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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 Working Draft 23 January 2003</h2>

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

        <dd><a
        href="http://www.w3.org/TR/2003/WD-rdf-primer-20030123/">http://www.w3.org/TR/2003/WD-rdf-primer-20030123/</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/2002/WD-rdf-primer-20021111/">http://www.w3.org/TR/2002/WD-rdf-primer-20021111/</a></dd>

        <dt>Editors:</dt>

        <dd>Frank Manola, The MITRE Corporation, <a
        href="mailto:fmanola@mitre.org">fmanola@mitre.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>

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      href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>
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      All Rights Reserved. W3C <a
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    <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. 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 can't be directly
    <em>retrieved</em> on the Web. RDF provides a common framework for
    expressing this information so it can be exchanged between
    applications without loss of meaning.</p>

    <p>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>

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

    <p>This is a W3C <a href="http://www.w3.org/2001/sw/RDFCore/">RDF
    Core Working Group</a> Last Call Working Draft produced as part of
    the W3C <a href="http://www.w3.org/2001/sw/">Semantic Web
    Activity</a> (<a href="http://www.w3.org/2001/sw/Activity">Activity
    Statement</a>).</p>

    <p>This document is in the Last Call review period, which ends on
    21 February 2003. This document has been endorsed by the RDF Core
    Working Group.</p>

    <p>This document incorporates material developed by the Working
    Group designed to provide the reader with the basic knowledge
    required to effectively use RDF in their particular
    applications.</p>

    <p>This document is being released for review by W3C Members and
    other interested parties to encourage feedback and comments,
    especially with regard to how the changes made affect existing
    implementations and content.</p>

    <p>In conformance with <a
    href="http://www.w3.org/Consortium/Process-20010719/#ipr">W3C
    policy</a> requirements, known patent and <acronym
    title="Intellectual Property Rights">IPR</acronym> constraints
    associated with this Working Draft are detailed on the <a
    href="http://www.w3.org/2001/sw/RDFCore/ipr-statements"
    rel="disclosure">RDF Core Working Group Patent Disclosure</a>
    page.</p>

    <p>Comments on this document are invited and should be sent to the
    public mailing list <a
    href="mailto:www-rdf-comments@w3.org">www-rdf-comments@w3.org</a>.
    An archive of comments is available at <a
    href="http://lists.w3.org/Archives/Public/www-rdf-comments/">http://lists.w3.org/Archives/Public/www-rdf-comments/</a>.</p>

    <p>This is a public W3C Last Call Working Draft for review by W3C
    Members and other interested parties. This section describes the
    status of this document at the time of its publication. It is a
    draft document and may be updated, replaced, or obsoleted by other
    documents at any time. It is inappropriate to use W3C Working
    Drafts as reference material or to cite as other than "work in
    progress". A list of current W3C Recommendations and other
    technical documents can be found at <a
    href="/TR/">http://www.w3.org/TR/</a>.</p>

    <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;5. <a href="#rdfschema">Defining RDF Vocabularies: RDF
    Schema</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.1 <a
    href="#schemaclasses">Defining Classes</a><br />
     &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5.2 <a
    href="#properties">Defining 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></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 can't be directly
      <em>retrieved</em> on the Web. Examples include information about
      items available from online 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 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 (URIs), 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 someone 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
        <tt>http://www.w3.org/People/EM/contact#me</tt></li>

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

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

        <li>values of those properties, e.g. <tt>mailto:em@w3.org</tt>
        as the value of the mailbox property (RDF also uses character
        strings such as "Eric Miller" as the values of some
        properties)</li>
      </ul>

      <p>RDF also provides an XML-based syntax (called RDF/XML) 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;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 <tt>mailbox</tt> and <tt>fullName</tt> (in an
      abbreviated form), and their respective values
      <tt>em@w3.org</tt>, and <tt>Eric Miller</tt>.</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, we can also describe cars, businesses, people, news
      events, etc. In addition, RDF properties themselves have URIs, to
      precisely identify the kind of relationship that exists 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 you understand RDF, but they may not always
      provide definitive or fully-complete answers. In such cases, you
      should refer to the relevant normative parts of the RDF
      specification. To help you do this, we provide links pointing to
      the relevant parts of the normative 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. In this section, we
      describe 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 that we want to state the fact that someone named
        John Smith created a particular Web page. A straightforward way
        to state this in English would be in the form of a simple
        statement such as:</p>

