description</code> 
       element to abbreviate the URIref of the subject of a statement.  
       <code>rdf:ID</code> can also be used in a 
       property element to automatically produce a reification of the triple that 
       the property element generates. <a href="#example20">Example 20</a> 
       shows how this could be used to produce the same graph
       as <a href="#example19">Example 19</a>:</p>  

        <div class="exampleOuter">
          <div class="c1">
            <a id="example20" name="example20">Example 20: Generating Reifications
            using <code>rdf:ID</code></a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
            xmlns:dc="http://purl.org/dc/elements/1.1/"
            xmlns:exterms="http://www.example.com/terms/"
            xml:base="http://www.example.com/2002/04/products"&gt;

  &lt;rdf:Description rdf:ID="item10245"&gt;
     &lt;exterms:weight rdf:ID="triple12345" rdf:datatype="&amp;xsd;decimal"&gt;2.4
     &lt;/exterms:weight&gt;
  &lt;/rdf:Description&gt;

  &lt;rdf:Description rdf:about="#triple12345"&gt;
     &lt;dc:creator rdf:resource="http://www.example.com/staffid/85740"/&gt;
  &lt;/rdf:Description&gt;

&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>


       <p>In this case, specifying the attribute <code>rdf:ID="triple12345"</code>
       in the <code>exterms:weight</code> element results in
       the original triple describing the tent's weight:</p>

        <div class="exampleOuter exampleInner">
<pre>
exproducts:item10245   exterms:weight   "2.4"^^xsd:decimal .
</pre>
        </div>

       <p>plus the reification triples:</p>

        <div class="exampleOuter exampleInner">
<pre>
exproducts:triple12345   rdf:type        rdf:Statement .
exproducts:triple12345   rdf:subject     exproducts:item10245 .
exproducts:triple12345   rdf:predicate   exterms:weight . 
exproducts:triple12345   rdf:object      "2.4"^^xsd:decimal .
</pre>
        </div>

       <p>The subject of these reification triples
       is a URIref formed by concatenating the base URI of the document 
       (given in the <code>xml:base</code> declaration), the character 
       "<code>#</code>" (to indicate that what follows is a fragment
       identifier), and the value of the <code>rdf:ID</code> attribute;
       that is, the triples have the same subject 
       <code>exproducts:triple12345</code> as in the previous examples.</p>

        <p>Note that asserting the reification is not
        the same as asserting the original statement, and
        neither implies the other. That is, when someone says that
        John said something about the weight of a tent, they are 
        not making a statement about the weight of a tent themselves, 
        they are making a statement about something John said.
        Conversely, when someone describes
        the weight of a tent, they are
        not also making a statement about a statement they made (since they
        may have no intention of talking about things called "statements").</p>


       <p>The text above deliberately referred in a number of places to 
       "the conventional use of reification".  As noted earlier, care is needed when using
       the RDF reification vocabulary because it is easy to imagine that
       the vocabulary defines some things that are not actually defined.
       While there are applications that successfully use reification, they do so by
       following some conventions, and making some assumptions, that are
       in addition to the actual meaning that RDF defines for the reification
       vocabulary, and the actual facilities that RDF provides to support it.</p> 

        <p>For one thing, it is important to
        note that in the conventional use of reification, the subject of
        the reification triples is assumed to identify
        a <em>particular instance</em>
        of a triple in a particular RDF document, rather than some
        arbitrary triple having the same subject, predicate, and
        object. This particular convention is used because
        reification is intended for expressing properties such
        as dates of composition and source information, as in the
        examples given already, and these properties need to be applied to specific
        instances of triples.  There could be several triples that have the same
        subject, predicate, and object and, although a graph is
        defined as a <em>set</em> of triples, several instances with the same
        triple structure might occur in different documents. Thus,
        to fully support this convention,
        there needs to be some means of associating the subject of the reification
        triples with <em>an individual triple in some document</em>. 
        However, RDF provides no way to do this.</p>

       <p>For instance, in the examples above, there is no explicit information 
       in either the triples or the RDF/XML that actually indicates that 
       the original statement describing the tent's weight is the resource
       <code>exproducts:triple12345</code>, the resource that is the subject of
       the four reification statements and the statement
       that John Smith created it.  
       This can be seen by looking at the drawn graph shown in 
       <a href="#figure17">Figure 17</a>.

       The original statement is certainly part of this graph, but
       as far as the information in the graph is concerned, 
       <code>exproducts:triple12345</code> is a separate resource, rather
       than identifying that part of the graph. 

       RDF does not provide a built-in way of indicating how a URIref
       like <code>exproducts:triple12345</code> is associated 
       with a particular statement or graph, any more than it provides 
       a built-in way of indicating how a URIref like <code>exproducts:item10245</code>
       is associated with an actual tent.  
       Associating specific URIrefs with specific resources (statements 
       in this case) must be done using mechanisms outside of RDF.</p>

          <p>Using <code>rdf:ID</code> as shown in <a href="#example20">Example 20</a>
          generates the reification 
          automatically, and provides a convenient way of indicating the 
          URIref to be used as the subject of the statements in the reification.  
          Moreover, it provides a partial "hook" relating the triples 
          in the reification with the piece of RDF/XML syntax that caused 
          them to be created, since the value <code>triple12345</code> of 
          the <code>rdf:ID</code> attribute is used to generate the URIref
          of the subject of the reification triples.  However, this 
          relationship is once again outside RDF, since there is nothing
          in the resulting triples that explicitly says that the original
          triple had the URIref <code>exproducts:triple12345</code> (RDF
          does not assume there is any relationship between a URIref
          and any RDF/XML that it might have been used or abbreviated in).</p>

        <p>The lack of a built-in means for assigning URIrefs to 
        statements does not mean that "provenance" information 
        of this kind cannot
        be expressed in RDF, just that it cannot be done using only the
        meaning RDF associates with the reification vocabulary. For
        example, if an RDF document (say, a Web page) has a URI,
        statements could be made about the resource identified by that URI
        and, based on some application-dependent understanding of how
        those statements should be interpreted, an application could act as if those
        statements "distribute" over (apply equally to) all the
        statements in the document. Also, if some mechanism exists
        (outside of RDF) to assign URIs to individual RDF statements,
        then statements could certainly be made about those individual
        statements, using their URIs to identify them. However, in these cases,
        it would also not be strictly necessary to use the reification vocabulary
        in the conventional way.</p>

        <p>To see this, assuming the original statement:</p> 

        <div class="exampleOuter exampleInner">
<pre>
exproducts:item10245   exterms:weight   "2.4"^^xsd:decimal .
</pre>
        </div>

        <p>had a URIref of
        <code>exproducts:triple12345</code>, the statement could be
        attributed to John Smith simply by the statement:</p>

        <div class="exampleOuter exampleInner">
<pre>
exproducts:triple12345   dc:creator   exstaff:85740 .
</pre>
        </div>

        <p>with no use of the reification vocabulary (although the
        description of <code>exproducts:triple12345</code> as having 
        <code>rdf:type</code> <code>rdf:Statement</code> might also be helpful).</p>

        <p>In addition, the reification vocabulary could be used
        directly according to the convention described
        above, along with an application-dependent understanding as to
        how to associate specific triples with their reifications.
        However, other applications receiving this RDF would not
        necessarily share this application-dependent understanding, and
        thus would not necessarily interpret the graphs
        appropriately.</p>

<p>It is also important to note that the interpretation of reification described here is not the same as "quotation", as found in some languages.  Instead, the reification describes the relationship between a particular instance of a triple and the resources the triple refers to.  The reification can be read intuitively as saying "this RDF triple talks about these things", rather than (as in quotation) "this RDF triple has this form."  For instance, in the reification example used in this section, the triple:</p> 

        <div class="exampleOuter exampleInner">
<pre>
exproducts:triple12345   rdf:subject   exproducts:item10245 .
</pre>
        </div>

<p>describing the <code>rdf:subject</code> of the original statement says that the subject of the statement is the resource (the tent) identified by the URIref <code>exproducts:item10245</code>.  It does <em>not</em> say that the subject of the statement is the URIref itself (i.e., a string beginning with certain characters), as quotation would do. </p>  

</div>

      </div>

      <div class="section">
        <h3><a name="rdfvalue" id="rdfvalue">4.4 More on Structured
        Values: rdf:value</a></h3>

        <p><a href="#structuredproperties">Section 2.3</a> noted
        that the RDF model intrinsically supports only
        <em>binary</em> relations; that is, a statement specifies a
        relation between two resources. For example, the statement:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:85740   exterms:manager   exstaff:62345 .
</pre>
        </div>

        <p>states that the relation <code>exterms:manager</code> holds between two
        employees (presumably one manages the other).</p>

        <p>However, in some cases it is necessary to represent
        information involving higher arity relations (relations between
        more than two resources) in RDF. 
        <a href="#structuredproperties">Section 2.3</a> discussed one example of
        this, where
        the problem was to represent the relationship between John
        Smith and his address information, and the value of John's
        address was a structured value of his street, city, state, and
        postal code. Writing this as a relation shows that this
        address is a 5-ary relation of the form:</p>

        <p class="ptriple"><code>address(exstaff:85740, "1501 Grant
        Avenue", "Bedford", "Massachusetts", "01730")</code></p>

        <p><a href="#structuredproperties">Section 2.3</a> noted that
        this kind of structured information can be represented
        in RDF by considering the aggregate thing be described
        (here, the group of components representing
        John's address) as a separate resource, and then making
        separate statements about that new resource, as in the
        triples:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:85740   exterms:address        _:johnaddress .
_:johnaddress   exterms:street         "1501 Grant Avenue" .
_:johnaddress   exterms:city           "Bedford" .
_:johnaddress   exterms:state          "Massachusetts" .
_:johnaddress   exterms:postalCode     "01730" .
</pre>
        </div>

        <p>(where <code>_:johnaddress</code> is the blank node identifier
        of the blank node representing John's address.)</p>

        <p>This is a general way to represent any n-ary relation in
        RDF: select one of the participants (John in this case) to
        serve as the subject of the original relation (<code>address</code>
        in this case), then specify an intermediate resource to
        represent the rest of the relation (either with or without
        assigning it a URI), then give that new resource properties
        representing the remaining components of the relation.</p>

        <p>In the case of John's address, none of the individual parts
        of the structured value could be considered the "main" value of
        the <code>exterms:address</code> property; all of the parts
        contribute equally to the value. However, in some cases one of
        the parts of the structured value is often thought of as the
        "main" value, with the other parts of the relation providing
        additional contextual or other information that qualifies the
        main value. For instance, in <a href="#example9">Example 9</a> in <a
        href="#newresources">Section 3.2</a>, the weight of a
        particular tent was given as the <span class="newstuff">
        decimal value <em>2.4</em> using a typed literal</span>,
        i.e.,</p>

        <div class="exampleOuter exampleInner">
<pre>
exproduct:item10245   exterms:weight   "2.4"^^xsd:decimal .
</pre>
        </div>

        <p>In fact, a more complete description of the weight would
        have been <em>2.4 kilograms</em> rather than just the decimal value
        <em>2.4</em>. To state
        this, the value of the <code>exterms:weight</code> property would
        need to have two components, the typed literal for the decimal value and an
        indication of the unit of measure (kilograms). In this
        situation the decimal value could be considered the "main"
        value of the <code>exterms:weight</code> property, because
        frequently the value would be recorded simply as the typed literal
        (as in the triple above), relying on an
        understanding of the context to fill in the unstated units
        information.</p>

        <p>In the RDF model a qualified property value of this kind can be
        considered as simply another kind of structured value. To
        represent this, a separate resource could be used to represent the
        structured value as a whole (the weight, in this case), and to
        serve as the object of the original statement. 
        That resource could then be given properties representing the individual parts of
        the structured value. In this case, there should be a property for the
        typed literal representing the decimal value, and a property for 
        the unit. RDF provides a predefined <code>rdf:value</code> property to describe
        the main value (if there is one) of a structured value. So in
        this case, the typed literal could be given as the value of the
        <code>rdf:value</code> property, and the resource
        <code>exunits:kilograms</code> as the value of an
        <code>exterms:units</code> property (assuming the resource
        <code>exunits:kilograms</code> is defined as part of example.org's 
        vocabulary). The resulting
        triples would be:</p>

        <div class="exampleOuter exampleInner">
<pre>
exproduct:item10245   exterms:weight   _:weight10245 .
_:weight10245         rdf:value        "2.4"^^xsd:decimal .
_:weight10245         exterms:units    exunits:kilograms .
</pre>
        </div>

        <p>which can be expressed using the RDF/XML shown in <a
        href="#example21">Example 21</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example21" name="example21">Example 21: RDF/XML
            using <code>rdf:value</code></a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
            xmlns:exterms="http://www.example.org/terms/"&gt;

  &lt;rdf:Description rdf:about="http://www.example.com/2002/04/products#item10245"&gt;
     &lt;exterms:weight rdf:parseType="Resource"&gt;
       &lt;rdf:value rdf:datatype="&amp;xsd;decimal"&gt;2.4&lt;/rdf:value&gt;
       &lt;exterms:units rdf:resource="http://www.example.org/units/kilograms"/&gt;
     &lt;/exterms:weight&gt;
  &lt;/rdf:Description&gt;

&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>


        <p class="newstuff">
        <a href="#example21">Example 21</a> also illustrates a second use of 
        the <code>rdf:parseType</code> attribute introduced in 
        <a href="#collections">Section 4.2</a>, 
        in this case, <code>rdf:parseType="Resource"</code>.  An 
        <code>rdf:parseType="Resource"</code> attribute is used to indicate 
        that the contents of an element are to be interpreted as the description 
        of a new (blank node) resource, without actually having to write a 
        nested <code>rdf:Description</code> element. In this case, the 
        <code>rdf:parseType="Resource"</code> attribute used in the 
        <code>exterms:weight</code> property element indicates that a 
        blank node is to be created as the value of the <code>exterms:weight</code> 
        property, and that the enclosed elements (<code>rdf:value</code> and 
        <code>exterms:units</code>) describe properties of that blank node. 
        Further details on <code>rdf:parseType="Resource"</code> are given 
        in <a href="#ref-rdf-syntax">[RDF-SYNTAX]</a>.</p>

        <p>The same approach can be used to represent quantities using
        any units of measure, as well as values taken from different
        classification schemes or rating systems, by using the
        <code>rdf:value</code> property to give the main value, and using
        additional properties to identify the classification scheme or
        other information that further describes the value.</p>

        <p>There is no need to use <code>rdf:value</code> for these purposes
        (e.g., a user-defined property name, such as
        <code>exterms:amount</code>, could have been used instead of <code>rdf:value</code>  
        in <a href="#example21">Example 21</a>), and RDF does not
        associate any special meaning with <code>rdf:value</code>. 
        <code>rdf:value</code> is simply provided as a convenience for use in these
        commonly-occurring situations.</p>

<p>However, even though much existing data in databases and on the Web 
(and in later Primer examples) takes the form of simple values for properties 
such as weights, costs, etc., the principle that such simple values are often 
insufficient to adequately describe these values is an important one. In a global 
environment such as the Web, it is generally <em>not</em> safe to make the 
assumption that anyone accessing a property value will understand the units 
being used (or other contextually-dependent information that may be involved). 
For example, a U.S. site might give a weight value in pounds, but someone accessing 
that data from outside the U.S. might assume that weights are given in kilograms. 
The correct interpretation of data in the Web environment may require that 
additional information (such as units information) be explicitly recorded. 
This can be done in many ways, such as using <code>rdf:value</code>, building 
units into property names (e.g., <code>exterms:weightInKg</code>), defining 
specialized datatypes that include units information (e.g., <code>extypes:kilograms</code>), 
or adding additional user-defined properties to specify this information 
(e.g., <code>exterms:unitOfWeight</code>), either in descriptions of individual 
items or products, in descriptions of sets of data (e.g., all the data in a 
catalog or on a site), or in schemas (see <a href="#rdfschema">Section 5</a>).</p>


      </div>

<div class="newstuff">
      <div class="section">
        <h3><a name="xmlliterals" id="xmlliterals">4.5 XML Literals</a></h3>

       <p>Sometimes the value of a property needs to be a fragment of XML, 
       or text that might contain XML markup.  For example, a publisher might 
       maintain RDF metadata that includes the titles of books and articles.  
       While such titles are often just simple strings of characters, this is 
       not always the case.  For instance, the titles of books on mathematics 
       may contain mathematical formulas that could be represented using 
       MathML <a href="#ref-mathml">[MATHML]</a>.  Titles might also include 
       markup for other reasons, such as for Ruby annotations 
       <a href="#ref-ruby">[RUBY]</a>, or for bidirectional rendering or 
       special glyph variants (see, e.g., <a href="#ref-charmod">[CHARMOD]</a>).</p>

       <p>RDF/XML provides a special notation to make it easy to write 
       literals of this kind.  This is done using a third value of the 
       <code>rdf:parseType</code> attribute.  Giving an element the attribute
       <code>rdf:parseType="Literal"</code> indicates that the contents of 
       the element are to be interpreted as an XML fragment.  
       <a href="#example22">Example 22</a> illustrates the use of 
       <code>rdf:parseType="Literal"</code>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example22" name="example22">Example 22: RDF/XML
            for an XML Literal</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
            xmlns:dc="http://purl.org/dc/elements/1.1/"
            xml:base="http://www.example.com/books"&gt;

  &lt;rdf:Description rdf:ID="book12345"&gt;
     &lt;dc:title rdf:parseType="Literal"&gt;
       &lt;span xml:lang="en"&gt;
         The &lt;em&gt;&amp;lt;br /&amp;gt;&lt;/em&gt; Element Considered Harmful.
       &lt;/span&gt;
     &lt;/dc:title&gt;
  &lt;/rdf:Description&gt;

&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

      <p>The RDF/XML in <a href="#example22">Example 22</a> describes a 
      graph containing a single triple with subject
      <code>ex:book12345</code>, and predicate <code>dc:title</code>.

       The <code>rdf:parseType="Literal"</code> attribute in the RDF/XML 
        indicates that all the XML within the <code>&lt;dc:title&gt;</code> 
        element is an XML fragment that is the value of the 
        <code>dc:title</code> property.

        In the graph, this value is a typed literal, whose datatype, 
        <code>rdf:XMLLiteral</code>, is defined in <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a> specifically to 
        represent fragments of XML (including character sequences that may
        or may not include XML markup).  The XML fragment is canonicalized according 
        to the XML Exclusive Canonicalization recommendation 
        <a href="#ref-xml-xc14n">[XML-XC14N]</a>.  This causes declarations 
        of used namespaces to be added to the fragment, 
        the uniform escaping or unescaping of characters, 
        the expansion of empty-element tags,
        and other transformations.  (For these reasons, and the fact that the
        triples notation itself requires further escaping, the
        actual typed literal is not shown here.  RDF/XML provides the 
        <code>rdf:parseType="Literal"</code> attribute so that RDF users will
        <em>not</em> have to deal directly with these transformations.  Those
        interested in the details should consult <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a> and <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>.)
        Contextual attributes, such as <code>xml:lang</code> and <code>xml:base</code>
        are not inherited from the RDF/XML document, and, if required, must, 
        as shown in the example, be explicitly specified in the XML fragment.</p>

        <p>This example illustrates that care must be taken in designing RDF data.  
        It might appear at first glance that titles are simple strings best 
        represented as plain literals, and only later might it be discovered 
        that some titles contain markup.  In cases where the value of a property 
        may sometimes contain markup and sometimes not, either 
        <code>rdf:parseType="Literal"</code> should be used throughout, 
        or software must handle both plain literals and literals of type
        <code>rdf:XMLLiteral</code> as values of the property.</p>

      </div>
</div>

    </div>

    <div class="section">
      <h2><a id="rdfschema" name="rdfschema"></a>5. Defining RDF
      Vocabularies: RDF Schema</h2>

      <p>RDF provides a way to express simple statements about
      resources, using named properties and values. However, RDF user
      communities also need the ability 
      <span class="newstuff">
      to define the <em>vocabularies</em>
      (terms) they intend to use in those statements, specifically, 
      </span>
      to indicate that they are
      describing specific kinds or classes of resources, and will use
      specific properties in describing those resources. For example,
      the company example.com from the examples in <a
      href="#newresources">Section 3.2</a> would want to describe
      classes such as <code>exterms:Tent</code>, and use properties such as
      <code>exterms:model</code>, <code>exterms:weightInKg</code>, and
      <code>exterms:packedSize</code> to describe them (QNames with
      various "example" namespace prefixes are used as the names of classes and
      properties here as a reminder that in RDF these names are
      actually <em>URI references</em>, as discussed in <a
      href="#basicconcepts">Section 2.1</a>). Similarly, people
      interested in describing bibliographic resources would want to
      describe classes such as <code>ex2:Book</code> or
      <code>ex2:MagazineArticle</code>, and use properties such as
      <code>ex2:author</code>, <code>ex2:title</code>, and <code>ex2:subject</code>
      to describe them. Other applications might need to describe
      classes such as <code>ex3:Person</code> and <code>ex3:Company</code>, and
      properties such as <code>ex3:age</code>, <code>ex3:jobTitle</code>,
      <code>ex3:stockSymbol</code>, and <code>ex3:numberOfEmployees</code>. 

<span class="newstuff">
      RDF
      itself provides no means for defining such application-specific
      classes and properties.
      Instead, such classes and properties are described as an RDF
      vocabulary, using extensions to RDF provided by the <a
      href="http://www.w3.org/TR/rdf-schema/">RDF Vocabulary
      Description Language 1.0: RDF Schema</a> <a
      href="#ref-rdf-vocabulary">[RDF-VOCABULARY]</a>, referred
      to here as <em>RDF Schema</em>.</span></p>

      <p>RDF Schema does not provide a vocabulary of
      application-specific classes like <code>exterms:Tent</code>,
      <code>ex2:Book</code>, or <code>ex3:Person</code>, and properties like
      <code>exterms:weightInKg</code>, <code>ex2:author</code> or
      <code>ex3:JobTitle</code>. Instead, it provides the facilities needed
      to <em>describe</em> such classes and properties, 
      and to indicate which classes and properties are
      expected to be used together (for example, to say that the
      property <code>ex3:jobTitle</code> will be used in describing a
      <code>ex3:Person</code>). In other words, RDF Schema provides a
      <em>type system</em> for RDF. The RDF Schema type system is
      similar in some respects to the type systems of object-oriented
      programming languages such as Java. For example, RDF Schema
      allows resources to be defined as instances of one or more
      <em>classes</em>. In addition, it allows classes to be organized
      in a hierarchical fashion; for example a class <code>ex:Dog</code>
      might be defined as a subclass of <code>ex:Mammal</code> which is a
      subclass of <code>ex:Animal</code>, meaning that any resource which
      is in class <code>ex:Dog</code> is also implicitly in class
      <code>ex:Animal</code> as well. However, RDF classes and properties are in
      some respects very different from programming language types. RDF
      class and property descriptions do not create a straightjacket
      into which information must be forced, but instead provide
      additional information about the RDF resources they describe.
      This information can be used in a variety of ways, which will be
      discussed in <a href="#interpretingschema">Section
      5.3</a>.</p>

      <p><span class="newstuff">
      The RDF Schema facilities are themselves provided in the form of 
      an RDF vocabulary;  that is, as a specialized set of predefined RDF 
      resources with their own special meanings.  The resources in the
      RDF Schema vocabulary have URIrefs with the prefix 
      <code>http://www.w3.org/2000/01/rdf-schema#</code> (conventionally
      associated with the QName prefix <code>rdfs:</code>).

      Vocabulary descriptions (schemas) written in the RDF Schema language
      are legal RDF graphs. Hence, RDF software that is not written
      to also process
      the additional RDF Schema vocabulary can still interpret a schema as a legal 
      RDF graph consisting of various resources and properties, 
      but will not "understand" the additional built-in 
      meanings of the RDF Schema terms.  To understand these additional 
      meanings, RDF software must
      be written to process an extended language that includes not
      only the <code>rdf:</code> vocabulary, but also the <code>rdfs:</code> 
      vocabulary, together with their built-in meanings. 
      This point will be illustrated in the next section.</span></p>

      <p>The following sections will illustrate RDF Schema's 
      basic resources and properties.</p>

      <div class="section">
        <h3><a name="schemaclasses" id="schemaclasses">5.1 Describing
        Classes</a></h3>

        <p>A basic step in any kind of description process is
        identifying the various kinds of things to be described. RDF
        Schema refers to these "kinds of things" as <em>classes</em>. A
        <em>class</em> in RDF Schema corresponds to the generic concept
        of a <em>Type</em> or <em>Category</em>, somewhat like the
        notion of a class in object-oriented programming languages such
        as Java. RDF classes can be used to represent almost any
        category of thing, such as Web pages, people, document types,
        databases or abstract concepts. Classes are described using the
        RDF Schema resources <code>rdfs:Class</code> and
        <code>rdfs:Resource</code>, and the properties <code>rdf:type</code>
        and <code>rdfs:subClassOf</code>.</p>

        <p>For example, suppose an organization
        <code>example.org</code> wanted to use RDF to provide
        information about different kinds of motor vehicles. In RDF
        Schema, <code>example.org</code> would first need a class to represent the category
        of things that are motor vehicles. The resources that belong to
        a class are called its <em>instances</em>. In this case, <code>example.org</code>
        intends for the instances of this class to be resources that are
        motor vehicles.</p>

        <p>In RDF Schema, a <em>class</em> is any resource having an
        <code>rdf:type</code> property whose value is the 
        resource <code>rdfs:Class</code>. So the motor vehicle class would
        be described by assigning the class a URIref, say
        <code>ex:MotorVehicle</code> (<span class="newstuff">using <code>ex:</code> 
        to stand for the URIref
        <code>http://www.example.org/schemas/vehicles</code>, which is used
        as the prefix for URIrefs from example.org's vocabulary</span>) 
        and describing that resource with an
        <code>rdf:type</code> property whose value is the 
        resource <code>rdfs:Class</code>. That is, <code>example.org</code> would write the RDF
        statement:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:MotorVehicle   rdf:type   rdfs:Class .
</pre>
        </div>

        <p>As indicated in <a href="#newresources">Section 3.2</a>,
        the property <code>rdf:type</code> is used to indicate that a
        resource is an instance of a class. So, having described
        <code>ex:MotorVehicle</code> as a class, resource <code>exthings:companyCar</code> 
        would be described as a motor vehicle by
        the RDF statement:</p>

        <div class="exampleOuter exampleInner">
<pre>
exthings:companyCar   rdf:type   ex:MotorVehicle .
</pre>
        </div>

        <p>(This statement uses a common convention that class names
        are written with an initial uppercase letter, while property
        and instance names are written with an initial lowercase
        letter. However, this convention is not required in RDF Schema.
        <span class="newstuff">The statement also assumes that <code>example.org</code>
        has decided to define separate vocabularies for classes of things, and
        instances of things.)</span></p>

        <p>The resource <code>rdfs:Class</code> itself has an
        <code>rdf:type</code> of <code>rdfs:Class</code>. A resource may be an
        instance of more than one class.</p>

        <p>After describing class <code>ex:MotorVehicle</code>, <code>example.org</code> might
        want to describe additional classes representing various
        specialized kinds of motor vehicle, e.g., passenger vehicles,
        vans, minivans, and so on. These classes can be described in the
        same way as class <code>ex:MotorVehicle</code>, by
        assigning a URIref for each new class, and writing RDF
        statements describing these resources as classes, e.g.,
        writing:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:Van     rdf:type   rdfs:Class .
ex:Truck   rdf:type   rdfs:Class .
</pre>
        </div>

        <p>and so on. However, these statements by themselves only describe
        the individual classes.  <code>example.org</code> may also want to 
        indicate their special
        relationship to class <code>ex:MotorVehicle</code>, i.e., that they
        are specialized <em>kinds</em> of MotorVehicle.</p>

        <p>This kind of specialization relationship between
        two classes is described using the predefined <code>rdfs:subClassOf</code> 
        property to relate the two classes.  
        For example, to state that <code>ex:Van</code> is a
        specialized kind of <code>ex:MotorVehicle</code>, <code>example.org</code>
        would write the RDF statement:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:Van   rdfs:subClassOf   ex:MotorVehicle .
</pre>
        </div>


        <p>The meaning of this <code>rdfs:subClassOf</code> relationship is
        that any instance of class <code>ex:Van</code> is also an instance 
        of class <code>ex:MotorVehicle</code>.
<span class="newstuff">  
        So if resource <code>exthings:companyVan</code> is an instance of
        <code>ex:Van</code> then, based on the declared 
        <code>rdfs:subClassOf</code> relationship, 
        RDF software written to understand the RDF Schema vocabulary
        can <a name="LCC-015" id="LCC-015">infer</a> the additional information 
        that <code>exthings:companyVan</code> 
        is also an instance of <code>ex:MotorVehicle</code>.</span></p>

        <p class="newstuff">This example of <code>exthings:companyVan</code>
        illustrates the point made earlier about RDF Schema defining
        an extended language.  RDF itself does not define the special 
        meaning of terms from the RDF Schema vocabulary such as <code>rdfs:subClassOf</code>.
        So if an RDF schema defines this <code>rdfs:subClassOf</code> relationship
        between <code>ex:Van</code> and <code>ex:MotorVehicle</code>, 
        RDF software not written to understand the RDF Schema terms
        would recognize this as a triple, 
        with predicate <code>rdfs:subClassOf</code>,  but it would not 
        understand the special significance of <code>rdfs:subClassOf</code>,
        and not be able to 
        draw the additional inference that <code>exthings:companyVan</code> 
        is also an instance of <code>ex:MotorVehicle</code>.</p>

        <p>The <code>rdfs:subClassOf</code> property is
        <em>transitive</em>. This means, for example, that given
        the RDF statements:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:Van       rdfs:subClassOf   ex:MotorVehicle .
ex:MiniVan   rdfs:subClassOf   ex:Van .
</pre>
        </div>

        <p class="newstuff">RDF Schema defines <code>ex:MiniVan</code> as also being a subclass of
        <code>ex:MotorVehicle</code>. As a result, RDF Schema defines resources that are
        instances of class <code>ex:MiniVan</code> as also being
        instances of class <code>ex:MotorVehicle</code> (as well as being instances of
        class <code>ex:Van</code>). A class may be a subclass of more than
        one class (for example, <code>ex:MiniVan</code> may be a subclass
        of both <code>ex:Van</code> and <code>ex:PassengerVehicle</code>). 
        RDF Schema defines all classes as subclasses of class
        <code>rdfs:Resource</code> (since the instances belonging to all
        classes are resources).</p>

        <p><a href="#figure18">Figure 18</a> shows the full class
        hierarchy being discussed in these examples.</p>

<div class="newstuff">
        <div class="figure">
          <img src="vehicleClassesJan12.png"
          alt="A Vehicle Class Hierarchy" /><br />
           <a id="figure18" name="figure18">Figure 18: A Vehicle Class
          Hierarchy</a>
        </div>

        <p>(To simplify the figure, the <code>rdf:type</code> properties 
        relating each of the classes to <code>rdfs:Class</code> are omitted in 
        <a href="#figure18">Figure 18</a>.  In fact, RDF Schema defines
        both the subjects and objects of statements that use the 
        <code>rdfs:subClassOf</code> property to be resources of type
        <code>rdfs:Class</code>, so this information could be inferred.
        However, in actually writing schemas, it is good practice to 
        explicitly provide this information.)</p> 
         
</div>

        <p>This schema could also be described by the triples:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:MotorVehicle       rdf:type          rdfs:Class .
ex:PassengerVehicle   rdf:type          rdfs:Class .
ex:Van                rdf:type          rdfs:Class .
ex:Truck              rdf:type          rdfs:Class .
ex:MiniVan            rdf:type          rdfs:Class .

ex:PassengerVehicle   rdfs:subClassOf   ex:MotorVehicle .
ex:Van                rdfs:subClassOf   ex:MotorVehicle .
ex:Truck              rdfs:subClassOf   ex:MotorVehicle .

ex:MiniVan            rdfs:subClassOf   ex:Van .
ex:MiniVan            rdfs:subClassOf   ex:PassengerVehicle .
</pre>
        </div>

        <p><a href="#example23">Example 23</a> shows how this schema
        could be written in RDF/XML.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example23" name="example23">Example 23: The Vehicle
            Class Hierarchy in RDF/XML</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
&lt;rdf:RDF   
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"  
  xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
  xml:base="http://example.org/schemas/vehicles"&gt;

&lt;rdf:Description rdf:ID="MotorVehicle"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:ID="PassengerVehicle"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/&gt;
  &lt;rdfs:subClassOf rdf:resource="#MotorVehicle"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:ID="Truck"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/&gt;
  &lt;rdfs:subClassOf rdf:resource="#MotorVehicle"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:ID="Van"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/&gt;
  &lt;rdfs:subClassOf rdf:resource="#MotorVehicle"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:ID="MiniVan"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/&gt;
  &lt;rdfs:subClassOf rdf:resource="#Van"/&gt;
  &lt;rdfs:subClassOf rdf:resource="#PassengerVehicle"/&gt;
&lt;/rdf:Description&gt;

&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

<div class="newstuff">
       <p>As discussed in <a href="#newresources">Section 3.2</a>
        in connection with <a href="#example13">Example 13</a>,
        RDF/XML provides an abbreviation for describing
        resources having an <code>rdf:type</code> property (<em>typed nodes</em>). 
        Since RDF Schema classes are RDF resources, 
        this abbreviation can be applied to the description of classes.
        Using this abbreviation, the schema could also be described 
        as shown in <a href="#example24">Example 24</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example24" name="example24">Example 24: The Vehicle
            Class Hierarchy Using the Typed Node Abbreviation</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
&lt;rdf:RDF   
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"  
  xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
  xml:base="http://example.org/schemas/vehicles"&gt;

&lt;rdfs:Class rdf:ID="MotorVehicle"/&gt;

&lt;rdfs:Class rdf:ID="PassengerVehicle"&gt;
  &lt;rdfs:subClassOf rdf:resource="#MotorVehicle"/&gt;
&lt;/rdfs:Class&gt;

&lt;rdfs:Class rdf:ID="Truck"&gt;
  &lt;rdfs:subClassOf rdf:resource="#MotorVehicle"/&gt;
&lt;/rdfs:Class&gt;

&lt;rdfs:Class rdf:ID="Van"&gt;
  &lt;rdfs:subClassOf rdf:resource="#MotorVehicle"/&gt;
&lt;/rdfs:Class&gt;

&lt;rdfs:Class rdf:ID="MiniVan"&gt;
  &lt;rdfs:subClassOf rdf:resource="#Van"/&gt;
  &lt;rdfs:subClassOf rdf:resource="#PassengerVehicle"/&gt;
&lt;/rdfs:Class&gt;

&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>Similar typed node abbreviations will be used throughout the rest of
         this section.</p>
</div>

        <p>The RDF/XML in <a href="#example23">Example 23</a>
        and <a href="#example24">Example 24</a> introduces names, such as
        <code>MotorVehicle</code>, for the resources (classes) that it
        describes using <code>rdf:ID</code>, to give the effect of
        "assigning" URIrefs relative to the schema document as 
        described in <a href="#newresources">Section 3.2</a>. 
        <span class="newstuff"><code>rdf:ID</code> is useful here because
        it both abbreviates the URIrefs, and also provides an additional 
        check that the value of the <code>rdf:ID</code> attribute is
        unique against the current base URI (usually the document URI).
        This helps pick up repeated <code>rdf:ID</code> values when defining
        the names of classes and properties in RDF schemas.</span>
        Relative
        URIrefs based on these names can then be used in other class
        definitions within the same schema (e.g., as 
        <code>#MotorVehicle</code> is used in the description of the other
        classes). The full URIref of this class, assuming that the
        schema itself was the resource
        <code>http://example.org/schemas/vehicles</code>, would be
        <code>http://example.org/schemas/vehicles#MotorVehicle</code>
        (shown in <a href="#figure18">Figure 18</a>). As noted in <a
        href="#newresources">Section 3.2</a>, to ensure that the
        references to these schema classes would be consistently
        maintained even if the schema were relocated or copied (or to
        simply assign a base URIref for the schema classes without
        assuming they are all published at a single location), the
        class descriptions could also include an explicit
        <code>xml:base="http://example.org/schemas/vehicles"</code>
        declaration.  <span class="newstuff">Use of an explicit 
        <code>xml:base</code> declaration is considered good practice,
        and one is provided in both examples.</span></p>

        <p>To refer to these classes in RDF instance data (e.g., data
        describing individual vehicles of these classes) located
        elsewhere, <code>example.org</code> would need to identify
        the classes <span class="newstuff">either by writing absolute URIrefs, 
        by using relative URIrefs together with an appropriate
        <code>xml:base</code> declaration, or by using QNames
        together with an appropriate namespace declaration that allows the
        QNames to be expanded to the proper URIrefs.</span>  For example, the resource
        <code>exthings:companyCar</code> could be described as an instance of the class
        <code>ex:MotorVehicle</code> described in the schema of 
        <a href="#example24">Example 24</a> by
        the RDF/XML shown in <a href="#example25">Example 25</a> :</p>

<div class="newstuff">
        <div class="exampleOuter">
          <div class="c1">
            <a id="example25" name="example25">Example 25: An Instance
            of <code>ex:MotorVehicle</code></a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;rdf:RDF   
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"  
  xmlns:ex="http://example.org/schemas/vehicles#"
  xml:base="http://example.org/things"&gt;

   &lt;ex:MotorVehicle rdf:ID="companyCar"/&gt;

&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

       <p>Note that the QName <code>ex:MotorVehicle</code>, when expanded using the
       namespace declaration <code>xmlns:ex="http://example.org/schemas/vehicles#"</code>,
       becomes the full URIref <code>http://example.org/schemas/vehicles#MotorVehicle</code>,
       which is the correct URIref for the <code>MotorVehicle</code> class 
       as shown in <a href="#figure18">Figure 18</a>.  The <code>xml:base</code> declaration
       <code>xml:base="http://example.org/things"</code> is provided
       to allow the <code>rdf:ID="companyCar"</code> to expand to the proper
       <code>exthings:companyCar</code> URIref (since a QName cannot be used as the
       value of the <code>rdf:ID</code> attribute).</p> 
</div>

      </div>

      <div class="section">
        <h3><a id="properties" name="properties">5.2 Describing
        Properties</a></h3>

        <p>In addition to describing the specific <em>classes</em> of
        things they want to describe, user communities also need to be
        able to describe specific <em>properties</em> that characterize
        those classes of things (such as <code>rearSeatLegRoom</code> to
        describe a passenger vehicle). In RDF Schema, properties are
        described using the RDF class <code>rdf:Property</code>,
        and the RDF Schema properties <code>rdfs:domain</code>,
        <code>rdfs:range</code>, and <code>rdfs:subPropertyOf</code>.</p>

        <p>All properties in RDF are described as instances of class
        <code>rdf:Property</code>. So a new property, such as
        <code>exterms:weightInKg</code>, is described by assigning the
        property a URIref, and describing that resource with an
        <code>rdf:type</code> property whose value is the resource
        <code>rdf:Property</code>, for example, by writing the RDF
        statement:</p>

        <div class="exampleOuter exampleInner">
<pre>
exterms:weightInKg   rdf:type   rdf:Property .
</pre>
        </div>

        <p>RDF Schema also provides vocabulary for describing how
        properties and classes are intended to be used together in RDF
        data. The most important information of this kind is supplied
        by using the RDF Schema properties <code>rdfs:range</code> and
        <code>rdfs:domain</code> to further describe application-specific
        properties.</p>

        <p>The <code>rdfs:range</code> property is used to indicate that
        the values of a particular property are instances of a
        designated class. For example, if <code>example.org</code> wanted to indicate that
        the property <code>ex:author</code> had values that are instances
        of class <code>ex:Person</code>, it would write the RDF
        statements:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:Person   rdf:type     rdfs:Class .
ex:author   rdf:type     rdf:Property .
ex:author   rdfs:range   ex:Person .
</pre>
        </div>

        <p>These statements indicate that <code>ex:Person</code> is a
        class, <code>ex:author</code> is a property, and that RDF
        statements using the <code>ex:author</code> property have instances
        of <code>ex:Person</code> as objects.</p>

        <p>A property, say <code>ex:hasMother</code>, can have zero, one,
        or more than one range property. If <code>ex:hasMother</code> has
        no range property, then nothing is said about the values
        of the <code>ex:hasMother</code> property. If <code>ex:hasMother</code>
        has one range property, say one specifying <code>ex:Person</code>
        as the range, this says that the values of the
        <code>ex:hasMother</code> property are instances of class
        <code>ex:Person</code>. If <code>ex:hasMother</code> has more than one
        range property, say one specifying <code>ex:Person</code> as its
        range, and another specifying <code>ex:Female</code> as its range,
        this says that the values of the <code>ex:hasMother</code> property
        are resources that are instances of <em>all</em> of the classes
        specified as the ranges, i.e., that any value of
        <code>ex:hasMother</code> is <em>both</em> a <code>ex:Female</code>
        <em>and</em> a <code>ex:Person</code>.</p>

<div class="newstuff">
        <p>This last point may not be obvious.  However, stating
        that the property <code>ex:hasMother</code> has the two ranges 
        <code>ex:Female</code> and <code>ex:Person</code> involves making
        two separate statements:</p>
        
        <div class="exampleOuter exampleInner">
<pre>
ex:hasMother   rdfs:range   ex:Female .
ex:hasMother   rdfs:range   ex:Person .
</pre>
        </div>

        <p>For any given statement using this property, say:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:frank   ex:hasMother   exstaff:frances .
</pre>
        </div>

        <p>in order for <em>both</em> the <code>rdfs:range</code> 
        statements to be
        correct, it must be the case that <code>exstaff:frances</code> is 
        <em>both</em> an instance of <code>ex:Female</code> and 
        of <code>ex:Person</code>.</p>
</div>

        <p>The <code>rdfs:range</code> property can also be used to
        indicate that the value of a property is given by a typed
        literal, as discussed in <a href="#typedliterals">Section
        2.4</a>. For example, if <code>example.org</code> wanted to indicate that the
        property <code>ex:age</code> had values from the XML Schema
        datatype <code>xsd:integer</code>, it would write the RDF
        statements:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:age   rdf:type     rdf:Property .
ex:age   rdfs:range   xsd:integer .
</pre>
        </div>

        <p>The datatype <code>xsd:integer</code> is identified by its
        URIref (the full URIref being
        <code>http://www.w3.org/2001/XMLSchema#integer</code>). This URIref
        can be used without explicitly stating in the schema that
        it identifies a datatype. However, it is often useful to
        explicitly state that a given URIref identifies a datatype.
        This can be done using the RDF Schema class
        <code>rdfs:Datatype</code>. To state that <code>xsd:integer</code> is a
        datatype, <code>example.org</code> would write the RDF statement:</p>

        <div class="exampleOuter exampleInner">
<pre>
xsd:integer   rdf:type   rdfs:Datatype .
</pre>
        </div>

        <p>This statement says that <code>xsd:integer</code> is the URIref
        of a datatype (which is assumed to conform to the requirements
        for RDF datatypes described in <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>). Such a statement
        does <em>not</em> constitute a <em>definition</em> of a
        datatype, e.g., in the sense that <code>example.org</code> is defining a new
        datatype. There is no way to define datatypes in RDF Schema. As noted
        in <a href="#typedliterals">Section 2.4</a>, datatypes are
        defined externally to RDF (and to RDF Schema), and 
        <em>referred to</em> in RDF statements
        by their URIrefs. This statement simply serves to document the
        existence of the datatype, and indicate explicitly that it is
        being used in this schema.</p>

        <p>The <code>rdfs:domain</code> property is used to indicate that a
        particular property applies to a designated class. For example,
        if <code>example.org</code> wanted to indicate that the property <code>ex:author</code>
        applies to instances of class <code>ex:Book</code>, it would write
        the RDF statements:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:Book     rdf:type      rdfs:Class .
ex:author   rdf:type      rdf:Property .
ex:author   rdfs:domain   ex:Book .
</pre>
        </div>

        <p>These statements indicate that <code>ex:Book</code> is a
        class, <code>ex:author</code> is a property, and that RDF
        statements using the <code>ex:author</code> property have instances
        of <code>ex:Book</code> as subjects.</p>

        <p>A given property, say <code>exterms:weight</code>, may have
        zero, one, or more than one domain property. If
        <code>exterms:weight</code> has no domain property, then 
        nothing is said about the resources that <code>exterms:weight</code>
        properties may be used with (any resource could have a
        <code>exterms:weight</code> property). If <code>exterms:weight</code>
        has one domain property, say one specifying <code>ex:Book</code> as
        the domain, this says that the <code>exterms:weight</code> property
        applies to instances of class <code>ex:Book</code>. If
        <code>exterms:weight</code> has more than one domain property, say
        one specifying <code>ex:Book</code> as the domain and another one
        specifying <code>ex:MotorVehicle</code> as the domain, this says
        that any resource that has a <code>exterms:weight</code> property
        is an instance of <em>all</em> of the classes specified as the
        domains, i.e., that any resource that has a
        <code>exterms:weight</code> property is both a <code>ex:Book</code>
        <em>and</em> a <code>ex:MotorVehicle</code> (illustrating the need
        for care in specifying domains and ranges).</p>

<div class="newstuff">
        <p>As in the case of <code>rdfs:range</code>, this last point may 
        not be obvious.  However, stating
        that the property <code>exterms:weight</code> has the two domains 
        <code>ex:Book</code> and <code>ex:MotorVehicle</code> involves making
        two separate statements:</p>
        
        <div class="exampleOuter exampleInner">
<pre>
exterms:weight   rdfs:domain   ex:Book .
exterms:weight   rdfs:domain   ex:MotorVehicle .
</pre>
        </div>

        <p>For any given statement using this property, say:</p>

        <div class="exampleOuter exampleInner">
<pre>
exthings:companyCar   exterms:weight   "2500"^^xsd:integer .
</pre>
        </div>

        <p>in order for <em>both</em> the <code>rdfs:domain</code> 
        statements to be
        correct, it must be the case that <code>exthings:companyCar</code> is 
        <em>both</em> an instance of <code>ex:Book</code> and 
        of <code>ex:MotorVehicle</code>.</p>
</div>

        <p>The use of these range and domain
        descriptions can be illustrated by extending the vehicle schema, adding two
        properties <code>ex:registeredTo</code> and
        <code>ex:rearSeatLegRoom</code>, a new class <code>ex:Person</code>,
        and explicitly describing the datatype <code>xsd:integer</code> as
        a datatype. The <code>ex:registeredTo</code> property applies to
        any <code>ex:MotorVehicle</code> and its value is a
        <code>ex:Person</code>. For the sake of this example,
        <code>ex:rearSeatLegRoom</code> applies only to instances of class
        <code>ex:PassengerVehicle</code>. The value is an
        <code>xsd:integer</code> giving the number of centimeters of rear
        seat legroom. These descriptions are shown in <a
        href="#example26">Example 26</a>:</p>

<div class="newstuff">
        <div class="exampleOuter">
          <div class="c1">
            <a id="example26" name="example26">Example 26: Some
            Property Descriptions for the Vehicle Schema</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdf:Property rdf:ID="registeredTo"&gt;
  &lt;rdfs:domain rdf:resource="#MotorVehicle"/&gt;
  &lt;rdfs:range rdf:resource="#Person"/&gt;
&lt;/rdf:Property&gt;

&lt;rdf:Property rdf:ID="rearSeatLegRoom"&gt;
  &lt;rdfs:domain rdf:resource="#PassengerVehicle"/&gt; 
  &lt;rdfs:range rdf:resource="&amp;xsd;integer"/&gt;
&lt;/rdf:Property&gt;