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

        <p>We've underlined parts of this statement to illustrate that,
        in order to describe the properties of something, we need ways
        to name, or identify, a number of things:</p>

        <ul>
          <li>We need a way to identify the thing we want to describe
          (the Web page, in this case)</li>

          <li>We need a way to identify a specific property (creator,
          in this case) of the thing that we want to describe</li>

          <li>We need a way to identify the thing we want to assign as
          the value of this property (who the creator is), for the
          thing we want to describe</li>
        </ul>

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

        <p>We could state other properties of this Web page 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, to
        specify the date the page was created, and the language in
        which the page is written, we could write the additional
        statements:</p>

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

        <p>RDF is based on the idea that the things we want to describe
        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"><tt><u>http://www.example.org/index.html</u>
        has a <u>creator</u> whose value is <u>John Smith</u></tt></p>

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

        <ul>
          <li>the <dfn>subject</dfn> is the URL
          <tt>http://www.example.org/index.html</tt></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, we need two
        things:</p>

        <ul>
          <li>a system of machine-processable identifiers that allows
          us to identify 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 we've seen, the Web already provides one form of
        identifier, the <dfn>Uniform Resource Locator</dfn> (URL). We
        used a URL in our 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, we
        would also like to be able to record information about many
        things that, unlike Web pages, don't 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, we can create a URI to refer to anything we want to talk
        about, 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 don't physically exist, like 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 <tt>http://www.example.org/index.html#section2</tt>
        consists of the URI <tt>http://www.example.org/index.html</tt>
        and (separated by the "#" character) the fragment identifier
        <tt>Section2</tt>. 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 <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/1998/REC-xml-19980210.html">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
        <tt>&lt;contact:fullName&gt;</tt> and
        <tt>&lt;contact:personalTitle&gt;</tt> to delimit the text
        content <tt>Eric Miller</tt> and <tt>Dr.</tt>, respectively.
        Such tags allow programs written with an understanding of what
        the tags mean to property interpret that content. <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>.</p>
      </div>

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

        <p>Now that we've introduced RDF's basic statement concepts,
        URI references for identifying things we want to talk about on
        the Web, and RDF/XML as a machine-processable way of
        representing RDF statements, we can describe how RDF lets us
        use URIs to make statements about resources. In the
        introduction, we said that RDF was based on the idea of
        expressing simple statements about resources, where those
        statements are built using subjects, predicates, and objects.
        In RDF, we could represent our original English statement:</p>

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

        <p>by an RDF statement having:</p>

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

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

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

        <p>Note how we have used URIrefs 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. We'll discuss this further 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, labeled with its URIref</li>

          <li>a node for the object, labeled with its URIref</li>

          <li>an arc for the predicate, labeled with its URIref,
          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 if we wanted to also represent the
        additional statements</p>

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

        <p>we could, by using suitable URIrefs to name the properties
        "creation-date" and "language", use the graph shown in <a
        href="#figure3">Figure 3</a>:</p>