&lt;rdfs:Class rdf:ID="Person"/&gt;

&lt;rdfs:Datatype rdf:about="&amp;xsd;integer"/&gt;
</pre>
          </div>
        </div>
</div>

        <p>Note that an <code>&lt;rdf:RDF&gt;</code>
        element is not used in <a href="#example26">Example 26</a>, because 
        it is
        assumed this RDF/XML is being added to the vehicle schema 
        described in <a href="#example24">Example 24</a>. This same
        assumption also allows the use of relative URIrefs like
        <code>#MotorVehicle</code> to refer to other classes from that
        schema.</p>

        <p>RDF Schema provides a way to specialize <em>properties</em>
        as well as classes. This specialization
        relationship between two properties is described using the predefined
        <code>rdfs:subPropertyOf</code> property. For example, if
        <code>ex:primaryDriver</code> and <code>ex:driver</code> are both
        properties, <code>example.org</code> could describe these properties, 
        and the fact that
        <code>ex:primaryDriver</code> is a specialization of
        <code>ex:driver</code>, by writing the RDF statements:</p>

        <div class="exampleOuter exampleInner">
<pre>
ex:driver          rdf:type             rdf:Property .
ex:primaryDriver   rdf:type             rdf:Property .
ex:primaryDriver   rdfs:subPropertyOf   ex:driver .
</pre>
        </div>

        <p>The meaning of this <code>rdfs:subPropertyOf</code> relationship
        is that if an instance <code>exstaff:fred</code> is an
        <code>ex:primaryDriver</code> of the instance
        <code>ex:companyVan</code>, <span class="newstuff">then 
        RDF Schema defines <code>exstaff:fred</code> as also being
        an <code>ex:driver</code> of
        <code>ex:companyVan</code>.</span> The RDF/XML describing these properties
        (assuming again that it is being added to the vehicle schema
        described in <a href="#example24">Example 24</a>) 
        is shown in <a href="#example27">Example
        27</a>.</p>

<div class="newstuff">
        <div class="exampleOuter">
          <div class="c1">
            <a id="example27" name="example27">Example 27: More
            Properties for the Vehicle Schema</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdf:Property rdf:ID="driver"&gt;
  &lt;rdfs:domain rdf:resource="#MotorVehicle"/&gt;
&lt;/rdf:Property&gt;

&lt;rdf:Property rdf:ID="primaryDriver"&gt;
  &lt;rdfs:subPropertyOf rdf:resource="#driver"/&gt;
&lt;/rdf:Property&gt;
</pre>
          </div>
        </div>
</div>

        <p>A property may be a subproperty of zero, one or more
        properties. All RDF Schema <code>rdfs:range</code> and
        <code>rdfs:domain</code> properties that apply to an RDF property
        also apply to each of its subproperties. So, in the above example,
<span class="newstuff">        
        RDF Schema defines <code>ex:primaryDriver</code>
        as also having an <code>rdfs:domain</code> of <code>ex:MotorVehicle</code>,
        because of its subproperty
        relationship to <code>ex:driver</code>.</span></p>

        <p><a href="#example28">Example 28</a> shows the RDF/XML for the
        full vehicle schema, containing all the descriptions
        given so far:</p>

<div class="newstuff">
        <div class="exampleOuter">
          <div class="c1">
            <a id="example28" name="example28">Example 28: The Full
            Vehicle Schema</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
&lt;rdf:RDF   
  xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"  
  xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
  xml:base="http://example.org/schemas/vehicles"&gt;

&lt;rdfs:Class rdf:ID="MotorVehicle"/&gt;

&lt;rdfs:Class rdf:ID="PassengerVehicle"&gt;
  &lt;rdfs:subClassOf rdf:resource="#MotorVehicle"/&gt;
&lt;/rdfs:Class&gt;

&lt;rdfs:Class rdf:ID="Truck"&gt;
  &lt;rdfs:subClassOf rdf:resource="#MotorVehicle"/&gt;
&lt;/rdfs:Class&gt;

&lt;rdfs:Class rdf:ID="Van"&gt;
  &lt;rdfs:subClassOf rdf:resource="#MotorVehicle"/&gt;
&lt;/rdfs:Class&gt;

&lt;rdfs:Class rdf:ID="MiniVan"&gt;
  &lt;rdfs:subClassOf rdf:resource="#Van"/&gt;
  &lt;rdfs:subClassOf rdf:resource="#PassengerVehicle"/&gt;
&lt;/rdfs:Class&gt;

&lt;rdfs:Class rdf:ID="Person"/&gt;

&lt;rdfs:Datatype rdf:about="&amp;xsd;integer"/&gt;

&lt;rdf:Property rdf:ID="registeredTo"&gt;
  &lt;rdfs:domain rdf:resource="#MotorVehicle"/&gt;
  &lt;rdfs:range rdf:resource="#Person"/&gt;
&lt;/rdf:Property&gt;

&lt;rdf:Property rdf:ID="rearSeatLegRoom"&gt;
  &lt;rdfs:domain rdf:resource="#PassengerVehicle"/&gt; 
  &lt;rdfs:range rdf:resource="&amp;xsd;integer"/&gt;
&lt;/rdf:Property&gt;

&lt;rdf:Property rdf:ID="driver"&gt;
  &lt;rdfs:domain rdf:resource="#MotorVehicle"/&gt;
&lt;/rdf:Property&gt;

&lt;rdf:Property rdf:ID="primaryDriver"&gt;
  &lt;rdfs:subPropertyOf rdf:resource="#driver"/&gt;
&lt;/rdf:Property&gt;

&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>
</div>

        <p>Having shown how to describe classes and properties
        using RDF Schema, instances using those classes and properties 
        can now be illustrated. For example, <a
        href="#example29">Example 29</a> describes an instance of the
        <code>ex:PassengerVehicle</code> class described in 
        <a href="#example28">Example 28</a>, together
        with some hypothetical values for its properties.</p>

<div class="newstuff">
        <div class="exampleOuter">
          <div class="c1">
            <a id="example29" name="example29">Example 29: An Instance
            of <code>ex:PassengerVehicle</code></a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;!DOCTYPE rdf:RDF [&lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt;]&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
            xmlns:ex="http://example.org/schemas/vehicles#"
            xml:base="http://example.org/things"&gt;

  &lt;ex:PassengerVehicle rdf:ID="johnSmithsCar"&gt;
       &lt;ex:registeredTo rdf:resource="http://www.example.org/staffid/85740"/&gt;
       &lt;ex:rearSeatLegRoom 
           rdf:datatype="&amp;xsd;integer"&gt;127&lt;/ex:rearSeatLegRoom&gt;
       &lt;ex:primaryDriver rdf:resource="http://www.example.org/staffid/85740"/&gt;
  &lt;/ex:PassengerVehicle&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>This example assumes that the instance is described in a
        separate document from the schema.  Since the 
        schema has an <code>xml:base</code> of 
        <code>http://example.org/schemas/vehicles</code>, the
        namespace declaration
        <code>xmlns:ex="http://example.org/schemas/vehicles#"</code> 
        is provided to allow
        QNames such as <code>ex:registeredTo</code> in the instance data to 
        be properly expanded to the URIrefs of the classes and 
        properties described in that schema.  An 
        <code>xml:base</code> declaration is also provided for this
        instance, to allow
        <code>rdf:ID="johnSmithsCar"</code> to expand to the proper URIref
        independently of the location of the actual document.</p>
</div>

        <p>Note that an <code>ex:registeredTo</code> property can be used in
        describing this instance of <code>ex:PassengerVehicle</code>,
        because <code>ex:PassengerVehicle</code> is a subclass of
        <code>ex:MotorVehicle</code>. Note also that a typed literal is used
        for the value of the <code>ex:rearSetLegRoom</code> property in this
        instance, rather than a plain literal (i.e., rather than stating the value
        as <code>&lt;ex:rearSeatLegRoom&gt;127&lt;/ex:rearSeatLegRoom&gt;</code>).
        Because the schema describes the range of this property as an
        <code>xsd:integer</code>, the value of the property should be a typed
        literal of that datatype in order to match the range description 
        <span class="newstuff">(i.e., the range declaration does not 
        automatically "assign" a
        datatype to a plain literal, and so a typed literal of the
        appropriate datatype must be explicitly provided).</span>

        Additional information, either in the schema, or in additional
        instance data, could also be provided to explicitly specify the
        <em>units</em> of the <code>ex:rearSetLegRoom</code> property
        (centimeters), as discussed in <a href="#rdfvalue">Section 4.4</a>.
</p>

      </div>

      <div class="section">
        <h3><a id="interpretingschema" name="interpretingschema">5.3
        Interpreting RDF Schema Declarations</a></h3>

        <p>As noted earlier, the RDF Schema type system is similar in
        some respects to the type systems of object-oriented
        programming languages such as Java. However, RDF differs from
        most programming language type systems in several important
        respects.</p>

        <p>One important difference is that instead of describing a
        class as having a collection of specific properties, an RDF
        schema describes properties as applying to specific classes of
        resources, using <em>domain</em> and <em>range</em> properties.
        For example, a typical object-oriented programming language
        might define a class <code>Book</code> with an attribute called
        <code>author</code> having values of type <code>Person</code>. A
        corresponding RDF schema would describe a class
        <code>ex:Book</code>, and, in a separate description, a property
        <code>ex:author</code> having a domain of <code>ex:Book</code> and a
        range of <code>ex:Person</code>.</p>

        <p>The difference between these approaches may seem to be only
        syntactic, but in fact there is an important difference. In the
        programming language class description, the attribute
        <code>author</code> is part of the description of class
        <code>Book</code>, and applies <em>only</em> to instances of class
        <code>Book</code>. Another class (say, <code>softwareModule</code>)
        might also have an attribute called <code>author</code>, but this
        would be considered a <em>different</em> attribute. In other
        words, the <em>scope</em> of an attribute description in most
        programming languages is restricted to the class or type in
        which it is defined. In RDF, on the other hand, property
        descriptions are, by default, <em>independent</em> of class
        definitions, and have, by default, <em>global</em> scope
        (although they may optionally be declared to apply only to
        certain classes using domain specifications).</p>

        <p>As a result, an RDF schema could describe a property
        <code>exterms:weight</code> without a domain being specified. This
        property could then be used to describe instances of any class
        that might be considered to have a weight. One benefit of the
        RDF property-based approach is that it becomes easier to extend
        the use of property definitions to situations that might not
        have been anticipated in the original description. At the same time,
        this is a "benefit" which must be used with care, to insure
        that properties are not mis-applied in inappropriate
        situations.</p>

        <p class="newstuff">Another result of the global scope of RDF property descriptions
        is that it is not possible in an RDF schema to define a specific property as
        having locally-different ranges depending on the class of the resource
        it is applied to. 
        For example, in defining the property <code>ex:hasParent</code>, it 
        would be desirable to be able to say that if the property is used
        to describe a resource of class <code>ex:Human</code>, then the range 
        of the property is also a resource of class <code>ex:Human</code>,
        while if the property is used
        to describe a resource of class <code>ex:Tiger</code>, then the range 
        of the property is also a resource of class <code>ex:Tiger</code>.
        This kind of definition is not possible in RDF Schema.  Instead, any
        range defined for an RDF property applies to <em>all</em> uses of the
        property, and so ranges should be defined with care.  However, while
        such locally-different ranges cannot be defined in RDF Schema,
        they can be defined in some of the richer schema languages discussed in 
        <a href="#richerschemas">Section 5.5</a>.</p>
        
        <p>Another important difference is that RDF Schema descriptions
        are not necessarily <em>prescriptive</em> in the way
        programming language type declarations typically are. For
        example, if a programming language declares a class
        <code>Book</code> with an <code>author</code> attribute having values
        of type <code>Person</code>, this is usually interpreted as a group
        of <em>constraints</em>. The language will not allow the
        creation of an instance of <code>Book</code> without an
        <code>author</code> attribute, and it will not allow an instance of
        <code>Book</code> with an <code>author</code> attribute that does not
        have a <code>Person</code> as its value. Moreover, if
        <code>author</code> is the <em>only</em> attribute defined for
        class <code>Book</code>, the language will not allow an instance of
        <code>Book</code> with some other attribute.</p>

        <p>RDF Schema, on the other hand, provides schema information
        as additional <em>descriptions</em> of resources, but does not
        prescribe how these descriptions should be used by an
        application. For example, suppose an RDF schema states that an
        <code>ex:author</code> property has an <code>rdfs:range</code> of class
        <code>ex:Person</code>. This is simply an RDF statement that RDF
        statements containing <code>ex:author</code> properties have
        instances of <code>ex:Person</code> as objects.</p>

        <p>This schema-supplied information might be used in different
        ways. One application might interpret this statement as
        specifying part of a template for RDF data it is creating, and
        use it to ensure that any <code>ex:author</code> property has a
        value of the indicated (<code>ex:Person</code>) class. That is,
        this application interprets the schema description as a
        <em>constraint</em> in the same way that a programming language
        might. However, another application might interpret this
        statement as providing additional information about data it is
        receiving, information which may not be provided explicitly in
        the original data. For example, this second application might
        receive some RDF data that includes an <code>ex:author</code>
        property whose value is a resource of unspecified class, and
        use this schema-provided statement to conclude that the
        resource must be an instance of class <code>ex:Person</code>. A
        third application might receive some RDF data that includes an
        <code>ex:author</code> property whose value is a resource of class
        <code>ex:Corporation</code>, and use this schema information as the
        basis of a warning that "there may be an inconsistency here,
        but on the other hand there may not be". Somewhere else there
        may be a declaration that resolves the apparent inconsistency
        (e.g., a declaration to the effect that "a Corporation is a
        (legal) Person").</p>

        <p>Moreover, depending on how the application interprets the
        property descriptions, a description of an instance might be
        considered valid either <em>without</em> some of the
        schema-specified properties (e.g., there might be an instance
        of <code>ex:Book</code> without an <code>ex:author</code> property,
        even if <code>ex:author</code> is described as having a domain of
        <code>ex:Book</code>), or with <em>additional</em> properties (there
        might be an instance of <code>ex:Book</code> with an
        <code>ex:technicalEditor</code> property, even though the schema
        describing class <code>ex:Book</code> does not describe such
        a property).</p>

        <p>In other words, statements in an RDF schema are always
        <em>descriptions</em>. They may also be <em>prescriptive</em>
        (introduce constraints), but only if the application
        interpreting those statements wants to treat them that way. All
        RDF Schema does is provide a way of stating this additional
        information. Whether this information conflicts with explicitly
        specified instance data is up to the application to determine
        and act upon.</p>
      </div>

      <div class="section">
        <h3><a id="otherschema" name="otherschema">5.4 Other Schema
        Information</a></h3>

        <p>RDF Schema provides a number of other built-in properties, which
        can be used to provide documentation and other information
        about an RDF schema or about instances. For example the
        <code>rdfs:comment</code> property can be used to provide a
        human-readable description of a resource. The
        <code>rdfs:label</code> property can be used to provide a more
        human-readable version of a resource's name. The
        <code>rdfs:seeAlso</code> property can be used to indicate a
        resource that might provide additional information about the
        subject resource. The <code>rdfs:isDefinedBy</code> property is a
        subproperty of <code>rdfs:seeAlso</code>, and can be used to
        indicate a resource that (in a sense not specified by RDF;
        e.g., the resource may not be an RDF schema) "defines" the
        subject resource. <a
        href="http://www.w3.org/TR/rdf-schema/">RDF Vocabulary
        Description Language 1.0: RDF Schema</a> <a
        href="#ref-rdf-vocabulary">[RDF-VOCABULARY]</a> 
        should be consulted for further discussion of these properties.</p>

        <p class="newstuff">As with a number of the built-in RDF properties such as 
        <code>rdf:value</code>, the uses described for these RDF Schema properties
        are only their <em>intended</em> uses.  <a
        href="#ref-rdf-semantics">[RDF-SEMANTICS]</a> defines no special
        meanings for these properties, and RDF Schema does
        not define any constraints based on these intended uses.  
        For example, there is no constraint specified that 
        the object of a <code>rdfs:seeAlso</code> property
        <em>must</em> provide additional information about the subject of the
        statement in which it appears.</p>

      </div>

      <div class="section">
        <h3><a id="richerschemas" name="richerschemas">5.5 Richer
        Schema Languages</a></h3>

        <p>RDF Schema provides basic capabilities for describing RDF
        vocabularies, but additional capabilities are also possible,
        and can be useful. These capabilities may be provided through
        further development of RDF Schema, or in other languages based
        on RDF. Other
        richer schema capabilities that have been identified as useful
        (but that are not provided by RDF Schema) include:</p>

        <ul type="disc">
          <li><em>cardinality constraints</em> on properties, e.g.,
          that a Person has <em>exactly one</em> biological
          father.</li>

          <li>specifying that a given property (such as
          <code>ex:hasAncestor</code>) is <em>transitive</em>, e.g., that if A
          <code>ex:hasAncestor</code> B, and B <code>ex:hasAncestor</code> C, then A
          <code>ex:hasAncestor</code> C.</li>

          <li>specifying that a given property is a unique identifier
          (or <em>key</em>) for instances of a particular class.</li>

          <li>specifying that two different classes (having different
          URIrefs) actually represent the same class.</li>

          <li>specifying that two different instances (having different
          URIrefs) actually represent the same individual.</li>

          <li><span class="newstuff">specifying constraints on the range or cardinality
          of a property that depend on the class of resource
          to which a property is applied, e.g., being able to
          say that for a soccer team the <code>ex:hasPlayers</code>
          property has 11 values, while for a basketball team
          the same property should have only 5 values.</span></li>

          <li>the ability to describe new classes in terms of
          combinations (e.g., unions and intersections) of other
          classes, or to say that two classes are disjoint (i.e., that
          no resource is an instance of both classes).</li>
        </ul>

        <p>The additional capabilities mentioned above, in addition to
        others, are the targets of <em>ontology</em> languages such as
        <a href="http://www.w3.org/TR/daml+oil-reference">DAML+OIL</a>
        <a href="#ref-damloil">[DAML+OIL]</a> and <a
        href="http://www.w3.org/TR/owl-ref/">OWL</a> <a
        href="#ref-owl">[OWL]</a>. Both these languages are based on
        RDF and RDF Schema (and both currently provide all the
        additional capabilities mentioned above). The intent of such
        languages is to provide additional machine-processable
        <em>semantics</em> for resources, that is, to make the machine
        representations of resources more closely resemble their
        intended real world counterparts. While such capabilities are
        not necessarily needed to build useful applications using RDF
        (see <a href="#applications">Section 6</a> for a description of
        a number of existing RDF applications), the development of such
        languages is a very active subject of work as part of the
        development of the <a
        href="http://www.w3.org/2001/sw/Activity">Semantic Web</a>.</p>
      </div>
    </div>

    <div class="section">
      <h2><a id="applications" name="applications"></a>6. Some RDF
      Applications: RDF in the Field</h2>

      <p>The previous sections have described the general
      capabilities of RDF and RDF Schema. While examples were used
      in those sections to illustrate those capabilities, and some
      of those examples may have suggested potential RDF applications,
      those sections did not actually discuss any <em>real</em> applications. 
      This
      section will describe some actual deployed RDF applications,
      showing how RDF supports various real-world requirements to
      represent and manipulate information about a wide variety of
      things.</p>

      <div class="section">
        <h3 id="dc"><a id="dublincore" name="dublincore">6.1 Dublin
        Core Metadata Initiative</a></h3>

        <p><em>Metadata</em> is <em>data about data</em>. Specifically,
        the term refers to data used to identify, describe, or locate
        information resources, whether these resources are physical or
        electronic. While structured metadata processed by computers is
        relatively new, the basic concept of metadata has been used for
        many years in helping manage and use large collections of
        information. Library card catalogs are a familiar example of
        such metadata.</p>

        <p>The Dublin Core is a set of "elements" (properties) for
        describing documents (and hence, for recording metadata). The
        element set was originally developed at the March 1995 Metadata
        Workshop in Dublin, Ohio. The Dublin Core has subsequently been
        modified on the basis of later Dublin Core Metadata workshops,
        and is currently maintained by the <a
        href="http://dublincore.org/">Dublin Core Metadata
        Initiative</a>. The goal of the Dublin Core is to provide a
        minimal set of descriptive elements that facilitate the
        description and the automated indexing of document-like
        networked objects, in a manner similar to a library card
        catalog. The Dublin Core metadata set is intended to be
        suitable for use by resource discovery tools on the Internet,
        such as the "Webcrawlers" employed by popular World Wide Web
        search engines. In addition, the Dublin Core is meant to be
        sufficiently simple to be understood and used by the wide range
        of authors and casual publishers who contribute information to
        the Internet. Dublin Core elements have become widely used in
        documenting Internet resources (the Dublin
        Core <code>creator</code> element has already been used in earlier examples). 
        The current
        elements of the Dublin Core are defined in the <a
        href="http://dublincore.org/documents/2003/06/02/dces/">Dublin Core
        Metadata Element Set, Version 1.1: Reference Description</a> <a
        href="#ref-dublin-core">[DC]</a>, and contain definitions for
        the following properties:</p>

        <ul type="disc">
          <li><strong>Title</strong>: A name given to the
          resource.</li>

          <li><strong>Creator</strong>: An entity primarily responsible
          for making the content of the resource.</li>

          <li><strong>Subject</strong>: The topic of the content of the
          resource.</li>

          <li><strong>Description</strong>: An account of the content
          of the resource.</li>

          <li><strong>Publisher</strong>: An entity responsible for
          making the resource available</li>

          <li><strong>Contributor</strong>: An entity responsible for
          making contributions to the content of the resource.</li>

          <li><strong>Date</strong>: A date associated with an event in
          the life cycle of the resource.</li>

          <li><strong>Type</strong>: The nature or genre of the content
          of the resource.</li>

          <li><strong>Format</strong>: The physical or digital
          manifestation of the resource.</li>

          <li><strong>Identifier</strong>: An unambiguous reference to
          the resource within a given context.</li>

          <li><strong>Source</strong>: A reference to a resource from
          which the present resource is derived.</li>

          <li><strong>Language</strong>: A language of the intellectual
          content of the resource.</li>

          <li><strong>Relation</strong>: A reference to a related
          resource.</li>

          <li><strong>Coverage</strong>: The extent or scope of the
          content of the resource.</li>

          <li><strong>Rights</strong>: Information about rights held in
          and over the resource.</li>
        </ul>

        <p>Information using the Dublin Core elements may be
        represented in any suitable language (e.g., in HTML <code>meta</code>
        elements). However, RDF is an ideal representation for Dublin
        Core information. The examples below represent the simple
        description of a set of resources in RDF using the Dublin Core
        vocabulary. Note that the specific Dublin Core RDF vocabulary
        shown here is not intended to be authoritative. The Dublin Core
        Reference Description <a href="#ref-dublin-core">[DC]</a> is
        the authoritative reference.</p>

        <p>The first example, <a href="#example30">Example 30</a>,
        describes a Web site home page using Dublin Core
        properties:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example30" name="example30">Example 30: A Web Page
            Described using Dublin Core Properties</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdf:RDF
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:dc="http://purl.org/dc/elements/1.1/"&gt;
    &lt;rdf:Description rdf:about="http://www.dlib.org"&gt;
      &lt;dc:title&gt;D-Lib Program - Research in Digital Libraries&lt;/dc:title&gt;
      &lt;dc:description&gt;The D-Lib program supports the community of people
       with research interests in digital libraries and electronic
       publishing.&lt;/dc:description&gt;
      &lt;dc:publisher&gt;Corporation For National Research Initiatives&lt;/dc:publisher&gt;
      &lt;dc:date&gt;1995-01-07&lt;/dc:date&gt;
      &lt;dc:subject&gt;
        &lt;rdf:Bag&gt;
          &lt;rdf:li&gt;Research; statistical methods&lt;/rdf:li&gt;
          &lt;rdf:li&gt;Education, research, related topics&lt;/rdf:li&gt;
          &lt;rdf:li&gt;Library use Studies&lt;/rdf:li&gt;
        &lt;/rdf:Bag&gt;
      &lt;/dc:subject&gt;
      &lt;dc:type&gt;World Wide Web Home Page&lt;/dc:type&gt;
      &lt;dc:format&gt;text/html&lt;/dc:format&gt;
      &lt;dc:language&gt;en&lt;/dc:language&gt;
    &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>Note that both RDF and the Dublin Core define an (XML)
        element called "Description" (although the Dublin Core element
        name is written in lowercase). Even if the initial letter were
        identically uppercase, the XML namespace mechanism enables 
        these two elements to be distinguished (one is
        <code>rdf:Description</code>, and the other is
        <code>dc:description</code>). Also, as a matter of interest, 
        accessing <a
        href="http://purl.org/dc/elements/1.1/">http://purl.org/dc/elements/1.1/</a>
        (the namespace URI used to identify the Dublin Core vocabulary in this
        example)
        in a Web browser (as of the current writing) will retrieve an
        RDF Schema declaration for <a
        href="#ref-dublin-core">[DC]</a>.</p>

        <p>The second example, <a href="#example31">Example 31</a>,
        describes a published magazine:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example31" name="example31">Example 31: Describing A
            Magazine Using Dublin Core</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdf:RDF
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:dc="http://purl.org/dc/elements/1.1/"
    xmlns:dcterms="http://purl.org/dc/terms/"&gt;
    &lt;rdf:Description rdf:about="http://www.dlib.org/dlib/may98/05contents.html"&gt;
      &lt;dc:title&gt;DLIB Magazine - The Magazine for Digital Library Research
        - May 1998&lt;/dc:title&gt;
      &lt;dc:description&gt;D-LIB magazine is a monthly compilation of
       contributed stories, commentary, and briefings.&lt;/dc:description&gt;
      &lt;dc:contributor&gt;Amy Friedlander&lt;/dc:contributor&gt;
      &lt;dc:publisher&gt;Corporation for National Research Initiatives&lt;/dc:publisher&gt;
      &lt;dc:date&gt;1998-01-05&lt;/dc:date&gt;
      &lt;dc:type&gt;electronic journal&lt;/dc:type&gt;
      &lt;dc:subject&gt;
        &lt;rdf:Bag&gt;
          &lt;rdf:li&gt;library use studies&lt;/rdf:li&gt;
          &lt;rdf:li&gt;magazines and newspapers&lt;/rdf:li&gt;
        &lt;/rdf:Bag&gt;
      &lt;/dc:subject&gt;
      &lt;dc:format&gt;text/html&lt;/dc:format&gt;
      &lt;dc:identifier rdf:resource="urn:issn:1082-9873"/&gt;
      &lt;dcterms:isPartOf rdf:resource="http://www.dlib.org"/&gt;
    &lt;/rdf:Description&gt;
 &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p><a href="#example31">Example 31</a> uses (in the
        third line from the bottom) the Dublin Core <em>qualifier</em>
        <code>isPartOf</code> (from a separate <span class="newstuff">vocabulary</span>) to indicate that
        this magazine is "part of" the previously-described Web
        site.</p>

        <p>The third example, <a href="#example32">Example 32</a>,
        describes a specific article in the magazine described in 
        <a href="#example31">Example 31</a>.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example32" name="example32">Example 32: Describing a
            Magazine Article</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdf:RDF
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:dc="http://purl.org/dc/elements/1.1/"
    xmlns:dcterms="http://purl.org/dc/terms/"&gt;
    &lt;rdf:Description rdf:about="http://www.dlib.org/dlib/may98/miller/05miller.html"&gt;
      &lt;dc:title&gt;An Introduction to the Resource Description Framework&lt;/dc:title&gt;
      &lt;dc:creator&gt;Eric J. Miller&lt;/dc:creator&gt;
      &lt;dc:description&gt;The Resource Description Framework (RDF) is an
       infrastructure that enables the encoding, exchange and reuse of
       structured metadata. rdf is an application of xml that imposes needed
       structural constraints to provide unambiguous methods of expressing
       semantics. rdf additionally provides a means for publishing both
       human-readable and machine-processable vocabularies designed to
       encourage the reuse and extension of metadata semantics among
       disparate information communities. the structural constraints rdf
       imposes to support the consistent encoding and exchange of
       standardized metadata provides for the interchangeability of separate
       packages of metadata defined by different resource description
       communities. &lt;/dc:description&gt;
      &lt;dc:publisher&gt;Corporation for National Research Initiatives&lt;/dc:publisher&gt;
      &lt;dc:subject&gt;
        &lt;rdf:Bag&gt;
          &lt;rdf:li&gt;machine-readable catalog record formats&lt;/rdf:li&gt;
          &lt;rdf:li&gt;applications of computer file organization and
           access methods&lt;/rdf:li&gt;
        &lt;/rdf:Bag&gt;
      &lt;/dc:subject&gt;
      &lt;dc:rights&gt;Copyright &copy; 1998 Eric Miller&lt;/dc:rights&gt;
      &lt;dc:type&gt;Electronic Document&lt;/dc:type&gt;
      &lt;dc:format&gt;text/html&lt;/dc:format&gt;
      &lt;dc:language&gt;en&lt;/dc:language&gt;
      &lt;dcterms:isPartOf rdf:resource="http://www.dlib.org/dlib/may98/05contents.html"/&gt;
    &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p><a href="#example32">Example 32</a> also uses the
        qualifier <code>isPartOf</code>, this time to indicate that this
        article is "part of" the previously-described magazine.</p>

<div class="newstuff">
        <p>Computer languages and file formats do
        not always make explicit provision for 
        <a name="LCC-029" id="LCC-029">embedding metadata</a> with the
        data it describes.  In many cases, the metadata has to be specified 
        as a separate resource and explicitly linked to the data 
        (this has been done for the RDF metadata that describes the Primer;
        there is an explicit link to this metadata at the end of the Primer).  
        However, applications and languages are increasingly making explicit 
        provision for embedding metadata directly with the data.  For example, the
        W3C's Scalable Vector Graphics language 
        <a href="#ref-svg">[SVG]</a> (another XML-based language) 
        provides an explicit <code>metadata</code> element for recording metadata
        along with other SVG data.  
        Any XML-based metadata language can be used inside this element.  
        <a href="#ref-svg">[SVG]</a> includes the example 
        shown in <a href="#example33">Example 33</a> of 
        how to embed metadata describing an SVG document in the SVG document itself.
        The example uses the Dublin Core vocabulary, and RDF/XML for recording
        the metadata.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example33" name="example33">Example 33: Including Metadata
            in an SVG Document</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;svg width="4in" height="3in" version="1.1"
    xmlns = 'http://www.w3.org/2000/svg'&gt;
    &lt;desc xmlns:myfoo="http://example.org/myfoo"&gt;
      &lt;myfoo:title&gt;This is a financial report&lt;/myfoo:title&gt;
      &lt;myfoo:descr&gt;The global description uses markup from the
        &lt;myfoo:emph&gt;myfoo&lt;/myfoo:emph&gt; namespace.&lt;/myfoo:descr&gt;
      &lt;myfoo:scene&gt;&lt;myfoo:what&gt;widget $growth&lt;/myfoo:what&gt;
      &lt;myfoo:contains&gt;$three $graph-bar&lt;/myfoo:contains&gt;
        &lt;myfoo:when&gt;1998 $through 2000&lt;/myfoo:when&gt; &lt;/myfoo:scene&gt;
   &lt;/desc&gt;
    &lt;metadata&gt;
      &lt;rdf:RDF
           xmlns:rdf = "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
           xmlns:rdfs = "http://www.w3.org/2000/01/rdf-schema#"
           xmlns:dc = "http://purl.org/dc/elements/1.1/" &gt;
        &lt;rdf:Description rdf:about="http://example.org/myfoo"
             dc:title="MyFoo Financial Report"
             dc:description="$three $bar $thousands $dollars $from 1998 $through 2000"
             dc:publisher="Example Organization"
             dc:date="2000-04-11"
             dc:format="image/svg+xml"
             dc:language="en" &gt;
          &lt;dc:creator&gt;
            &lt;rdf:Bag&gt;
              &lt;rdf:li&gt;Irving Bird&lt;/rdf:li&gt;
              &lt;rdf:li&gt;Mary Lambert&lt;/rdf:li&gt;
            &lt;/rdf:Bag&gt;
          &lt;/dc:creator&gt;
        &lt;/rdf:Description&gt;
      &lt;/rdf:RDF&gt;
    &lt;/metadata&gt;
&lt;/svg&gt;
</pre>
          </div>
        </div>
        
        <p>Adobe's <a href="http://www.adobe.com/products/xmp/main.html">
        Extensible Metadata Platform (XMP)</a> 
        is another example of technology that allows metadata
        about a file to be embedded into the file itself. 
        XMP uses RDF/XML as the basis of its metadata representation.  
        A number of Adobe products already support XMP.</p>
</div>

      </div>

      <div class="section">
        <h3><a id="prism" name="prism">6.2 PRISM</a></h3>

        <p><a
        href="http://www.prismstandard.org/">PRISM:
        Publishing Requirements for Industry Standard Metadata</a> <a
        href="#ref-prism">[PRISM]</a> is a metadata specification
        developed in the publishing industry. Magazine publishers and
        their vendors formed the PRISM Working Group to
        identify the industry's needs for metadata and define a
        specification to meet them. Publishers want to use existing
        content in many ways in order to get a greater return on the
        investment made in creating it. Converting magazine articles to
        HTML for posting on the Web is one example. Licensing it to
        aggregators like <a href="http://www.lexisnexis.com/">LexisNexis</a> 
        is another. All of these are "first
        uses" of the content; typically they all go live at the time
        the magazine hits the stands. The publishers also want their
        content to be "evergreen". It might be used in new issues, such
        as in a retrospective article. It could be used by other
        divisions in the company, such as in a book compiled from the
        magazine's photos, recipes, etc. Another use is to license it
        to outsiders, such as in a reprint of a product review, or in a
        retrospective produced by a different publisher. This overall
        goal requires a metadata approach that emphasizes
        <em>discovery</em>, <em>rights tracking</em>, and
        <em>end-to-end metadata</em>.</p>

        <p><em>Discovery:</em> Discovery is a general term for finding
        content which encompasses searching, browsing, content routing,
        and other techniques. Discussions of discovery frequently
        center on a consumer searching a public Web site. However,
        discovering content is much broader than that. The audience may
        consist of consumers, or it may consist of internal users such
        as researchers, designers, photo editors, licensing agents,
        etc. To assist discovery, PRISM provides properties to describe
        the topics, formats, genre, origin, and contexts of a resource.
        It also provides means for categorizing resources using
        multiple subject description taxonomies.</p>

        <p><em>Rights Tracking:</em> Magazines frequently contain
        material licensed from others. Photos from a stock photo agency
        are the most common type of licensed material, but articles,
        sidebars, and all other types of content may be licensed.
        Simply knowing if content was licensed for one-time use,
        requires royalty payments, or is wholly-owned by the publisher
        is a struggle. PRISM provides elements for basic tracking of
        such rights. A separate vocabulary defined in the
        PRISM specification supports description of places, times, and
        industries where content may or may not be used.</p>

        <p><em>End-to-end metadata:</em> Most published content already
        has metadata created for it. Unfortunately, when content moves
        between systems, the metadata is frequently discarded, only to
        be re-created later in the production process at considerable
        expense. PRISM aims to reduce this problem by providing a
        specification that can be used in multiple stages in the
        content production pipeline. An important feature of the PRISM
        specification is its use of other existing specifications.
        Rather than create an entirely new thing, the group decided to
        use existing specifications as much as possible, and only
        define new things where needed. For this reason, the PRISM
        specification uses XML, RDF, Dublin Core, and well as various
        ISO formats and vocabularies.</p>

        <p>A PRISM description may be as simple as a few Dublin Core
        properties with plain literal values. <a
        href="#example34">Example 34</a> describes a photograph, giving
        basic information on its title, photographer, format, etc.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example34" name="example34">Example 34: A PRISM
            Description of a Photograph</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:dc="http://purl.org/dc/elements/1.1/"
         xml:lang="en-US"&gt;

 &lt;rdf:Description rdf:about="http://travel.example.com/2000/08/Corfu.jpg"&gt;
  &lt;dc:title&gt;Walking on the Beach in Corfu&lt;/dc:title&gt;
  &lt;dc:description&gt;Photograph taken at 6:00 am on Corfu with two models
  &lt;/dc:description&gt;
  &lt;dc:creator&gt;John Peterson&lt;/dc:creator&gt;
  &lt;dc:contributor&gt;Sally Smith, lighting&lt;/dc:contributor&gt;
  &lt;dc:format&gt;image/jpeg&lt;/dc:format&gt;
 &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>PRISM also augments the Dublin Core to allow more detailed
        descriptions. The augmentations are defined <span class="newstuff">as three new
        vocabularies</span>, generally cited using the prefixes <code>prism:</code>,
        <code>pcv:</code>, and <code>prl:</code>.</p>

        <p><code>prism:</code> This prefix refers to the main PRISM
        <span class="newstuff">vocabulary, whose terms use the URI prefix
        <code>http://prismstandard.org/namespaces/basic/1.0/</code></span>. Most
        of the properties in this vocabulary are more specific versions of properties from
        the Dublin Core. For example, more specific versions of
        <code>dc:date</code> are provided by properties like
        <code>prism:publicationTime</code>, <code>prism:releaseTime</code>,
        <code>prism:expirationTime</code>, etc.</p>

        <p><code>pcv:</code> This prefix refers to the PRISM Controlled
        Vocabulary (pcv) <span class="newstuff">vocabulary, whose terms use the URI prefix
        <code>http://prismstandard.org/namespaces/pcv/1.0/</code></span>.
        Currently, common practice for describing the subject(s) of an
        article is by supplying descriptive keywords. Unfortunately,
        simple keywords do not make a great difference in retrieval
        performance, due to the fact that different people will use
        different keywords <a href="#ref-bates96">[BATES96]</a>. Best
        practice is to code the articles with subject terms from a
        "controlled vocabulary". The vocabulary should provide as many
        synonyms as possible for its terms in the vocabulary. This way
        the controlled terms provide a meeting ground for the keywords
        supplied by the searcher and the indexer. The pcv vocabulary
        provides properties for specifying
        terms in a vocabulary, the relations between terms, and
        alternate names for the terms.</p>

        <p><code>prl:</code> This prefix refers to the PRISM Rights
        Language <span class="newstuff">vocabulary, whose terms use the URI prefix
        <code>http://prismstandard.org/namespaces/prl/1.0/</code></span>. Digital
        Rights Management is an area undergoing considerable upheaval.
        There are a number of proposals for rights management
        languages, but none are clearly favored throughout the
        industry. Because there was no clear choice to recommend, the
        PRISM Rights Language (PRL) was defined as an interim measure.
        It provides properties which let people say if an item can or
        cannot be "used", depending on conditions of time, geography,
        and industry. This is believed to be an 80/20 trade-off which
        will help publishers begin to save money when tracking rights.
        It is not intended to be a general rights language, or allow
        publishers to automatically enforce limits on consumer uses of
        the content.</p>

        <p>PRISM uses RDF because of its abilities for dealing with
        descriptions of varying complexity. Currently, a great deal of
        metadata uses simple character string (plain literal) values,
        such as:</p>

        <div class="exampleOuter exampleInner">
<pre>
&lt;dc:coverage&gt;Greece&lt;/dc:coverage&gt;
</pre>
        </div>

        <p>Over time the developers of PRISM expect uses of the PRISM
        specification to become more sophisticated, moving from simple
        literal values to more structured values. In fact, that range
        of values is a situation being faced now. Some publishers
        already use sophisticated controlled vocabularies, others are
        barely using manually-supplied keywords. To illustrate this,
        some examples of the different kinds of values that can be
        given for the <code>dc:coverage</code> property are:</p>

        <div class="exampleOuter exampleInner">
<pre>
&lt;dc:coverage&gt;Greece&lt;/dc:coverage&gt;

&lt;dc:coverage rdf:resource="http://prismstandard.org/vocabs/ISO-3166/GR"/&gt;
</pre>
        </div>

        <p>(i.e., using either a plain literal or a URIref to identify
        the country) and</p>

        <div class="exampleOuter exampleInner">
<pre>
&lt;dc:coverage&gt;
  &lt;pcv:Descriptor rdf:about="http://prismstandard.org/vocabs/ISO-3166/GR"&gt;
    &lt;pcv:label xml:lang="en"&gt;Greece&lt;/pcv:label&gt;
    &lt;pcv:label xml:lang="fr"&gt;Gr&egrave;ce&lt;/pcv:label&gt;
  &lt;/pcv:Descriptor&gt;
&lt;/dc:coverage&gt;
</pre>
        </div>

        <p>(using a structured value to provide both a URIref and names
        in various languages).</p>

        <p>Note also that there are properties whose meanings are
        similar, or subsets of other properties. For example, the
        geographic subject of a resource could be given with</p>

        <div class="exampleOuter exampleInner">
<pre>
&lt;prism:subject&gt;Greece&lt;/prism:subject&gt;
&lt;dc:coverage&gt;Greece&lt;/dc:coverage&gt;
</pre>
        </div>

        <p>or</p>

        <div class="exampleOuter exampleInner">
<pre>
&lt;prism:location&gt;Greece&lt;/prism:location&gt;
</pre>
        </div>

        <p>Any of those properties might use the simple literal value,
        or a more complex structured value. Such a range of
        possibilities cannot be adequately described by DTDs, or even
        by the newer XML Schemas. While there is a wide range of
        syntactic variations to deal with, RDF's graph model has a
        simple structure - a set of triples. Dealing with the metadata
        in the triples domain makes it much easier for older software
        to accommodate content with new extensions.</p>

        <p>This section closes with two final examples. <a
        href="#example35">Example 35</a> says that the image
        (<code>.../Corfu.jpg</code>) cannot be used (<code>#none</code>) in the
        tobacco industry (code 21 in SIC, the Standard Industrial
        Classifications).</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example35" name="example35">Example 35: A PRISM
            Description of an Image</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdf:RDF xmlns:prism="http://prismstandard.org/namespaces/basic/1.0/"
         xmlns:prl="http://prismstandard.org/namespaces/prl/1.0/"
         xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:dc="http://purl.org/dc/elements/1.1/"&gt;

 &lt;rdf:Description rdf:about="http://travel.example.com/2000/08/Corfu.jpg"&gt;
  &lt;dc:rights rdf:parseType="Resource"
         xml:base="http://prismstandard.org/vocabularies/1.0/usage.xml"&gt;
     &lt;prl:usage rdf:resource="#none"/&gt;
     &lt;prl:industry rdf:resource="http://prismstandard.org/vocabs/SIC/21"/&gt;
  &lt;/dc:rights&gt;
 &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p><a href="#example36">Example 36</a> says that the
        photographer for the Corfu image was employee 3845, better
        known as John Peterson. It also says that the geographic
        coverage of the photo is Greece. It does so by providing, not
        just a code from a controlled vocabulary, but a cached version
        of the information for that term in the vocabulary.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example36" name="example36">Example 36: Additional
            Information about the Image from Example 35</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:pcv="http://prismstandard.org/namespaces/pcv/1.0/"
         xmlns:dc="http://purl.org/dc/elements/1.1/"
         xml:base="http://travel.example.com/"&gt;

  &lt;rdf:Description rdf:about="/2000/08/Corfu.jpg"&gt;
    &lt;dc:identifier rdf:resource="/content/2357845" /&gt;
    &lt;dc:creator&gt;
      &lt;pcv:Descriptor rdf:about="/emp3845"&gt;
        &lt;pcv:label&gt;John Peterson&lt;/pcv:label&gt;
      &lt;/pcv:Descriptor&gt;
    &lt;/dc:creator&gt;
    &lt;dc:coverage&gt;
      &lt;pcv:Descriptor
          rdf:about="http://prismstandard.org/vocabs/ISO-3166/GR"&gt;
        &lt;pcv:label xml:lang="en"&gt;Greece&lt;/pcv:label&gt;
        &lt;pcv:label xml:lang="fr"&gt;Grece&lt;/pcv:label&gt;
      &lt;/pcv:Descriptor&gt;
    &lt;/dc:coverage&gt;
  &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>
      </div>

      <div class="section">
        <h3><a id="xpackage" name="xpackage">6.3 XPackage</a></h3>

<div class="newstuff">
        <p>Many situations involve the need to maintain information
        about structured groupings of resources and their associations
        that are, or may be, used as a unit. The <a
        href="http://www.xpackage.org/specification/">XML Package
        (XPackage) specification</a> <a
        href="#ref-xpackage">[XPACKAGE]</a> provides a framework for
        defining such groupings, called <em>packages</em>. XPackage
        specifies a framework for describing the resources included in
        such packages, the properties of those resources, their method
        of inclusion, and their relationships with each other. XPackage
        applications include specifying the style sheets used by a
        document, declaring the images shared by multiple documents,
        indicating the author and other metadata of a document,
        describing how namespaces are used by XML resources, and
        providing a manifest for bundling resources into a single
        archive file.</p>

        <p>The XPackage framework is based upon XML, RDF, and the <a
        href="http://www.w3.org/TR/xlink/">XML Linking Language</a> <a
        href="#ref-xlink">[XLINK]</a>, and provides multiple RDF
        vocabularies: one for general packaging descriptions, and
        several other vocabularies for providing supplemental
        resource information useful to package processors.</p>

        <p>One application of XPackage is the description of XHTML
        documents and their supporting resources. An XHTML document
        retrieved from a Web site may rely on other resources such as
        style sheets and image files that also need to be retrieved.
        However, the identities of these supporting resources may not
        be obvious without processing the entire document. Other
        information about the document, such as the name of its author,
        may also not be available without processing the document.
        XPackage allows such descriptive information to be stored in a
        standard way in a package description document containing RDF.
        The outer elements of a package description document describing
        such an XHTML document might look like <a
        href="#example37">Example 37</a> (with namespace declarations
        removed for simplicity):</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example37" name="example37">Example 37: Outer
            Elements of an XPackage Package Description Document</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt;
&lt;xpackage:description&gt;
  &lt;rdf:RDF&gt;

    (description of individual resources go here)

  &lt;/rdf:RDF&gt;
&lt;/xpackage:description&gt;
</pre>
          </div>
        </div>

        <p>Resources (such as the XHTML document, style sheets, and
        images) are described within this package description document
        using standard RDF/XML syntax.

        Each resource description element may include
        RDF properties from various vocabularies (XPackage uses the
        term "ontology" for what RDF calls a "vocabulary"). 
        Besides the main packaging vocabulary, XPackage itself 
        specifies several supplemental vocabularies, including:</p>

        <ul>

        <li>a vocabulary (using prefix <code>file:</code>) for describing files
        (with properties such as <code>file:size</code>)</li> 

        <li>a vocabulary (using prefix <code>mime:</code>) for providing MIME information 
        (with properties such as <code>mime:contentType</code>)</li> 

        <li>a vocabulary (using prefix <code>unicode:</code>) for providing character usage information
        (with properties such as <code>unicode:script</code>)</li> 

        <li>a vocabulary (using prefix <code>x:</code>) for describing XML-based resources
        (with properties such as <code>x:namespace</code> and <code>x:style</code>)</li>
  
        </ul>

        <p>In <a
        href="#example38">Example 38</a>, the document's MIME content
        type ("application/xhtml+xml") is defined using a standard
        XPackage property from the XPackage MIME vocabulary,
        <code>mime:contentType</code>. Another property, the document's
        author (in this case, "Garret Wilson"), is described using a
        property from the Dublin Core vocabulary, defined outside
        of XPackage, resulting in a <code>dc:creator</code>
        property. </p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example38" name="example38">Example 38: A
            Description of an XHTML Document</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt; 
&lt;xpackage:description 
           xmlns:xpackage="http://xpackage.org/namespaces/2003/xpackage#"
           xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
           xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
           xmlns:dc="http://purl.org/dc/elements/1.1/"
           xmlns:mime="http://xpackage.org/namespaces/2003/mime#" 
           xmlns:x="http://xpackage.org/namespaces/2003/xml#"
           xmlns:xlink="http://www.w3.org/1999/xlink"&gt;  
    &lt;rdf:RDF&gt;

    &lt;!--doc.html--&gt;
    &lt;rdf:Description rdf:about="urn:example:xhtmldocument-doc"&gt;
      &lt;rdfs:comment&gt;The XHTML document.&lt;/rdfs:comment&gt;
      &lt;xpackage:location xlink:href="doc.html"/&gt;
      &lt;mime:contentType&gt;application/xhtml+xml&lt;/mime:contentType&gt;
      &lt;x:namespace rdf:resource="http://www.w3.org/1999/xhtml"/&gt;
      &lt;x:style rdf:resource="urn:example:xhtmldocument-stylesheet"/&gt;
      &lt;dc:creator&gt;Garret Wilson&lt;/dc:creator&gt;
      &lt;xpackage:manifest rdf:parseType="Collection"&gt;
         &lt;rdf:Description rdf:about="urn:example:xhtmldocument-stylesheet"/&gt;
         &lt;rdf:Description rdf:about="urn:example:xhtmldocument-image"/&gt;
      &lt;/xpackage:manifest&gt;
    &lt;/rdf:Description&gt;

    &lt;/rdf:RDF&gt; 
&lt;/xpackage:description&gt;
</pre>
          </div>
        </div>

        <p>The <code>xpackage:manifest</code> property indicates that both
        the style sheet and image resources are necessary for
        processing; those resources are described separately within the
        package description document. The example style sheet resource
        description in <a href="#example39">Example 39</a> lists its
        location within the package ("stylesheet.css") using the 
        general XPackage vocabulary
        <code>xpackage:location</code> property (which is compatible with
        XLink), and shows through use of the XPackage MIME vocabulary
        <code>mime:contentType</code> property that it is a CSS
        style sheet ("text/css").</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example39" name="example39">Example 39: A Style Sheet
            Resource Description</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt; 
&lt;xpackage:description 
           xmlns:xpackage="http://xpackage.org/namespaces/2003/xpackage#"
           xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
           xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
           xmlns:dc="http://purl.org/dc/elements/1.1/"
           xmlns:mime="http://xpackage.org/namespaces/2003/mime#" 
           xmlns:x="http://xpackage.org/namespaces/2003/xml#"
           xmlns:xlink="http://www.w3.org/1999/xlink"&gt;  
    &lt;rdf:RDF&gt; 

    &lt;!--stylesheet.css--&gt;
    &lt;rdf:Description rdf:about="urn:example:xhtmldocument-css"&gt;
      &lt;rdfs:comment&gt;The document style sheet.&lt;/rdfs:comment&gt;
      &lt;xpackage:location xlink:href="stylesheet.css"/&gt;
      &lt;mime:contentType&gt;text/css&lt;/mime:contentType&gt;
    &lt;/rdf:Description&gt;

    &lt;/rdf:RDF&gt; 
&lt;/xpackage:description&gt;
</pre>
          </div>
        </div>

        <p>The full version of this example may be found in <a
        href="#ref-xpackage">[XPACKAGE]</a>.</p>
</div>
      </div>

      <div class="section">
        <h3><a id="rss" name="rss">6.4 RSS 1.0: RDF Site
        Summary</a></h3>

        <p>People sometimes need to access a wide variety of information on
        the Web on a day-to-day basis, such as schedules, to-do lists, news
        headlines, search results, "What's New", etc.  As the sources 
        and diversity of the information on the Web increases, it becomes
        increasingly difficult to manage this information and integrate
        it into a coherent whole. <a href="http://purl.org/rss/1.0">RSS
        1.0</a> ("RDF Site Summary") is an RDF vocabulary that provides
        a lightweight, yet powerful way of describing information for
        timely, large-scale distribution and reuse.  RSS 1.0 is also perhaps 
        the most widely deployed RDF application on the Web.</p>

        <p>To give a simple example, the <a
        href="http://www.w3.org/">W3C home page</a> is a primary point of contact
        with the public and serves in part to disseminate information
        about the deliverables of the Consortium. 
        An example of the W3C home page as of a certain date is shown
        in <a href="#figure19">Figure 19</a>.  The center column of
        news items changes frequently. To support the timely
        dissemination of this information, the W3C Team has implemented
        an RDF Site Summary (<a href="http://purl.org/rss/1.0/">RSS
        1.0</a>) news feed that makes the content in the center column
        available to others to reuse as they will. News syndication
        sites may merge the headlines into a summary of the day's
        latest news, others may display the headlines as links as a
        service to their readers, and, increasingly, individuals may
        subscribe to this feed with a desktop application. These
        desktop <em>RSS readers</em> allow their users to keep track of
        potentially hundreds of sites, without having to visit each one
        in their browser.</p>

        <div class="figure">
          <a href="w3c-rss.crop.gif"><img src="fig18dec16.gif"
          alt="The W3C Home Page" /></a><br />
          <br />
           <a id="figure19" name="figure19">Figure 19: The W3C Home
          Page</a>
        </div>

        <p>Numerous sites all over the Web provide RSS 1.0 feeds. <a
        href="#example40">Example 40</a> is an example of the <a
        href="http://www.w3.org/2000/08/w3c-synd/home.rss">W3C
        feed</a> (from a different date):</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example40" name="example40">Example 40: An Example
            of the W3C RSS 1.0 Feed</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0" encoding="utf-8"?&gt;

&lt;rdf:RDF xmlns="http://purl.org/rss/1.0/"
    xmlns:dc="http://purl.org/dc/elements/1.1/" 
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"&gt;

  &lt;channel rdf:about="http://www.w3.org/2000/08/w3c-synd/home.rss"&gt;
    &lt;title&gt;The World Wide Web Consortium&lt;/title&gt;
    &lt;description&gt;Leading the Web to its Full Potential...&lt;/description&gt;
    &lt;link&gt;http://www.w3.org/&lt;/link&gt;

    &lt;dc:date&gt;2002-10-28T08:07:21Z&lt;/dc:date&gt;

    &lt;items&gt;
        &lt;rdf:Seq&gt;
            &lt;rdf:li rdf:resource="http://www.w3.org/News/2002#item164"/&gt;
            &lt;rdf:li rdf:resource="http://www.w3.org/News/2002#item168"/&gt;
            &lt;rdf:li rdf:resource="http://www.w3.org/News/2002#item167"/&gt;
        &lt;/rdf:Seq&gt;
    &lt;/items&gt;

  &lt;/channel&gt;

  &lt;item rdf:about="http://www.w3.org/News/2002#item164"&gt;
    &lt;title&gt;User Agent Accessibility Guidelines Become a W3C 
       Proposed Recommendation&lt;/title&gt;
    &lt;description&gt;17 October 2002: W3C is pleased to announce the 
       advancement of User Agent Accessibility Guidelines 1.0 to 
       Proposed Recommendation. Comments are welcome through 14 November. 
       Written for developers of user agents, the guidelines lower 
       barriers to Web accessibility for people with disabilities 
       (visual, hearing, physical, cognitive, and neurological). 
       The companion Techniques Working Draft is updated. Read about 
       the Web Accessibility Initiative. (News archive)&lt;/description&gt;
    &lt;link&gt;http://www.w3.org/News/2002#item164&lt;/link&gt;
    &lt;dc:date&gt;2002-10-17&lt;/dc:date&gt;
  &lt;/item&gt;

  &lt;item rdf:about="http://www.w3.org/News/2002#item168"&gt;
    &lt;title&gt;Working Draft of Authoring Challenges for Device 
       Independence Published&lt;/title&gt;
    &lt;description&gt;25 October 2002: The Device Independence 
       Working Group has released the first public Working Draft of 
       Authoring Challenges for Device Independence. The draft describes 
       the considerations that Web authors face in supporting access to 
       their sites from a variety of different devices. It is written 
       for authors, language developers, device experts and developers 
       of Web applications and authoring systems. Read about the Device 
       Independence Activity (News archive)&lt;/description&gt;
    &lt;link&gt;http://www.w3.org/News/2002#item168&lt;/link&gt;
    &lt;dc:date&gt;2002-10-25&lt;/dc:date&gt;
  &lt;/item&gt;

  &lt;item rdf:about="http://www.w3.org/News/2002#item167"&gt;
    &lt;title&gt;CSS3 Last Call Working Drafts Published&lt;/title&gt;
    &lt;description&gt;24 October 2002: The CSS Working Group has 
       released two Last Call Working Drafts and welcomes comments 
       on them through 27 November. CSS3 module: text is a set of 
       text formatting properties and addresses international contexts. 
       CSS3 module: Ruby is properties for ruby, a short run of text 
       alongside base text typically used in East Asia. CSS3 module: 
       The box model for the layout of textual documents in visual 
       media is also updated. Cascading Style Sheets (CSS) is a 
       language used to render structured documents like HTML and 
       XML on screen, on paper, and in speech. Visit the CSS home 
       page. (News archive)&lt;/description&gt;
    &lt;link&gt;http://www.w3.org/News/2002#item167&lt;/link&gt;
    &lt;dc:date&gt;2002-10-24&lt;/dc:date&gt;
  &lt;/item&gt;

&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>As <a href="#example40">Example 40</a> shows, the format is
        designed for content that can be packaged into easily
        distinguishable sections. News sites, Web logs, sports scores,
        stock quotes, and the like are all use-cases for RSS 1.0.</p>

        <p>The RSS feed can be requested by any application able to
        "speak" HTTP. More recently, however, RSS 1.0 applications are
        splitting into three different categories:</p>

        <ul>
          <li>On-line aggregators - Sites such as <a
          href="http://www.oreillynet.com/meerkat/index.php?&amp;c=4743&amp;t=ALL">
          Meerkat</a> and <a
          href="http://www.newsisfree.com/sources/info/906/">NewsIsFree</a>,
          shown side-by-side in <a href="#figure20">Figure 20</a> (each
          mirroring W3C's column of news). These gather feeds from
          thousands of sources, separate each of the
          <code>&lt;item&gt;</code>s out, and add them together again into
          one large group. The whole group is then made searchable. In
          this way, one can search for the latest news on, for example,
          "Java" from perhaps thousands of sites, without having to
          search them all.</li>

          <li>Desktop Readers - Utilities such as <a
          href="http://www.disobey.com/amphetadesk/">Amphetadesk</a>
          and <a
          href="http://ranchero.com/netnewswire/">NetNewsWire
          Lite</a> allow their users to subscribe to hundreds of feeds
          from their desktop. Readers customarily refresh each feed
          once an hour, allowing users to stay up to date.</li>

          <li>Scripts - RSS's original purpose was to allow Webmasters
          to include the content of another's site within their own.
          RSS 1.0 is still used in this way, with many sites (<a
          href="http://slashdot.org">Slashdot</a> for example)
          incorporating RSS feeds on their front page.</li>
        </ul>

        <div class="figure">
          <a href="meerkat-rss.crop.gif"><img src="fig19Adec16.gif"
          alt="Meerkat RSS" /></a> <a
          href="newsisfree-rss.crop.gif"><img src="fig19Bdec16.gif"
          alt="NewsIsFree RSS" /></a><br />
          <br />
           <a id="figure20" name="figure20">Figure 20: MeerKat and
          NewsIsFree</a>
        </div>

        <p>RSS 1.0 is extensible by design. By importing additional RDF
        vocabularies (or <em>modules</em> as they are known within the
        RSS development community), the RSS 1.0 author can provide
        large amounts of metadata and handling instructions to the
        recipient of the file. Modules can, as with more general RDF
        vocabularies, be written by anyone. Currently there are <a
        href="http://web.resource.org/rss/1.0/">3 official modules</a>
        and <a
        href="http://web.resource.org/rss/1.0/modules/proposed.html">19
        proposed modules</a> readily recognized by the community at
        large. These modules range from the complete <a
        href="http://web.resource.org/rss/1.0/modules/dc/">Dublin Core
        module</a> to more specialized RSS-centric modules such as the
        <a
        href="http://web.resource.org/rss/1.0/modules/aggregation/">Aggregation
        module</a>.</p>

        <p>Care should be taken when discussing "RSS" in the scope of
        RDF. There are currently two RSS specification strands. One
        strand (RSS 0.91,0.92,0.93,0.94 and 2.0) does not use RDF. The
        other strand (RSS 0.9 and 1.0) does.</p>
      </div>

      <div class="section">
        <h3><a id="cimxml" name="cimxml"></a>6.5 CIM/XML</h3>

        <p>Electric utilities use power system models for a number of
        different purposes. For example, simulations of power systems
        are necessary for planning and security analysis. Power system
        models are also used in actual operations, e.g., by the Energy
        Management Systems (EMS) used in energy control centers. An
        operational power system model can consist of thousands of
        classes of information. In addition to using these models
        in-house, utilities need to exchange system modeling
        information, both in planning, and for operational purposes,
        e.g., for coordinating transmission and ensuring reliable
        operations. However, individual utilities use different
        software for these purposes, and as a result the system models
        are stored in different formats, making the exchange of these
        models difficult.</p>

        <p>In order to support the exchange of power system models,
        utilities needed to agree on common definitions of power system
        entities and relationships. To support this, the <a
        href="http://www.epri.com/">Electric Power Research
        Institute</a> (EPRI) a non-profit energy research consortium,
        developed a Common
        Information Model (CIM) <a href="#ref-cim">[CIM]</a>. 
        The CIM specifies common semantics
        for power system resources, their attributes, and
        relationships. In addition, to further support the ability to
        electronically exchange CIM models, the power industry has
        developed <a
        href="http://www.langdale.com.au/CIMXML/">CIM/XML</a>, a
        language for expressing CIM models in XML. CIM/XML is an RDF
        application, using RDF and RDF Schema to organize its XML
        structures. The <a href="http://www.nerc.com/">North American
        Electric Reliability Council</a> (NERC) (an industry-supported
        organization formed to promote the reliability of electricity
        delivery in North America) has adopted CIM/XML as the standard
        for exchanging models between power transmission system
        operators. The CIM/XML format is also going through an IEC
        international standardization process. An excellent discussion
        of CIM/XML can be found in <a href="#ref-devos">[DWZ01]</a>.
        [NB: This power industry CIM should not be confused with the
        CIM developed by the <a href="http://www.dmtf.org/">Distributed 
        Management Task Force</a> for
        representing management information for distributed software,
        network, and enterprise environments. The DMTF CIM also has an
        XML representation, but does not currently use RDF, although
        independent research is underway in that direction.]</p>

        <p>The CIM can represent all of the major objects of an
        electric utility as object classes and attributes, as well as
        their relationships. CIM uses these object classes and
        attributes to support the integration of independently
        developed applications between vendor specific EMS systems, or
        between an EMS system and other systems that are concerned with
        different aspects of power system operations, such as
        generation or distribution management.</p>

        <p>The CIM is specified as a set of class diagrams using the
        <a href="http://www.uml.org/">Unified Modeling Language</a>
        (UML). The base class of the CIM is
        the <code>PowerSystemResource</code> class, with other more
        specialized classes such as <code>Substation</code>,
        <code>Switch</code>, and <code>Breaker</code> being defined as
        subclasses. CIM/XML represents the CIM as an RDF Schema
        vocabulary, and uses RDF/XML as the language for exchanging
        specific system models. <a href="#example41">Example 41</a>
        shows examples of CIM/XML class and property definitions:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example41" name="example41">Example 41: Examples of
            CIM/XML Class and Property Definitions</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdfs:Class rdf:ID="PowerSystemResource"&gt; 
  &lt;rdfs:label xml:lang="en"&gt;PowerSystemResource&lt;/rdfs:label&gt; 
  &lt;rdfs:comment&gt;"A power system component that can be either an
    individual element such as a switch or a set of elements 
    such as a substation. PowerSystemResources that are sets 
    could be members of other sets. For example a Switch is a 
    member of a Substation and a Substation could be a member 
    of a division of a Company"&lt;/rdfs:comment&gt; 
&lt;/rdfs:Class&gt;

&lt;rdfs:Class rdf:ID="Breaker"&gt; 
  &lt;rdfs:label xml:lang="en"&gt;Breaker&lt;/rdfs:label&gt; 
  &lt;rdfs:subClassOf rdf:resource="#Switch" /&gt; 
  &lt;rdfs:comment&gt;"A mechanical switching device capable of making, 
     carrying, and breaking currents under normal circuit conditions 
     and also making, carrying for a specified time, and breaking 
     currents under specified abnormal circuit conditions e.g. those 
     of short circuit. The typeName is the type of breaker, e.g., 
     oil, air blast, vacuum, SF6."&lt;/rdfs:comment&gt; 
&lt;/rdfs:Class&gt;

&lt;rdf:Property rdf:ID="Breaker.ampRating"&gt; 
   &lt;rdfs:label xml:lang="en"&gt;ampRating&lt;/rdfs:label&gt; 
   &lt;rdfs:domain rdf:resource="#Breaker" /&gt; 
   &lt;rdfs:range rdf:resource="#CurrentFlow" /&gt; 
   &lt;rdfs:comment&gt;"Fault interrupting rating in amperes"&lt;/rdfs:comment&gt; 
&lt;/rdf:Property&gt;
</pre>
          </div>
        </div>

        <p>CIM/XML uses only a subset of the complete RDF/XML syntax,
        in order to simplify expressing the models. In addition,
        CIM/XML implements some extensions to the RDF Schema vocabulary.
        These extensions support the description of inverse
        roles and multiplicity (cardinality) constraints describing how
        many instances of a given property are allowed for a given
        resource (allowable values for a
        multiplicity declaration are zero-or-one, exactly-one,
        zero-or-more, one-or-more). The properties in <a
        href="#example42">Example 42</a> illustrate these
        extensions (which are identified by a <code>cims:</code> QName prefix):</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example42" name="example42">Example 42: Some CIM/XML
            Extensions of RDF Schema</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdf:Property rdf:ID="Breaker.OperatedBy"&gt; 
   &lt;rdfs:label xml:lang="en"&gt;OperatedBy&lt;/rdfs:label&gt; 
   &lt;rdfs:domain rdf:resource="#Breaker" /&gt; 
   &lt;rdfs:range rdf:resource="#ProtectionEquipment" /&gt; 
   &lt;cims:inverseRoleName rdf:resource="#ProtectionEquipment.Operates" /&gt; 
   &lt;cims:multiplicity rdf:resource="http://www.cim-logic.com/schema/990530#M:0..n" /&gt;
   &lt;rdfs:comment&gt;"Circuit breakers may be operated by 
       protection relays."&lt;/rdfs:comment&gt;
&lt;/rdf:Property&gt;

&lt;rdf:Property rdf:ID="ProtectionEquipment.Operates"&gt; 
   &lt;rdfs:label xml:lang="en"&gt;Operates&lt;/rdfs:label&gt; 
   &lt;rdfs:domain rdf:resource="#ProtectionEquipment" /&gt; 
   &lt;rdfs:range rdf:resource="#Breaker" /&gt; 
   &lt;cims:inverseRoleName rdf:resource="#Breaker.OperatedBy" /&gt; 
   &lt;cims:multiplicity rdf:resource="http://www.cim-logic.com/schema/990530#M:0..n" /&gt;
   &lt;rdfs:comment&gt;"Circuit breakers may be operated by 
       protection relays."&lt;/rdfs:comment&gt;
&lt;/rdf:Property&gt;
</pre>
          </div>
        </div>

        <p>EPRI has conducted successful interoperability tests using
        CIM/XML to exchange real-life, large-scale models (involving,
        in the case of one test, data describing over 2000 substations)
        between a variety of vendor products, and validating that these
        models would be correctly interpreted by typical utility
        applications. Although the CIM was originally intended for EMS
        systems, it is also being extended to support power
        distribution and other applications as well.</p>

        <p>The <a href="http://www.omg.org/">Object Management
        Group</a> has adopted an object interface standard to access
        CIM power system models called the Data Access Facility <a
        href="#ref-daf">[DAF]</a>. Like the CIM/XML language, the DAF
        is based on the RDF model and shares the same CIM schema.
        However, while CIM/XML enables a model to be exchanged as a
        document, DAF enables an application to access the model as a
        set of objects.</p>

        <p>CIM/XML illustrates the useful role RDF can play in
        supporting XML-based exchange of information that is naturally
        expressed as entity-relationship or object-oriented classes,
        attributes, and relationships (even when that information will
        not necessarily be Web-accessible). In these cases, RDF
        provides a basic structure for the XML in support of
        identifying objects, and using them in structured
        relationships. This connection is illustrated by a number of
        applications using RDF/XML for information interchange, as well
        as a number of projects investigating linkages between RDF (or
        ontology languages such as OWL) and UML (and its XML
        representations).

        CIM/XML's need to extend RDF Schema to support cardinality
        constraints and inverse relationships also illustrates the kinds
        of requirements
        that have led to the development of more powerful
        RDF-based schema/ontology languages such as DAML+OIL and OWL
        described in <a href="#richerschemas">Section 5.5</a>. Such
        languages may be appropriate in supporting many similar
        modeling applications in the future.</p>

        <p>Finally, CIM/XML also illustrates an important fact for
        those looking for additional examples of "RDF in the Field":
        sometimes languages are described as "XML" languages, or
        systems are described as using "XML", and the "XML" they are
        actually using is RDF/XML, i.e., they are RDF applications.
        Sometimes it is necessary to go fairly far into the description
        of the language or system in order to find this out (in some
        examples that have been found, RDF is never explicitly
        mentioned at all, but sample data clearly shows it is RDF/XML).
        Moreover, in applications such as CIM/XML, the RDF that is
        created will not be readily found on the Web, since it is
        intended for information exchange between software components
        rather than for general access (although future scenarios could
        be imagined in which more of this type of RDF would become
        Web-accessible).</p>
      </div>

      <div class="section">
        <h3><a id="geneont" name="geneont"></a>6.6 Gene Ontology
        Consortium</h3>

        <p>Structured metadata using controlled vocabularies such as <a
        href="http://www.snomed.org/">SNOMED RT</a> (Systematized
        Nomenclature of Medicine Reference Terminology) and <a
        href="http://www.nlm.nih.gov/mesh/meshhome.html">MeSH</a>
        (Medical Subject Headings) plays an important role in medicine,
        enabling more efficient literature searches and aiding in the
        distribution and exchange of medical knowledge <a
        href="#ref-cowan">[COWAN]</a>. At the same time, the field of
        medicine is rapidly changing, and with that comes the need to
        develop additional vocabularies.</p>

        <p>The objective of the <a
        href="http://www.geneontology.org/">Gene Ontology (GO)
        Consortium</a> <a href="#ref-go">[GO]</a> is to provide controlled vocabularies to
        describe specific aspects of gene products. Collaborating
        databases annotate their gene products (or genes) with GO
        terms, providing references and indicating what kind of
        evidence is available to support the annotations. The use of
        common GO terms by these databases facilitates uniform queries
        across them. The GO ontologies are structured to allow both
        attribution and querying to be performed at different levels of
        granularity. The GO vocabularies are dynamic, since knowledge
        of gene and protein roles in cells is accumulating and
        changing.</p>

        <p>The three organizing principles of the GO are <em>molecular
        function</em>, <em>biological process</em>, and <em>cellular component</em>. 
        A gene
        product has one or more molecular functions and is used in one
        or more biological processes; it may be, or may be associated
        with, one or more cellular components. Definitions of the terms
        within all three of these ontologies are contained in a single
        (text) definition file. XML formatted
        versions, containing all three ontology files and all available
        definitions, are generated monthly.</p>

        <p>Function, process and component are represented as directed
        acyclic graphs (DAGs) or networks. A child term may be an
        "instance" of its parent term (isa relationship) or a component
        of its parent term (part-of relationship). A child term may
        have more than one parent term and may have a different class
        of relationship with its different parents. Synonyms and
        cross-references to external databases are also represented in
        the ontologies.

        <span class="newstuff">GO uses RDF/XML facilities to represent the relationships
        between terms in the XML versions of
        the ontologies, because of its flexibility in representing these
        graph structures, as well as its widespread tool support.
        At the same time, GO currently uses <em>non</em>-RDF nested XML structures
        within the term descriptions, so the language used is not
        pure RDF/XML.</span></p>

        <p><a href="#example43">Example 43</a> shows some sample GO
        information from the <a
        href="http://www.geneontology.org/GO.doc.html">GO
        documentation</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example43" name="example43">Example 43: Sample GO
            Information</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;?xml version="1.0" encoding="UTF-8"?&gt; 
&lt;!DOCTYPE go:go&gt; 
&lt;go:go xmlns:go="http://www.geneontology.org/xml-dtd/go.dtd#" 
       xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"&gt; 
  &lt;go:version timestamp="Wed May 9 23:55:02 2001" /&gt; 

  &lt;rdf:RDF&gt; 
     &lt;go:term rdf:about="http://www.geneontology.org/go#GO:0003673"&gt; 
        &lt;go:accession&gt;GO:0003673&lt;/go:accession&gt; 
        &lt;go:name&gt;Gene_Ontology&lt;/go:name&gt; 
        &lt;go:definition&gt;&lt;/go:definition&gt; 
     &lt;/go:term&gt; 

     &lt;go:term rdf:about="http://www.geneontology.org/go#GO:0003674"&gt; 
        &lt;go:accession&gt;GO:0003674&lt;/go:accession&gt; 
        &lt;go:name&gt;molecular_function&lt;/go:name&gt; 
        &lt;go:definition&gt;The action characteristic of a gene product.&lt;/go:definition&gt; 
        &lt;go:part-of rdf:resource="http://www.geneontology.org/go#GO:0003673" /&gt; 
        &lt;go:dbxref&gt; 
           &lt;go:database_symbol&gt;go&lt;/go:database_symbol&gt; 
           &lt;go:reference&gt;curators&lt;/go:reference&gt; 
        &lt;/go:dbxref&gt; 
     &lt;/go:term&gt; 

     &lt;go:term rdf:about="http://www.geneontology.org/go#GO:0016209"&gt; 
        &lt;go:accession&gt;GO:0016209&lt;/go:accession&gt; 
        &lt;go:name&gt;antioxidant&lt;/go:name&gt; 
        &lt;go:definition&gt;&lt;/go:definition&gt; 
        &lt;go:isa rdf:resource="http://www.geneontology.org/go#GO:0003674" /&gt; 
        &lt;go:association&gt; 
           &lt;go:evidence evidence_code="ISS"&gt; 
              &lt;go:dbxref&gt; 
                 &lt;go:database_symbol&gt;fb&lt;/go:database_symbol&gt; 
                 &lt;go:reference&gt;fbrf0105495&lt;/go:reference&gt; 
              &lt;/go:dbxref&gt; 
           &lt;/go:evidence&gt; 
           &lt;go:gene_product&gt; 
              &lt;go:name&gt;CG7217&lt;/go:name&gt; 
              &lt;go:dbxref&gt; 
                 &lt;go:database_symbol&gt;fb&lt;/go:database_symbol&gt; 
                 &lt;go:reference&gt;FBgn0038570&lt;/go:reference&gt; 
              &lt;/go:dbxref&gt; 
           &lt;/go:gene_product&gt; 
        &lt;/go:association&gt; 
        &lt;go:association&gt; 
           &lt;go:evidence evidence_code="ISS"&gt; 
              &lt;go:dbxref&gt; 
                 &lt;go:database_symbol&gt;fb&lt;/go:database_symbol&gt; 
                 &lt;go:reference&gt;fbrf0105495&lt;/go:reference&gt; 
              &lt;/go:dbxref&gt; 
           &lt;/go:evidence&gt; 
           &lt;go:gene_product&gt; 
              &lt;go:name&gt;Jafrac1&lt;/go:name&gt; 
              &lt;go:dbxref&gt; 
                 &lt;go:database_symbol&gt;fb&lt;/go:database_symbol&gt; 
                 &lt;go:reference&gt;FBgn0040309&lt;/go:reference&gt; 
              &lt;/go:dbxref&gt; 
           &lt;/go:gene_product&gt; 
        &lt;/go:association&gt; 
      &lt;/go:term&gt; 
  &lt;/rdf:RDF&gt; 
&lt;/go:go&gt; 
</pre>
          </div>
        </div>

        <p><a href="#example43">Example 43</a> illustrates that
        <code>go:term</code> is the basic element. In some cases, the GO has 
        defined its own terms rather than using RDF Schema. For
        example, term <code>GO:0016209</code> has the element
        <code>&lt;go:isa
        rdf:resource="http://www.geneontology.org/go#GO:0003674"
        /&gt;</code>. This tag represents the relationship
        "<code>GO:0016209</code> isa <code>GO:0003674</code>", or, in English,
        "Antioxidant is a molecular function." Another specialized
        relationship is <code>go:part-of</code>. For example,
        <code>GO:0003674</code> has the element <code>&lt;go:part-of
        rdf:resource="http://www.geneontology.org/go#GO:0003673"
        /&gt;</code>. This says that "Molecular function is part of the
        Gene Ontology".</p>

        <p>Every annotation must be attributed to a source, which may
        be a literature reference, another database or a computational
        analysis. The annotation must indicate what kind of evidence is
        found in the cited source to support the association between
        the gene product and the GO term. A simple controlled
        vocabulary is used to record evidence. Examples include:</p>

        <ul>
          <li>ISS means "inferred from sequence similarity [with
          &lt;database:sequence_id&gt;]"</li>

          <li>IDA means "inferred from direct assay"</li>

          <li>TAS means "traceable author statement"</li>
        </ul>

        <p>The <code>go:dbxref</code> element represents the term in an
        external database, and <code>go:association</code> represents the
        gene associations of each term. <code>go:association</code> can
        have both <code>go:evidence</code>, which holds a
        <code>go:dbxref</code> to the evidence supporting the association,
        and a <code>go:gene_product</code>, which contains the gene symbol
        and <code>go:dbxref</code>. 

<span class="newstuff">
        These elements illustrate
        that the GO XML syntax is not "pure" RDF/XML, since the 
        nesting of other elements within these elements does not
        conform to the alternate node/predicate arc "stripes" described in
        Sections 2.1 and 2.2 of <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>.</span></p>

        <p>The GO illustrates a number of interesting points. First, it
        shows that the value of using XML for information exchange can
        be enhanced by structuring that XML using RDF. This is
        particularly true for data that has an overall graph or network
        structure, rather than being a strict hierarchy. The GO is also
        another example in which data using RDF will not necessarily appear
        for direct use on the Web (although the files are
        Web-accessible). It is also another example of data which is,
        on the surface, described as "XML", but on closer examination
        uses RDF/XML facilities (albeit not "pure" RDF/XML). 
        Finally, the GO illustrates the role RDF can
        play as a basis for representing ontologies. This role will be
        further enhanced once richer RDF-based languages for specifying
        ontologies, such as the DAML+OIL or OWL languages discussed in
        <a href="#richerschemas">Section 5.5</a>, become more widely
        used.  <span class="newstuff">In fact, a <a href="http://gong.man.ac.uk/">Gene Ontology 
        Next Generation</a> project is
        currently developing a representation of the GO ontologies in
        these richer languages.</span></p>
      </div>

      <div class="section">
        <h3><a id="devcap" name="devcap">6.7 Describing Device
        Capabilities and User Preferences</a></h3>

        <p>In recent years a large number of new mobile devices for
        browsing the Web have appeared. Many of these devices have
        highly divergent capabilities including a wide range of input
        and output capabilities as well as different levels of language
        support. Mobile devices may also have widely differing network
        connectivity capabilities. Users of these new devices expect a
        usable presentation regardless of the device's capabilities or
        the current network characteristics. Likewise, users want their
        dynamically changing preferences (e.g. turn audio on/off) to be
        considered when content or an application is presented. The
        reality, however, is that device heterogeneity, and the lack of
        a standard way for users to convey their preferences to the
        server, may result in: content that cannot be stored on the
        device, content that cannot be displayed, or content that
        violates the desires of the user. Additionally, the resulting
        content may take too long to convey over the network to the
        client device.</p>

        <p>A solution for addressing these problems is for a client to
        encode its <em>delivery context</em> - the device's
        capabilities, the user's preferences, the network
        characteristics, etc. - in such a way that a server can use the
        context to customize content for the device and user (see <a
        href="#ref-diprinc">[DIPRINC]</a> for a definition of delivery
        context). The W3C's Composite Capabilities/Preferences Profile
        (CC/PP) specification <a href="#ref-ccpp">[CC/PP]</a> helps to
        address this problem by defining a generic framework for
        describing a delivery context.</p>

        <p>The CC/PP framework defines a relatively simple structure -
        a two-level hierarchy of components and attribute/value pairs.
        A <em>component</em> may be used to capture a part of a
        delivery context (e.g. network characteristics, software
        supported by a device, or the hardware characteristics of a
        device). A component may contain one or more
        <em>attributes</em>. For example a component that encodes user
        preferences may contain an attribute to specify whether or not
        <em>AudioOutput</em> is desired.</p>

        <p>CC/PP defines its structure (the hierarchy described above)
        using RDF Schema (see <a href="#ref-ccpp">[CC/PP]</a> for
        details of the structure schema). A CC/PP <em>vocabulary</em>
        defines specific components and their attributes. <a
        href="#ref-ccpp">[CC/PP]</a>, however, does not define such
        vocabularies. Instead, vocabularies are defined by other
        organizations or applications (as described below). <a
        href="#ref-ccpp">[CC/PP]</a> also does not define a protocol
        for transporting an instance of a CC/PP vocabulary.</p>

        <p>An instance of a CC/PP vocabulary is called a
        <em>profile</em>. CC/PP attributes are encoded as RDF
        properties in a profile. <a href="#example44">Example 44</a>
        shows a profile fragment of user preferences for a user that
        prefers an audio presentation:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example44" name="example44">Example 44: A CC/PP
            Profile Fragment</a>
          </div>

          <div class="exampleInner">
<pre>
 &lt;ccpp:component&gt;
  &lt;rdf:Description rdf:ID="UserPreferences"&gt;
   &lt;rdf:type rdf:resource="http://www.example.org/profiles/prefs/v1_0#UserPreferences"/&gt;
   &lt;ex:AudioOutput&gt;Yes&lt;/ex:AudioOutput&gt;
   &lt;ex:Graphics&gt;No&lt;/ex:Graphics&gt;
   &lt;ex:Languages&gt;
    &lt;rdf:Seq&gt;
     &lt;rdf:li&gt;en-cockney&lt;/rdf:li&gt;
     &lt;rdf:li&gt;en&lt;/rdf:li&gt;
    &lt;/rdf:Seq&gt;
   &lt;/ex:Languages&gt;
  &lt;/rdf:Description&gt;
 &lt;/ccpp:component&gt;
</pre>
          </div>
        </div>

        <p>There are several advantages to using RDF in this
        application. First, a profile encoded via CC/PP may include
        attributes that were defined in schemas created by different
        organizations. RDF is a natural fit for these profiles because
        no single organization is likely to create a <em>super</em>
        schema for the aggregated profile data. A second advantage of
        RDF is that it facilitates (by virtue of its graph-based data
        model) the insertion of arbitrary attributes (RDF properties)
        into a profile. This is particularly useful for profiles that
        include frequently changing data such as location
        information.</p>

        <p>The Open Mobile Alliance has defined the User Agent Profile
        (UAProf) <a href="#ref-uaprof">[UAPROF]</a> - a CC/PP-based
        framework that includes a vocabulary for describing device
        capabilities, user agent capabilities, network characteristics,
        etc., as well as a protocol for transporting a profile. UAProf
        defines six components including: <em>HardwarePlatform</em>,
        <em>SoftwarePlatform</em>, <em>NetworkCharacteristics</em> and
        <em>BrowserUA</em>. It also defines several attributes for each
        of its components although a component's attributes are not
        fixed - they may be supplemented or overridden. <a
        href="#example45">Example 45</a> shows a fragment of UAProf's
        <em>HardwarePlatform</em> component:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example45" name="example45">Example 45: A Fragment
            of UAProf's HardwarePlatform Component</a>
          </div>

          <div class="exampleInner">
<pre>
 &lt;prf:component&gt;
  &lt;rdf:Description rdf:ID="HardwarePlatform"&gt;
   &lt;rdf:type rdf:resource="http://www.openmobilealliance.org/profiles/UAPROF/ccppschema-20021113#HardwarePlatform"/&gt;
   &lt;prf:ScreenSizeChar&gt;15x6&lt;/prf:ScreenSizeChar&gt;
   &lt;prf:BitsPerPixel&gt;2&lt;/prf:BitsPerPixel&gt;
   &lt;prf:ColorCapable&gt;No&lt;/prf:ColorCapable&gt;
   &lt;prf:BluetoothProfile&gt;
    &lt;rdf:Bag&gt;
     &lt;rdf:li&gt;headset&lt;/rdf:li&gt;
     &lt;rdf:li&gt;dialup&lt;/rdf:li&gt;
     &lt;rdf:li&gt;lanaccess&lt;/rdf:li&gt;
    &lt;/rdf:Bag&gt;
   &lt;/prf:BluetoothProfile&gt;
  &lt;/rdf:Description&gt;
 &lt;/prf:component&gt;
</pre>
          </div>
        </div>

        <p>The UAProf protocol supports both <em>static</em> profiles
        and <em>dynamic</em> profiles. A <em>static</em> profile is
        accessed via a URI. This has several advantages: a client's
        request to a server only contains a URI rather a potentially
        verbose XML document (thus minimizing over the air traffic);
        the client does not have to store and/or create the profile;
        the implementation burden on a client is relatively
        light-weight. <em>Dynamic</em> profiles are created on-the-fly
        and consequently do not have an associated URI. They may
        consist of a profile fragment containing a <em>difference</em>
        from a static profile, but they may also contain unique data
        that is not included in the client's static profile. A request
        may contain any number of static profiles and dynamic profiles.
        However, the ordering of the profiles is important as later
        profiles override earlier profiles in the request. See <a
        href="#ref-uaprof">[UAPROF]</a> for more information about
        UAProf's protocol and its rules for resolving multiple
        profiles.</p>

        <p>Several other communities (i.e. 3GPP's TS 26.234 <a
        href="#ref-3gpp">[3GPP]</a> and the WAP Forum's Multimedia
        Messaging Service Client Transactions Specification <a
        href="#ref-mms">[MMS-CTR]</a>) have defined vocabularies based
        on CC/PP. As a result, a profile may take advantage of the
        distributed nature of RDF and include components defined from
        various vocabularies. <a href="#example46">Example 46</a> 
        shows such a profile:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example46" name="example46">Example 46: A Profile
            Using Several Vocabularies</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:prf="http://www.wapforum.org/profiles/UAPROF/ccppschema-20010330#"
         xmlns:mms="http://www.wapforum.org/profiles/MMS/ccppschema-20010111#"
     xmlns:pss="http://www.3gpp.org/profiles/PSS/ccppschema-YYYYMMDD#"&gt;

 &lt;rdf:Description rdf:ID="SomeDevice"&gt;
  &lt;prf:component&gt;
   &lt;rdf:Description rdf:ID="Streaming"&gt;
    &lt;rdf:type rdf:resource="http://www.3gpp.org/profiles/PSS/ccppschema-PSS5#Streaming"/&gt;
    &lt;pss:AudioChannels&gt;Stereo&lt;/pss:AudioChannels&gt;
    &lt;pss:VideoPreDecoderBufferSize&gt;30720&lt;/pss:VideoPreDecoderBufferSize&gt;
    &lt;pss:VideoInitialPostDecoderBufferingPeriod&gt;0&lt;/pss:VideoInitialPostDecoderBufferingPeriod&gt;
    &lt;pss:VideoDecodingByteRate&gt;16000&lt;/pss:VideoDecodingByteRate&gt;
   &lt;/rdf:Description&gt;
  &lt;/prf:component&gt;
 
  &lt;prf:component&gt;
   &lt;rdf:Description rdf:ID="MmsCharacteristics"&gt;
    &lt;rdf:type rdf:resource="http://www.wapforum.org/profiles/MMS/ccppschema-20010111#Streaming"/&gt;
    &lt;mms:MmsMaxMessageSize&gt;2048&lt;/mms:MmsMaxMessageSize&gt;
    &lt;mms:MmsMaxImageResolution&gt;80x60&lt;/mms:MmsMaxImageResolution&gt;
    &lt;mms:MmsVersion&gt;2.0&lt;/mms:MmsVersion&gt;
   &lt;/rdf:Description&gt;
  &lt;/prf:component&gt;

  &lt;prf:component&gt;
   &lt;rdf:Description rdf:ID="PushCharacteristics"&gt;
    &lt;rdf:type rdf:resource="http://www.openmobilealliance.org/profiles/UAPROF/ccppschema-20010330#PushCharacteristics"/&gt;
    &lt;prf:Push-MsgSize&gt;1024&lt;/prf:Push-MsgSize&gt;
    &lt;prf:Push-MaxPushReq&gt;5&lt;/prf:Push-MaxPushReq&gt;
    &lt;prf:Push-Accept&gt;
     &lt;rdf:Bag&gt;
      &lt;rdf:li&gt;text/html&lt;/rdf:li&gt;
      &lt;rdf:li&gt;text/plain&lt;/rdf:li&gt;
      &lt;rdf:li&gt;image/gif&lt;/rdf:li&gt;
     &lt;/rdf:Bag&gt;
    &lt;/prf:Push-Accept&gt;
   &lt;/rdf:Description&gt;
  &lt;/prf:component&gt;

 &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>The definition of a delivery context and the data within a
        context will continually evolve. Consequently, RDF's inherent
        extensibility, and thus support for dynamically changing
        vocabularies, make RDF a good framework for encoding a delivery
        context.</p>
      </div>
    </div>

    <div class="section">
      <h2><a id="otherparts" name="otherparts">7. Other Parts of the
      RDF Specification</a></h2>

      <p><a href="#intro">Section 1</a> indicated that the RDF
      Specification consists of a number of documents (in addition to
      this Primer):</p>

      <ul>
        <li><a href="http://www.w3.org/TR/rdf-concepts/">RDF Concepts
        and Abstract Syntax</a> <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a></li>

        <li><a href="http://www.w3.org/TR/rdf-syntax-grammar/">RDF/XML
        Syntax Specification</a> <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a></li>

        <li><a href="http://www.w3.org/TR/rdf-schema/">RDF Vocabulary
        Description Language 1.0: RDF Schema</a> <a
        href="#ref-rdf-vocabulary">[RDF-VOCABULARY]</a></li>

        <li><a href="http://www.w3.org/TR/rdf-mt/">RDF Semantics</a> <a
        href="#ref-rdf-semantics">[RDF-SEMANTICS]</a></li>

        <li><a href="http://www.w3.org/TR/rdf-testcases/">RDF Test
        Cases</a> <a href="#ref-rdf-tests">[RDF-TESTS]</a></li>
      </ul>

      <p>The Primer has already discussed the subjects of several of
      these documents, basic RDF concepts (in <a
      href="#statements">Section 2</a>), the RDF/XML syntax (in <a
      href="#rdfxml">Section 3</a>) and RDF Schema (in <a
      href="#rdfschema">Section 5</a>). This section briefly
      describes the remaining documents (even though there have already
      been numerous references to <a href="#ref-rdf-semantics">[RDF-SEMANTICS]</a>
      as well), in order to explain their role
      in the complete specification of RDF.</p>

      <div class="section">
        <h3><a id="semantics" name="semantics">7.1 RDF
        Semantics</a></h3>

        <p>As discussed in the preceding sections, RDF is intended to
        be used to express statements about resources in the form of a
        graph, using specific vocabularies (names of resources,
        properties, classes, etc.). RDF is also intended to be the
        foundation for more advanced languages, such as those discussed
        in <a href="#richerschemas">Section 5.5</a>. In order to serve
        these purposes, the "meaning" of an RDF graph must be defined
        in a very precise manner.</p>

        <p>Exactly what constitutes the "meaning" of an RDF graph in a
        very general sense may depend on many factors, including 
        conventions within a user community to interpret user-defined
        RDF classes and properties in specific ways, 
        comments in natural language, or links to other
        content-bearing documents. As noted briefly in 
        <a href="#rdfmodel">Section 2.2</a>, much of
        the meaning conveyed in these forms will not be directly
        accessible to machine processing, although this meaning may be
        used by human interpreters of the RDF information, or by
        programmers writing software to perform various kinds of
        processing on that RDF information. However, RDF statements
        also have a <em>formal</em> meaning which determines, with
        mathematical precision, the conclusions (or
        <em>entailments</em>) that machines can draw from a given RDF graph.
        The <a href="http://www.w3.org/TR/rdf-mt/">RDF Semantics</a> <a
        href="#ref-rdf-semantics">[RDF-SEMANTICS]</a> document defines this
        formal meaning, using a technique called <em>model theory</em>
        for specifying the semantics of a formal language.
<span class="newstuff"> 
        <a href="#ref-rdf-semantics">[RDF-SEMANTICS]</a> also defines the
        semantic extensions to the RDF language represented by RDF Schema, and by 
        individual datatypes.  
</span>
        In other
        words, the RDF model theory provides the formal underpinnings
        for all RDF concepts. Based on the
        semantics defined in the model theory, it is simple to
        translate an RDF graph into a logical expression with
        essentially the same meaning.</p>
      </div>

      <div class="section">
        <h3><a id="testcases" name="testcases">7.2 Test Cases</a></h3>

        <p>The <a href="http://www.w3.org/TR/rdf-testcases/">RDF Test
        Cases</a> <a href="#ref-rdf-tests">[RDF-TESTS]</a> supplement
        the textual RDF specifications with test cases (examples)
        corresponding to particular technical issues addressed by the
        RDF Core Working Group. To help describe these examples, the
        Test Cases document introduces a notation called <a
        href="http://www.w3.org/TR/rdf-testcases/#ntriples">
        N-Triples</a>, which provides the basis for the triples
        notation used throughout this Primer. The test cases are
        published in machine-readable form at Web locations referenced
        by the Test Cases document, so developers can use these as the
        basis for automated testing of RDF software.</p>

        <p>The test cases are divided into a number of categories:</p>

        <ul>
          <li>Positive and Negative Parser Tests: These test whether
          RDF/XML parsers produce a correct N-Triples output graph from
          legal RDF/XML input documents, or correctly report errors if
          the input documents are not legal RDF/XML.</li>

          <li>Positive and Negative Entailment Tests: These test
          whether proper entailments (conclusions) are or are not drawn
          from sets of specified RDF statements.</li>

          <li>Datatype-aware Entailment Tests: These are positive or
          negative entailment tests that involve the use of datatypes,
          and hence require additional support for the specific
          datatypes involved in the tests.</li>

          <li>Miscellaneous Tests: These are tests that do not fall
          into one of the other categories.</li>
        </ul>

        <p>The test cases are not a complete specification of RDF, and
        are not intended to take precedence over the other
        specification documents. However, they are intended to
        illustrate the intent of the RDF Core Working Group with
        respect to the design of RDF, and developers may find these
        test cases helpful should the wording of the specifications be
        unclear on any point of detail.</p>
      </div>
    </div>

    <div class="section">
      <h2><a name="references" id="references">8. References</a></h2>

      <div class="section">
        <h3><a name="normative-references"
        id="normative-references">8.1 Normative References</a></h3>

        <dl>
          <dt><a id="ref-rdf-concepts"
          name="ref-rdf-concepts"></a>[RDF-CONCEPTS]</dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/">Resource
          Description Framework (RDF): Concepts and Abstract
          Syntax</a></cite>, Klyne G., Carroll J. (Editors), W3C Recommendation, 10 February 2004. <a href ="http://www.w3.org/TR/2004/REC-rdf-primer-20040210/">This version</a> is http://www.w3.org/TR/2004/REC-rdf-primer-20040210/. The <a
           href="http://www.w3.org/TR/rdf-concepts/">latest version</a>
          is http://www.w3.org/TR/rdf-concepts/.</dd>

          <dt><a id="ref-rdf-mime-type"
          name="ref-rdf-mime-type"></a>[RDF-MIME-TYPE]</dt>
  <dd>
    <cite><a href="http://www.iana.org/assignments/media-types/">MIME Media Types</a></cite>, The Internet Assigned Numbers Authority (IANA).  This document is http://www.iana.org/assignments/media-types/ .  The <a href="http://www.w3.org/2001/sw/RDFCore/mediatype-registration">registration for <code>application/rdf+xml</code></a> is archived at http://www.w3.org/2001/sw/RDFCore/mediatype-registration .
  </dd>

          <dt><a id="ref-rdfms" name="ref-rdfms">[RDF-MS]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/">Resource
          Description Framework (RDF) Model and Syntax
          Specification</a></cite>, Lassila O., Swick R. (Editors),
          World Wide Web Consortium, 22 February 1999. <a href = "http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/">This version</a> is http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/. The <a
          href="http://www.w3.org/TR/REC-rdf-syntax/">latest
          version</a> is http://www.w3.org/TR/REC-rdf-syntax/.</dd>

          <dt><a id="ref-rdf-semantics"
          name="ref-rdf-semantics"></a>[RDF-SEMANTICS]</dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2004/REC-rdf-mt-20040210/">RDF
          Semantics</a></cite>, Hayes P. (Editor), W3C Recommendation,
10 February 2004. <a href =
"http://www.w3.org/TR/2004/REC-rdf-mt-20040210/">This
version</a> is
http://www.w3.org/TR/2004/REC-rdf-mt-20040210/. The <a
href="http://www.w3.org/TR/rdf-mt/">latest version</a>
          is http://www.w3.org/TR/rdf-mt/.</dd>

          <dt><a id="ref-rdf-syntax"
          name="ref-rdf-syntax"></a>[RDF-SYNTAX]</dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/">
          RDF/XML Syntax Specification (Revised)</a></cite>, Beckett
          D.  (Editor), W3C Recommendation, 10 February 2004. <a href = "http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/">This version</a> http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/. The <a
          href="http://www.w3.org/TR/rdf-syntax-grammar/">latest
          version</a> is
          http://www.w3.org/TR/rdf-syntax-grammar/.</dd>

          <dt><a id="ref-rdf-tests"
          name="ref-rdf-tests"></a>[RDF-TESTS]</dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2004/REC-rdf-testcases-20040210/">RDF
          Test Cases</a></cite>, Grant J., Beckett D. (Editors), W3C
Recommendation, 10 February 2004.  <a href = "http://www.w3.org/TR/2004/REC-rdf-testcases-20040210/">This version</a> is http://www.w3.org/TR/2004/REC-rdf-testcases-20040210/. The <a
          href="http://www.w3.org/TR/rdf-testcases/">latest
          version</a> is http://www.w3.org/TR/rdf-testcases/.</dd>

          <dt><a id="ref-rdf-vocabulary"
          name="ref-rdf-vocabulary"></a>[RDF-VOCABULARY]</dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2004/REC-rdf-schema-20040210/">RDF
          Vocabulary Description Language 1.0: RDF Schema</a></cite>,
          Brickley D., Guha R.V. (Editors), W3C Recommendation, 10 February 2004.
<a href="http://www.w3.org/TR/2004/REC-rdf-schema-20040210/">This version</a> is http://www.w3.org/TR/2004/REC-rdf-schema-20040210/. The <a
          href="http://www.w3.org/TR/rdf-schema/">latest version</a>
          is http://www.w3.org/TR/rdf-schema/.</dd>

      <dt><a id="ref-unicode" name="ref-unicode"></a>[UNICODE]</dt>

      <dd><cite>The Unicode Standard, Version 3</cite>, The Unicode
      Consortium, Addison-Wesley, 2000. ISBN 0-201-61633-5, as updated
      from time to time by the publication of new versions. (See <a
      href=
      "http://www.unicode.org/unicode/standard/versions/">http://www.unicode.org/unicode/standard/versions/</a>
      for the latest version and additional information on versions of
      the standard and of the Unicode Character Database).</dd>

          <dt><a id="ref-uri" name="ref-uri">[URIS]</a></dt>

          <dd><cite><a
          href="http://www.isi.edu/in-notes/rfc2396.txt">RFC 2396 -
          Uniform Resource Identifiers (URI): Generic
          Syntax</a></cite>, Berners-Lee T., Fielding R., Masinter L.,
          IETF, August 1998, http://www.isi.edu/in-notes/rfc2396.txt.</dd>

          <dt><a id="ref-xml" name="ref-xml">[XML]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2000/REC-xml-20001006">Extensible
          Markup Language (XML) 1.0, Second Edition</a></cite>, Bray
          T., Paoli J., Sperberg-McQueen C.M., Maler E. (Editors),
          World Wide Web Consortium, 6 October 2000. <a
          href="http://www.w3.org/TR/2000/REC-xml-20001006">This
          version</a> is http://www.w3.org/TR/2000/REC-xml-20001006.
          The <a href="http://www.w3.org/TR/REC-xml">latest version</a>
          is http://www.w3.org/TR/REC-xml.</dd>

          <dt><a id="ref-xml-base"
          name="ref-xml-base">[XML-BASE]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2001/REC-xmlbase-20010627/">XML
          Base</a></cite>, Marsh J. (Editor), World Wide Web Consortium, 27 June 2001. <a
          href="http://www.w3.org/TR/2001/REC-xmlbase-20010627/">This
          version</a> is
          http://www.w3.org/TR/2001/REC-xmlbase-20010627/. The <a
          href="http://www.w3.org/TR/xmlbase/">latest version</a> is
          http://www.w3.org/TR/xmlbase/.</dd>

          <dt><a id="ref-namespaces"
          name="ref-namespaces">[XML-NS]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/TR/1999/REC-xml-names-19990114/">Namespaces
          in XML</a></cite>, Bray T., Hollander D., Layman A.
          (Editors), World Wide Web Consortium, 14 January 1999. <a
          href="http://www.w3.org/TR/1999/REC-xml-names-19990114/">This
          version</a> is
          http://www.w3.org/TR/1999/REC-xml-names-19990114/. The <a
          href="http://www.w3.org/TR/REC-xml-names/">latest version</a>
          is http://www.w3.org/TR/REC-xml-names/.</dd>

          <dt><a id="ref-xml-xc14n" 
          name="ref-xml-xc14n">[XML-XC14N]</a></dt>

          <dd><cite><a href="http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/">Exclusive 
          XML Canonicalization Version 1.0</a></cite>, Boyer J., Eastlake D.E. 3rd, 
          Reagle J. (Authors/Editors), World Wide Web Consortium, 18 July 2002.  <a 
          href="http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/">This 
          version</a> is http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/.  
          The <a href="http://www.w3.org/TR/xml-exc-c14n/">latest version</a> is 
          http://www.w3.org/TR/xml-exc-c14n/.</dd>

        </dl>
      </div>

      <div class="section">
        <h3><a name="informational-references"
        id="informational-references">8.2 Informational
        References</a></h3>

        <dl>
          <dt><a id="ref-3gpp" name="ref-3gpp">[3GPP]</a></dt>

          <dd><cite><a href="http://www.3gpp.org/specs/specs.htm">3GPP
          TS 26.234.</a></cite> 3rd Generation Partnership Project;
          Technical Specification Group Services and System Aspects;
          Transparent end-to-end packet switched streaming service;
          Protocols and codecs V5.2.0 (2002-09). <a
          href="http://www.3gpp.org/specs/specs.htm">This document</a>
          is available at http://www.3gpp.org/specs/specs.htm via
          directory
          ftp://ftp.3gpp.org/specs/2002-09/Rel-5/26_series/.</dd>

          <dt><a id="ref-address-schemes"
          name="ref-address-schemes">[ADDRESS-SCHEMES]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/Addressing/schemes.html">Addressing
          Schemes</a></cite>, Connolly D., 2001. <a
          href="http://www.w3.org/Addressing/schemes.html">This
          document</a> is
          http://www.w3.org/Addressing/schemes.html.</dd>

          <dt><a id="ref-bates96" name="ref-bates96">[BATES96]</a></dt>

          <dd><cite><a
          href="http://is.gseis.ucla.edu/research/mjbates.html">Indexing
          and Access for Digital Libraries and the Internet: Human,
          Database, and Domain Factors</a></cite>, Bates M.J., 1996. <a
          href="http://is.gseis.ucla.edu/research/mjbates.html">This
          document</a> is
          http://is.gseis.ucla.edu/research/mjbates.html.</dd>

          <dt><a id="ref-berners-lee98"
          name="ref-berners-lee98">[BERNERS-LEE98]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/DesignIssues/RDFnot.html">What the
          Semantic Web can represent</a></cite>, Berners-Lee T., 1998.
          <a href="http://www.w3.org/DesignIssues/RDFnot.html">This
          document</a> is
          http://www.w3.org/DesignIssues/RDFnot.html.</dd>

          <dt><a id="ref-ccpp" name="ref-ccpp">[CC/PP]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2004/REC-CCPP-struct-vocab-20040115/">
          Composite Capability/Preference Profiles (CC/PP): Structure
          and Vocabularies</a></cite>, Klyne G., Reynolds F., Woodrow
          C., Ohto H., Hjelm J., Butler M., Tran, L., W3C Recommendation, 15 January 2004. <a href = "http://www.w3.org/TR/2004/REC-CCPP-struct-vocab-20040115/">This version</a> is http://www.w3.org/TR/2004/REC-CCPP-struct-vocab-20040115/. The
          <a href="http://www.w3.org/TR/CCPP-struct-vocab/">latest
          version</a> is http://www.w3.org/TR/CCPP-struct-vocab/.</dd>

          <dt><a id="ref-cg" name="ref-cg">[CG]</a></dt>

          <dd><cite>Conceptual Graphs</cite>, Sowa J., ISO working
          document ISO/JTC1/SC32/WG2 N 000, 2 April 2001 (work in
          progress). Available at <a
          href="http://users.bestweb.net/~sowa/cg/cgstand.htm">http://users.bestweb.net/~sowa/cg/cgstand.htm</a>.</dd>

          <dt><a id="ref-charmod" name="ref-charmod">[CHARMOD]</a></dt>

          <dd><cite><a href="http://www.w3.org/TR/2002/WD-charmod-20020220/">Character 
          Model for the World Wide Web 1.0</a></cite>, D&uuml;rst M., Yergeau F., 
          Ishida R., Wolf M., Freytag A., Texin T. (Editors), World Wide Web Consortium, 
          20 February 2002 (work in progress).  
          <a href="http://www.w3.org/TR/2002/WD-charmod-20020220/">This version</a> 
          is http://www.w3.org/TR/2002/WD-charmod-20020220/. The 
          <a href="http://www.w3.org/TR/charmod/">latest version</a> is
          http://www.w3.org/TR/charmod/.</dd>

          <dt><a id="ref-cim" name="ref-cim">[CIM]</a></dt>

          <dd><cite>Common Information Model (CIM): CIM 10 Version</cite>, 
          EPRI, Palo Alto, CA: 2001, 1001976. <a
          href="http://www.epri.com/attachments/286161_1001976(1).pdf">This document</a>
          is available at 
          http://www.epri.com/attachments/286161_1001976(1).pdf (267pp.).</dd>

          <dt><a id="ref-cowan" name="ref-cowan">[COWAN]</a></dt>

          <dd><cite><a
          href="http://seminars.seyboldreports.com/2002_new_york/files/presentations/014/cowan_john.ppt">
          Metadata, Reuters Health Information, and Cross-Media
          Publishing</a></cite> , Cowan J., 2002. Presentation at
          Seybold New York 2002 Enterprise Publishing Conference. <a
          href="http://seminars.seyboldreports.com/2002_new_york/files/presentations/014/cowan_john.ppt">
          This document</a> is
          http://seminars.seyboldreports.com/seminars/2002_new_york/presentations/014/cowan_john.ppt.
          An accompanying <a
          href="http://seminars.seyboldreports.com/2002_new_york/files/transcripts/doc/transcript_EP7.doc">
          transcript</a> is
          http://seminars.seyboldreports.com/2002_new_york/files/transcripts/doc/transcript_EP7.doc</dd>

          <dt><a id="ref-daf" name="ref-daf">[DAF]</a></dt>

          <dd><cite><a
          href="http://www.omg.org/technology/documents/formal/UMS_Data_Access_Facility.htm">Utility
          Management System (UMS) Data Access Facility</a></cite>, version 2.0,
          Object Management Group, November 2002. <a
          href="http://www.omg.org/technology/documents/formal/UMS_Data_Access_Facility.htm">This
          document</a> is available at
          http://www.omg.org/technology/documents/formal/UMS_Data_Access_Facility.htm.</dd>

          <dt><a id="ref-damloil"
          name="ref-damloil">[DAML+OIL]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/TR/daml+oil-reference">DAML+OIL
          (March 2001) Reference Description</a></cite>, Connolly D.,
          van Harmelen F., Horrocks I., McGuinness D.L.,
          Patel-Schneider P.F., Stein L.A., World Wide Web Consortium,
          18 December 2001. <a
          href="http://www.w3.org/TR/daml+oil-reference">This
          document</a> is http://www.w3.org/TR/daml+oil-reference.</dd>

          <dt><a id="ref-dublin-core"
          name="ref-dublin-core">[DC]</a></dt>

          <dd><cite><a
          href="http://dublincore.org/documents/2003/06/02/dces/">Dublin Core
          Metadata Element Set, Version 1.1: Reference
          Description</a></cite>, 02 June 2003. <a
          href="http://dublincore.org/documents/2003/06/02/dces/">This
          version</a> is http://dublincore.org/documents/2003/06/02/dces/.
          The <a
          href="http://dublincore.org/documents/dces/">latest
          version</a> is http://dublincore.org/documents/dces/.</dd>

          <dt><a id="ref-diprinc" name="ref-diprinc">[DIPRINC]</a></dt>

          <dd><cite><a href="http://www.w3.org/TR/di-princ/">Device
          Independence Principles.</a></cite> Gimson, R., Finkelstein,
          S., Maes, S., Suryanarayana, L., World Wide Web Consortium,
          18 September 2001 (work in progress). <a
          href="http://www.w3.org/TR/2001/WD-di-princ-20010918/">This
          version</a> is
          http://www.w3.org/TR/2001/WD-di-princ-20010918. The <a
          href="http://www.w3.org/TR/di-princ/">latest version</a> is
          http://www.w3.org/TR/di-princ/.</dd>

          <dt><a id="ref-devos" name="ref-devos">[DWZ01]</a></dt>

          <dd><cite><a href="http://www.langdale.com.au/PICA/">XML for
          CIM Model Exchange</a></cite> , deVos A., Widergreen S.E.,
          Zhu J., Proc. IEEE Conference on Power Industry Computer
          Systems, Sydney, Australia, 2001. <a
          href="http://www.langdale.com.au/PICA/">This document</a> is
          http://www.langdale.com.au/PICA/.</dd>

          <dt><a id="ref-go" name="ref-go">[GO]</a></dt>

          <dd><cite><a href="http://www.geneontology.org/GO_nature_genetics_2000.pdf">Gene 
          Ontology: tool for the unification of biology</a></cite>, 
          The Gene Ontology Consortium, <em>Nature Genetics</em>, Vol. 25: 25-29, May 2000.
          Available at <a href="http://www.geneontology.org/GO_nature_genetics_2000.pdf">http://www.geneontology.org/GO_nature_genetics_2000.pdf</a></dd>

          <dt><a id="ref-gray" name="ref-gray">[GRAY]</a></dt>

          <dd><cite>Logic, Algebra and Databases</cite>, Gray P., Ellis
          Horwood Ltd., 1984. ISBN 0-85312-709-3, 0-85312-803-0,
          0-470-20103-7, 0-470-20259-9.</dd>

          <dt><a id="ref-hayes" name="ref-hayes">[HAYES]</a></dt>

          <dd><cite>In Defense of Logic</cite>, Hayes P., Proceedings
          from the International Joint Conference on Artificial
          Intelligence, 1975, San Francisco. Morgan Kaufmann Inc.,
          1977. Also in <cite>Computation and Intelligence: Collected
          Readings</cite>, Luger G. (ed), AAAI press/MIT press, 1995.
          ISBN 0-262-62101-0.</dd>

          <dt><a id="ref-kif" name="ref-kif">[KIF]</a></dt>

          <dd><cite>Knowledge Interchange Format</cite>, Genesereth M.,
          draft proposed American National Standard NCITS.T2/98-004.
          Available at <a
          href="http://logic.stanford.edu/kif/dpans.html">http://logic.stanford.edu/kif/dpans.html</a>.</dd>

          <dt><a id="ref-lbase"
          name="ref-lbase"></a>[LBASE]</dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2003/NOTE-lbase-20031010/">LBase: 
          Semantics for Languages of the Semantic Web</a></cite>, Guha R. V., Hayes P., 
          W3C Note, 10 October 2003. <a
          href="http://www.w3.org/TR/2003/NOTE-lbase-20031010/">This
          version</a> is http://www.w3.org/TR/2003/NOTE-lbase-20031010/.
          The <a href="http://www.w3.org/TR/lbase/">latest version</a>
          is http://www.w3.org/TR/lbase/.</dd>

          <dt><a id="ref-luger" name="ref-luger">[LUGER]</a></dt>

          <dd><cite>Artificial Intelligence: Structures and Strategies
          for Complex Problem Solving</cite> (3rd ed.), Luger G.,
          Stubblefield W., Addison Wesley Longman, 1998. ISBN
          0-805-31196-3.</dd>

          <dt><a id="ref-mathml" name="ref-mathml">[MATHML]</a></dt>

          <dd><cite><a href="http://www.w3.org/TR/2001/REC-MathML2-20010221/">Mathematical
          Markup Language (MathML) Version 2.0</a></cite>, Carlisle D., Ion P., 
          Miner R., Poppelier N. (Editors); Ausbrooks R., Buswell S., Dalmas S., 
          Devitt S., Diaz A., Hunter R., Smith B., Soiffer N., Sutor R., 
          Watt S. (Principal Authors), World Wide Web Consortium, 21 February 2001.  
          <a href="http://www.w3.org/TR/2001/REC-MathML2-20010221/">This 
          version</a> is http://www.w3.org/TR/2001/REC-MathML2-20010221/.  
          The <a href="http://www.w3.org/TR/MathML2/">latest version</a> is
          http://www.w3.org/TR/MathML2/.</dd>

          <dt><a id="ref-mms" name="ref-mms">[MMS-CTR]</a></dt>

          <dd><cite><a
          href="http://www.openmobilealliance.org/">Multimedia
          Messaging Service Client Transactions
          Specification.</a></cite> WAP-206-MMSCTR-20020115-a. 
          This document is available at
          http://www.openmobilealliance.org/.</dd>

          <dt><a id="ref-nameaddress"
          name="ref-nameaddress">[NAMEADDRESS]</a></dt>

          <dd><cite><a href="http://www.w3.org/Addressing/">Naming and
          Addressing: URIs, URLs, ...</a></cite>, Connolly D., 2002. <a
          href="http://www.w3.org/Addressing/">This document</a> is
          http://www.w3.org/Addressing/.</dd>

          <dt><a id="ref-owl" name="ref-owl">[OWL]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2004/REC-owl-ref-20040210/">OWL Web
          Ontology Language Reference</a></cite>, Dean M., 
          Schreiber G (Editors); van Harmelen F., Hendler J., Horrocks I., 
          McGuinness D.L., Patel-Schneider P.F., Stein L.A. (Authors), 
          W3C Recommendation, 10 February 2004. The <a
          href="http://www.w3.org/TR/owl-ref/">latest version</a> is
          http://www.w3.org/TR/owl-ref/.</dd>

          <dt><a id="ref-prism" name="ref-prism">[PRISM]</a></dt>

          <dd><cite><a
          href="http://www.prismstandard.org/">PRISM:
          Publishing Requirements for Industry Standard
          Metadata</a></cite>, Version 1.1, 19 February 2002. The <a
          href="http://www.prismstandard.org/">latest version</a> 
          of the PRISM specification is available at
          http://www.prismstandard.org/.</dd>

          <dt><a id="ref-rdf-issue"
          name="ref-rdf-issue">[RDFISSUE]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/2000/03/rdf-tracking/">RDF Issue
          Tracking</a></cite>, McBride B., 2002. <a
          href="http://www.w3.org/2000/03/rdf-tracking/">This
          document</a> is http://www.w3.org/2000/03/rdf-tracking/.</dd>

          <dt><a id="ref-rdf-s"
          name="ref-rdf-s">[RDF-S]</a></dt>

          <dd><cite><a href="http://www.w3.org/TR/2000/CR-rdf-schema-20000327/">
          Resource Description Framework (RDF) Schema Specification 1.0</a>
          </cite>, Brickley D., Guha, R.V. (Editors), World Wide Web Consortium. 
          27 March 2000. <a href="http://www.w3.org/TR/2000/CR-rdf-schema-20000327/">
          This version</a> is http://www.w3.org/TR/2000/CR-rdf-schema-20000327/.</dd>

          <dt><a id="ref-rss" name="ref-rss">[RSS]</a></dt>

          <dd><cite><a href="http://purl.org/rss/1.0/spec">RDF Site
          Summary (RSS) 1.0</a></cite>, Beged-Dov G., Brickley D.,
          Dornfest R., Davis I., Dodds L., Eisenzopf J., Galbraith D.,
          Guha R.V., MacLeod K., Miller E., Swartz A., van der Vlist
          E., 2000. <a href="http://purl.org/rss/1.0/spec">This
          document</a> is http://purl.org/rss/1.0/spec.</dd>

          <dt><a id="ref-ruby" name="ref-ruby">[RUBY]</a></dt>

          <dd><cite><a href="http://www.w3.org/TR/2001/REC-ruby-20010531/">Ruby 
          Annotation</a></cite>, Sawicki M., Suignard M., Ishikawa M., D&uuml;rst M., 
          Texin T. (Editors), World Wide Web Consortium, 31 May 2001.  
          <a href="http://www.w3.org/TR/2001/REC-ruby-20010531/">This version</a> 
          is http://www.w3.org/TR/2001/REC-ruby-20010531/.  The 
          <a href="http://www.w3.org/TR/ruby/">latest version</a> is 
          http://www.w3.org/TR/ruby/.</dd>

          <dt><a id="ref-sowa" name="ref-sowa">[SOWA]</a></dt>

          <dd><cite>Knowledge Representation: Logical, Philosophical
          and Computational Foundations</cite>, Sowa J., Brookes/Cole,
          2000. ISBN 0-534-94965-7.</dd>

          <dt><a id="ref-svg" name="ref-svg">[SVG]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2003/REC-SVG11-20030114/">Scalable
          Vector Graphics (SVG) 1.1 Specification</a></cite>, Ferraiolo J.,
          Fujisawa J., Jackson D. (Editors), World Wide Web Consortium, 14
          January 2003. <a
          href="http://www.w3.org/TR/2003/REC-SVG11-20030114/">This
          version</a> is http://www.w3.org/TR/2003/REC-SVG11-20030114/.
          The <a href="http://www.w3.org/TR/SVG11/">latest version</a>
          is http://www.w3.org/TR/SVG11/.</dd>

          <dt><a id="ref-uaprof" name="ref-uaprof">[UAPROF]</a></dt>

          <dd><cite><a href="http://www.openmobilealliance.org/">User
          Agent Profile.</a></cite> OMA-WAP-UAProf-v1_1. This document
          is available at http://www.openmobilealliance.org/.</dd>

          <dt><a id="ref-webdata" name="ref-webdata">[WEBDATA]</a></dt>

          <dd><cite><a href="http://www.w3.org/1999/04/WebData">Web
          Architecture: Describing and Exchanging Data</a></cite>,
          Berners-Lee T., Connolly D., Swick R., World Wide Web Consortium, 7 June 1999. <a
          href="http://www.w3.org/1999/04/WebData">This document</a> is
          http://www.w3.org/1999/04/WebData.</dd>

          <dt><a id="ref-xlink" name="ref-xlink">[XLINK]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2001/REC-xlink-20010627/">XML
          Linking Language (XLink) Version 1.0</a></cite>, DeRose S.,
          Maler E., Orchard D. (Editors), World Wide Web Consortium, 27
          June 2001. <a
          href="http://www.w3.org/TR/2001/REC-xlink-20010627/">This
          version</a> is http://www.w3.org/TR/2001/REC-xlink-20010627/.
          The <a href="http://www.w3.org/TR/xlink/">latest version</a>
          is http://www.w3.org/TR/xlink/.</dd>

          <dt><a id="ref-xmlschema2"
          name="ref-xmlschema2">[XML-SCHEMA2]</a></dt>

          <dd><cite><a
          href="http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/">XML
          Schema Part 2: Datatypes</a></cite>, Biron P., Malhotra A.
          (Editors), World Wide Web Consortium. 2 May 2001. <a
          href="http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/">This
          version</a> is
          http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/. The <a
          href="http://www.w3.org/TR/xmlschema-2/">latest version</a>
          is http://www.w3.org/TR/xmlschema-2/.</dd>

          <dt><a id="ref-xpackage"
          name="ref-xpackage">[XPACKAGE]</a></dt>

          <dd><cite><a
          href="http://www.xpackage.org/specification/xpackage-draft-20030306.html">
          XML Package (XPackage) 1.0</a></cite> , Wilson G., Editor's Working Draft, 
          6 March 2003. <a
          href="http://www.xpackage.org/specification/xpackage-draft-20030306.html">
          This version</a> is
          http://www.xpackage.org/specification/xpackage-draft-20030306.html.
          The <a href="http://www.xpackage.org/specification/">latest
          version</a> is http://www.xpackage.org/specification/.</dd>
        </dl>
      </div>
    </div>

    <div class="section">
      <h2><a id="acknowledgements" name="acknowledgements">9.
      Acknowledgments</a></h2>

      <p>This document has benefited from inputs from many members of
      the <a href="http://www.w3.org/2001/sw/RDFCore/">RDF Core Working
      Group</a>. Specific thanks are due to Art Barstow, Dave Beckett,
      Dan Brickley, Ron Daniel, Ben Hammersley, Martyn Horner, Graham
      Klyne, Sean Palmer, Patrick Stickler, Aaron Swartz, Ralph Swick,
      and Garret Wilson who, together with the many people who
      commented on earlier versions of the Primer, provided valuable
      contributions to this document.</p>

      <p>In addition, this document contains a significant contribution
      from Pat Hayes, Sergey Melnik, and Patrick Stickler, who led the
      development of the RDF datatype facilities described in the RDF
      family of specifications.</p>

      <p>Frank Manola also thanks 
      <a href="http://www.mitre.org/">The MITRE Corporation</a>, Frank's employer 
      during most of the preparation of this document, for its support of
      his RDF Core Working Group activities under a MITRE Sponsored Research
      grant.</p>
    </div>
    <hr />


    <div class="section">
      <h2><a id="identifiers" name="identifiers"></a>Appendix
      A: More on Uniform Resource Identifiers (URIs)</h2>

      <p class="newstuff">Note:  This section is intended to provide a 
      brief introduction to URIs.  The definitive specification
      of URIs is <a
      href="http://www.isi.edu/in-notes/rfc2396.txt">RFC 2396</a> <a
      href="#ref-uri">[URIS]</a>, which should be consulted for further
      details. Additional discussion of URIs
      can also be found in <a href="http://www.w3.org/Addressing/">Naming
      and Addressing: URIs, URLs, ...</a> <a
      href="#ref-nameaddress">[NAMEADDRESS]</a>.</p>

      <p>As discussed in <a href="#basicconcepts">Section 2.1</a>, the Web
      provides a general form of identifier, called the <a
      href="http://www.isi.edu/in-notes/rfc2396.txt">Uniform Resource
      Identifier</a> (URI), for identifying (naming) resources on the
      Web. Unlike URLs, URIs are not limited to identifying things that
      have network locations, or use other computer access mechanisms.
      A number of different <em>URI schemes</em> (URI forms) have been
      already been developed, and are being used, for various purposes.
      Examples include:</p>

      <ul>
        <li><code>http:</code> (Hypertext Transfer Protocol, for Web
        pages)</li>

        <li><code>mailto:</code> (email addresses), e.g.,
        <code>mailto:em@w3.org</code></li>

        <li><code>ftp:</code> (File Transfer Protocol)</li>

        <li><code>urn:</code> (Uniform Resource Names, intended to be
        persistent location-independent resource identifiers), e.g.,
        <code>urn:isbn:0-520-02356-0</code> (for a book)</li>
      </ul>

      <p>A list of existing URI schemes can be found in <a
      href="http://www.w3.org/Addressing/schemes.html">Addressing
      Schemes</a> <a href="#ref-address-schemes">[ADDRESS-SCHEMES]</a>,
      and it is a good idea to consider adapting one of the existing
      schemes for any specialized identification purposes,
      rather than trying to invent a new one.</p>

      <p>No one person or organization controls who makes URIs or how
      they can be used. While some URI schemes, such as URL's
      <code>http:</code>, depend on centralized systems such as DNS, other
      schemes, such as <code>freenet:</code>, are completely decentralized.
      This means that, as with any other kind of name, no one needs
      special authority or permission to create a URI for something.
      Also, anyone can create URIs to refer to things they do not own, just as in
      ordinary language anyone can use whatever name they like for things
      they do not own.</p>

      <p>As also noted in <a href="#basicconcepts">Section 2.1</a>,
      RDF uses <em>URI references</em> <a href="#ref-uri">[URIS]</a> to
      name subjects, predicates, and objects in RDF statements. A URI
      reference (or <em>URIref</em>) is a URI, together with an
      optional <em>fragment identifier</em> at the end. For example,
      the URI reference
      <code>http://www.example.org/index.html#section2</code> consists of
      the URI <code>http://www.example.org/index.html</code> and (separated
      by the "#" character) the fragment identifier
      <code>Section2</code>.

RDF URIrefs can contain
Unicode <a href="#ref-unicode">[UNICODE]</a> characters (see <a href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>), allowing many languages to be reflected in URIrefs.

</p>

      <p>URIrefs may be either <em>absolute</em> or <em>relative</em>.
      An <em>absolute</em> URIref refers to a resource independently of
      the context in which the URIref appears, e.g., the URIref
      <code>http://www.example.org/index.html</code>. A <em>relative</em>
      URIref is a shorthand form of an absolute URIref, where some
      prefix of the URIref is missing, and information from the context
      in which the URIref appears is required to fill in the missing
      information. For example, the relative URIref
      <code>otherpage.html</code>, when appearing in a resource
      <code>http://www.example.org/index.html</code>, would be filled out
      to the absolute URIref
      <code>http://www.example.org/otherpage.html</code>. A URIref without
      a URI part is considered a reference to the current document (the
      document in which it appears). So, an empty URIref within a
      document is considered equivalent to the URIref of the document
      itself. A URIref consisting of just a fragment identifier is
      considered equivalent to the URIref of the document in which it
      appears, with the fragment identifier appended to it. For
      example, within <code>http://www.example.org/index.html</code>, if
      <code>#section2</code> appeared as a URIref, it would be considered
      equivalent to the absolute URIref
      <code>http://www.example.org/index.html#section2</code>.</p>

      <p><a href="#ref-rdf-concepts">[RDF-CONCEPTS]</a> notes that RDF
      graphs (the abstract models) do not use relative URIrefs, i.e.,
      the subjects, predicates, and objects (and datatypes in typed
      literals) in RDF statements must always be identified
      independently of any context. However, a specific concrete RDF
      syntax, such as RDF/XML, may allow relative URIrefs to be used as
      a shorthand for absolute URIrefs in certain situations. RDF/XML
      does permit such use of relative URIrefs, and some of the RDF/XML
      examples in this Primer illustrate such uses. 
      <a href="#ref-rdf-syntax">[RDF-SYNTAX]</a> should be 
      consulted for further details.</p>

      <p>Both RDF and Web browsers use URIrefs to identify things.
      However, RDF and browsers interpret URIrefs in slightly different
      ways. This is because RDF uses URIrefs <em>only</em> to identify
      things, while browsers also use URIrefs to <em>retrieve</em>
      things. Often there is no effective difference, but in some cases
      the difference can be significant. One obvious difference is that when
      a URIref is used in a browser, there is the expectation that it
      identifies a resource that can actually be retrieved: that
      something is actually "at" the location identified by the URI.
      However, in RDF a URIref may be used to identify something, such
      as a person, that <em>cannot</em> be retrieved on the Web. People
      sometimes use RDF together with a convention that, when a URIref
      is used to identify an RDF resource, a page containing
      descriptive information about that resource will be placed on the
      Web "at" that URI, so that the URIref can be used in a browser to
      retrieve that information. This can be a useful convention in
      some circumstances, although it creates a difficulty in
      distinguishing the identity of the original resource from the
      identity of the Web page describing it (a subject discussed
      further in <a href="#structuredproperties">Section 2.3</a>).
      However, this convention is not an explicit part of the
      definition of RDF, and RDF itself does not assume that a URIref
      identifies something that can be retrieved.</p>

      <p>Another difference is in the way URIrefs with fragment
      identifiers are handled. Fragment identifiers are often seen in
      the URLs that identify HTML documents, where they serve to
      identify a specific place within the document identified by the
      URL. In normal HTML usage, where URI references are used to
      retrieve the indicated resources, the two URIrefs:</p>

      <p><code>http://www.example.org/index.html</code><br />
       <code>http://www.example.org/index.html#Section2</code></p>

      <p>are related (they both refer to the same document, the second
      one identifying a location within the first one). However, as
      noted already, RDF uses URI references purely to
      <em>identify</em> resources, not to retrieve them, and RDF
      assumes no particular relationship between these two URIrefs. As
      far as RDF is concerned, they are syntactically different URI
      references, and hence may refer to unrelated things. This
      does not mean that the HTML-defined containment relationship might
      not exist, just that RDF does not assume that a relationship
      exists based only on the fact that the URI parts of the URI
      references are the same.</p>  

<div class="newstuff">
      <p>Carrying this point further, RDF
      does not assume that there is any relationship between URI references
      that share a common leading string, whether there is a fragment
      identifier or not.  For example, as far as RDF is concerned, the
      two URIrefs:</p>

      <p><code>http://www.example.org/foo.html</code><br />
       <code>http://www.example.org/bar.html</code></p>

      <p>have no particular relationship even though both of them start
      with the string <code>http://www.example.org/</code>.  To RDF, they 
      are simply different resources, because their URIrefs are different.
      (They may in fact be two files located in the same directory, but 
      RDF does not assume this or any other relationship exists.)</p>
</div>

    </div>


    <div class="section">
      <h2><a id="documents" name="documents"></a>Appendix B:
      More on the Extensible Markup Language (XML)</h2>

      <p class="newstuff">Note:  This section is intended to provide a 
      brief introduction to XML.  The definitive specification
      of XML is <a href="#ref-xml">[XML]</a>, which should be consulted for 
      further details. </p>

      <p>The <a
      href="http://www.w3.org/TR/2000/REC-xml-20001006">Extensible
      Markup Language</a> <a href="#ref-xml">[XML]</a> was designed to
      allow anyone to design their own document format and then write a
      document in that format. Like HTML documents (Web pages), XML
      documents contain text. This text consists primarily of plain
      text content, and markup in the form of <em>tags</em>. This
      markup allows a processing program to interpret the various
      pieces of content (called <em>elements</em>). 

Both XML content and (with certain exceptions) tags can contain Unicode <a href="#ref-unicode">[UNICODE]</a> characters, allowing information from many languages to be directly represented.

      In HTML, the set of
      permissible tags, and their interpretation, is defined by the
      HTML specification. However, XML allows users to define their own
      markup languages (tags and the structures in which they can
      appear) adapted to their own specific requirements 

      (the RDF/XML language described in <a href="#rdfxml">Section
      3</a> is one such XML markup language).  For example,
      the following is a simple passage marked up using an XML-based
      markup language:</p>

      <div class="exampleOuter exampleInner">
<pre>
&lt;sentence&gt;&lt;person webid="http://example.com/#johnsmith"&gt;I&lt;/person&gt; 
just got a new pet &lt;animal&gt;dog&lt;/animal&gt;.&lt;/sentence&gt;
</pre>
      </div>

      <p>Elements delimited by tags (<code>&lt;sentence&gt;</code>,
      <code>&lt;person&gt;</code>, etc.) are introduced to reflect a
      particular structure associated with the passage. The tags allow
      a program written with an understanding of these particular
      elements, and the way they are structured, to properly interpret
      the passage. For example, one of the elements in this example is
      <code>&lt;animal&gt;dog&lt;/animal&gt;</code>. This consists of the
      <em>start-tag</em> <code>&lt;animal&gt;</code>, the element
      <em>content</em>, and a matching <em>end-tag</em>
      <code>&lt;/animal&gt;</code>. This <code>animal</code> element, together
      with the <code>person</code> element, are nested as part of the
      content of the <code>sentence</code> element. The nesting is possibly
      clearer (and closer to some of the more "structured" XML
      contained in the rest of this Primer) if the sentence is
      written:</p>

      <div class="exampleOuter exampleInner">
<pre>
&lt;sentence&gt;
    &lt;person webid="http://example.com/#johnsmith"&gt;I&lt;/person&gt; 
    just got a new pet 
    &lt;animal&gt;dog&lt;/animal&gt;.
&lt;/sentence&gt;
</pre>
      </div>

      <p>In some cases, an element may have no content. This can be
      written either by enclosing no content within the pair of
      delimiting start- and end-tags, as in
      <code>&lt;animal&gt;&lt;/animal&gt;</code>, or by using a shorthand
      form of tag called an <em>empty-element tag</em>, as in
      <code>&lt;animal/&gt;</code>.</p>

      <p>In some cases, a start-tag (or empty-element tag) may contain
      qualifying information other than the tag name, in the form of
      <em>attributes</em>. For example, the start-tag of the
      <code>&lt;person&gt;</code> element contains the attribute
      <code>webid="http://example.com/#johnsmith"</code> (presumably
      identifying the specific person referred to). An attribute
      consists of a name, an equal sign, and a value (enclosed in
      quotes).</p>

      <p>This particular markup language uses the words "sentence,"
      "person," and "animal" as tag names in an attempt to convey some
      of the meaning of the elements; and they <em>would</em> convey
      meaning to an English-speaking person reading it, or to a program
      specifically written to interpret this vocabulary. However, there
      is no built-in meaning here. For example, to non-English
      speakers, or to a program not written to understand this markup,
      the element <code>&lt;person&gt;</code> may mean absolutely nothing.
      Take the following passage, for example:</p>

      <div class="exampleOuter exampleInner">
<pre>
&lt;dfgre&gt;&lt;reghh bjhbw="http://example.com/#johnsmith"&gt;I&lt;/reghh&gt; 
just got a new pet &lt;yudis&gt;dog&lt;/yudis&gt;.&lt;/dfgre&gt;
</pre>
      </div>

      <p>To a machine, this passage has exactly the same structure as
      the previous example. However, it is no longer clear to an
      English-speaker what is being said, because the tags are no
      longer English words. Moreover, others may have used the same
      words as tags in their own markup languages, but with completely
      different intended meanings. For example, "sentence" in another
      markup language might refer to the amount of time that a
      convicted criminal must serve in a penal institution. So
      additional mechanisms must be provided to help keep XML
      vocabulary straight.</p>

      <p>To prevent confusion, it is necessary to uniquely identify
      markup elements. This is done in XML using <a
      href="http://www.w3.org/TR/REC-xml-names/">XML Namespaces</a> <a
      href="#ref-namespaces">[XML-NS]</a>. A <em>namespace</em> is just
      a way of identifying a part of the Web (space) which acts as a
      qualifier for a specific set of names. A namespace is created for
      an XML markup language by creating a URI for it. By qualifying
      tag names with the URIs of their namespaces, anyone can create
      their own tags and properly distinguish them from tags with
      identical spellings created by others. A convention that is
      sometimes followed is to
      create a Web page to describe the markup language (and the
      intended meaning of the tags) and use the URL of that Web page as
      the URI for its namespace. However, this is just a convention, and
      neither XML nor RDF assumes that a namespace URI identifies a 
      retrievable Web resource.  
      The following example illustrates the
      use of an XML namespace.</p>

      <div class="exampleOuter exampleInner">
<pre>
&lt;user:sentence xmlns:user="http://example.com/xml/documents/"&gt;
   &lt;user:person user:webid="http://example.com/#johnsmith"&gt;I&lt;/user:person&gt; 
just got a new pet &lt;user:animal&gt;dog&lt;/user:animal&gt;.
&lt;/user:sentence&gt;
</pre>
      </div>

      <p>In this example, the attribute
      <code>xmlns:user="http://example.com/xml/documents/"</code> declares a
      namespace for use in this piece of XML. It maps the
      <em>prefix</em> <code>user</code> to the namespace URI
      <code>http://example.com/xml/documents/</code>. The XML content can
      then use <em>qualified names</em> (or <em>QNames</em>) like
      <code>user:person</code> as tags. A QName contains a prefix that
      identifies a namespace, followed by a colon, and then a <em>local
      name</em> for an XML tag or attribute name. By using namespace
      URIs to distinguish specific groups of names, and qualifying tags
      with the URIs of the namespaces they come from, as in this
      example, there is no need to worry about tag names conflicting. Two
      tags having the same spelling are considered the same only if
      they also have the same namespace URIs.</p>

      <p>Every XML document is required to be <dfn>well-formed</dfn>.  
      This means the XML document must satisfy a number of syntactic 
      conditions, for example, that every start-tag must have a matching 
      end-tag, and that elements must be properly nested within other elements 
      (elements may not overlap). The complete set of well-formedness 
      conditions is defined in <a href="#ref-xml">[XML]</a>.</p>  

      <p>In addition, an XML document may optionally include an XML 
      <dfn>document type declaration</dfn> to define additional constraints 
      on the structure of the document, and to support the use of 
      predefined units of text within the document. 

      The document type declaration (introduced with <code>DOCTYPE</code>) 
      contains or points to declarations that define a grammar for the 
      document.  This grammar is known as a <dfn>document type definition</dfn>, 
      or <dfn>DTD</dfn>.  

      The declarations in a DTD specify such things as
      which XML elements and attributes 
      may appear in XML documents corresponding to the DTD, the relationships 
      of these elements and attributes (e.g., which elements can be nested within 
      which other elements, or which attributes may appear with which elements), 
      and whether elements or attributes are required or optional.  

      The document type declaration can point to a set of declarations located
      outside the document (called the <dfn>external subset</dfn>, which can
      be used to allow common declarations to be shared among multiple 
      documents), can include 
      the declarations directly in the document (called the 
      <dfn>internal subset</dfn>), or can have both internal and external
      DTD subsets.  
      The complete DTD for a document consists of both subsets taken together.  
      A simple example of an XML document with a document type declaration 
      is shown in <a href="#example47">Example 47</a>:</p> 

      <div class="exampleOuter">
        <div class="c1">
          <a id="example47" name="example47">Example 47: An XML Document
          With a Document Type Declaration</a>
        </div>

        <div class="exampleInner">
<pre>
&lt;?xml version="1.0"?&gt; 
&lt;!DOCTYPE greeting SYSTEM "http://www.example.org/dtds/hello.dtd"&gt; 
&lt;greeting&gt;Hello, world!&lt;/greeting&gt; 
</pre>
        </div>
      </div>

      <p>In this case, the document has only an external DTD subset, and the
      <dfn>system identifier</dfn> <code>http://www.example.org/dtds/hello.dtd</code> 
      provides its location (a URIref).</p>  

      <p>An XML document 
      is <dfn>valid</dfn> if it has an associated document type declaration 
      and the document complies with the constraints defined by the document 
      type declaration.</p>  


<div class="newstuff">

      <p>An RDF/XML document is only required to be well-formed XML;  it is not 
      intended to be validated against an XML DTD (or an XML Schema), and 
      <a href="#ref-rdf-syntax">[RDF-SYNTAX]</a> does not specify a normative 
      DTD that could be used for validating arbitrary RDF/XML (an appendix of 
      <a href="#ref-rdf-syntax">[RDF-SYNTAX]</a> does provide a non-normative 
      example schema for RDF/XML). As a result, more detailed
      discussion of XML DTD grammars is beyond the scope of this Primer.
      Further information on XML DTDs and XML validation can be found in
      <a href="#ref-xml">[XML]</a>, and the numerous books on XML.</p>

      <p>However, there is one use of XML document type declarations 
      that <em>is</em> relevant to RDF/XML, and that is their use in defining XML
      <em>entities</em>.  An XML entity 
      declaration essentially associates a name with a string of characters.  
      When the entity name is used elsewhere within an XML document, XML 
      processors replace the entity name with the corresponding string. 
      This provides a way to abbreviate long strings such as URIrefs, and can
      help make XML documents containing such strings more readable.   
      Using a document type declaration just to declare
      XML entities is allowed, and can be useful, even when (as in
      RDF/XML) the documents are not intended to be validated.</p>

   
      <p>In RDF/XML documents, entities are generally declared within the 
      document itself, i.e., using only an internal DTD subset (one reason
      for this is that RDF/XML is not intended to be validated, and non-validating
      XML processors are not required to process external DTD subsets).
      For example, providing the document type declaration 
      shown in <a href="#example48">Example 48</a>  
      at the beginning of an RDF/XML document allows the URIrefs in that document
      for the <code>rdf</code>, 
      <code>rdfs</code>, and <code>xsd</code> namespaces to be abbreviated as 
      <code>&amp;rdf;</code>, <code>&amp;rdfs;</code>, and <code>&amp;xsd;</code> respectively,
      as shown in the example.</p>

      <div class="exampleOuter">
        <div class="c1">
          <a id="example48" name="example48">Example 48: Some XML Entity
          Declarations</a>
        </div>

        <div class="exampleInner">
<pre>
&lt;?xml version='1.0'?&gt; 

&lt;!DOCTYPE rdf:RDF [ 
    &lt;!ENTITY rdf "http://www.w3.org/1999/02/22-rdf-syntax-ns#"&gt; 
    &lt;!ENTITY rdfs "http://www.w3.org/2000/01/rdf-schema#"&gt; 
    &lt;!ENTITY xsd "http://www.w3.org/2001/XMLSchema#"&gt; 
]&gt; 

&lt;rdf:RDF 
    xmlns:rdf = "&amp;rdf;" 
    xmlns:rdfs = "&amp;rdfs;" 
    xmlns:xsd = "&amp;xsd;">

...RDF statements...
 
&lt;/rdf:RDF&gt;</pre>
        </div>
      </div>


      </div>

    </div>


    <div class="section">
      <h2><a id="changes" name="changes"></a>Appendix C: Changes</h2>

      <p>Only minor editorial and typographic changes have been made since
      the <a href="http://www.w3.org/TR/2003/PR-rdf-primer-20031215/">
      Proposed Recommendation version</a>.  Older changes are detailed in its
      <a href="http://www.w3.org/TR/2003/PR-rdf-primer-20031215/#changes">
      change log</a>.</p>

   </div>

    <hr />

    <div class="metadata">
      <p><a href="metadata.rdf"><img border="0"
      src="http://www.w3.org/RDF/icons/rdf_metadata_button.40"
      alt="RDF/XML Metadata" /></a></p>
    </div>
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