        <div class="figure">
          <img src="fig3dec16.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 the objects
        of RDF statements may be either resources identified by
        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. Literals may not be the subjects or
        predicates of RDF statements. (The simple character string
        literals we will use for now 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> we will introduce 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>.) In drawing RDF
        graphs, nodes that represent resources identified by URIrefs
        are shown as ellipses, while nodes that represent literals are
        shown as boxes (labeled by the literal itself).</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 node labels (either URIref or literal), in that order.
        The triples representing the three statements shown in <a
        href="#figure3">Figure 3</a> would be written in full 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://www.example.org/terms/language&gt; "English" .
</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, <tt>http://www.example.org/index.html</tt>
        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. For
        convenience, we will use a shorthand way of writing triples in
        the rest of this Primer, and also in other RDF specifications.
        In this shorthand, we can substitute a <em>qualified name</em>
        (or <em>QName</em>) without angle brackets as an abbreviation
        of a full URI reference. A QName contains a prefix that has
        been assigned to a namespace URI, followed by a colon, and then
        a <em>local name</em> (QNames are discussed further in <a
        href="#documents">Appendix B</a>). So, for example, if the
        QName prefix <tt>foo</tt> is assigned to the namespace URI
        <tt>http://example.org/somewhere/</tt>, then the QName
        <tt>foo:bar</tt> is shorthand for the URIref
        <tt>http://example.org/somewhere/bar</tt>. We will also make
        extensive use in these examples of several "well-known" QName
        prefixes (which we will use without explicitly specifying them
        each time), defined as follows:<br />
        <br />
         prefix <tt>rdf:</tt>, namespace URI:
        <tt>http://www.w3.org/1999/02/22-rdf-syntax-ns#</tt><br />
         prefix <tt>rdfs:</tt>, namespace URI:
        <tt>http://www.w3.org/2000/01/rdf-schema#</tt><br />
         prefix <tt>dc:</tt>, namespace URI:
        <tt>http://purl.org/dc/elements/1.1/</tt><br />
         prefix <tt>daml:</tt>, namespace URI:
        <tt>http://www.daml.org/2001/03/daml+oil#</tt><br />
         prefix <tt>ex:</tt>, namespace URI:
        <tt>http://www.example.org/</tt> (or
        <tt>http://www.example.com/</tt>)<br />
         prefix <tt>xsd:</tt>, namespace URI:
        <tt>http://www.w3.org/2001/XMLSchema#</tt></p>

        <p>We will also use variations on the "example" prefix
        <tt>ex:</tt> as needed in the examples, where this will not
        cause confusion, for example,<br />
        <br />
         prefix <tt>exterms:</tt>, namespace URI:
        <tt>http://www.example.org/terms/</tt> (for terms used by our
        example organization),<br />
         prefix <tt>exstaff:</tt>, namespace URI:
        <tt>http://www.example.org/staffid/</tt> (for our example
        organization's staff identifiers),<br />
         prefix <tt>ex2:</tt>, namespace URI:
        <tt>http://www.domain2.example.org/</tt> (for a second example
        organization), and so on.</p>

        <p>Using our new shorthand, we can write the previous set of
        triples 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 exterms:language "English" .
</pre>
        </div>

        <p>The examples we've just given of RDF statements begin to
        illustrate some of the advantages of using URIrefs as RDF's
        basic way of identifying things. For instance, instead of
        identifying the creator of the Web page in our first example by
        the character string "John Smith", we've assigned him a URIref,
        in this case (using a URIref based on his employee number)
        <tt>http://www.example.org/staffid/85740</tt> . An advantage of
        using a URIref in this case is that we can be more precise in
        our identification. That is, the creator of the page isn't 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 we have a URIref for the
        creator of the page, it is a full-fledged resource, and we can
        record additional information about him, such as his name, and
        age, as in the graph shown in <a href="#figure4">Figure
        4</a>:</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 allows us to distinguish the properties we use from
        properties someone else may use that would otherwise be
        identified by the same character string. For instance, in our
        example, 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
        wouldn't necessarily be able to distinguish these uses.
        However, if example.org writes
        <tt>http://www.example.org/terms/name</tt> for its "name"
        property, and the other person writes
        <tt>http://www.domain2.example.org/genealogy/terms/name</tt>
        for hers, we can keep straight the fact that there are distinct
        properties involved (even if a program cannot automatically
        determine the distinct meanings). Another reason why it is
        important to use URIrefs to identify properties is that it
        allows us to treat RDF properties as resources themselves.
        Since properties are resources, we can record descriptive
        information 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 allows us to begin to develop and use a shared
        vocabulary on the Web, reflecting (and creating) a shared
        understanding of the concepts we talk about. For example, in
        the triple</p>

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

        <p>the predicate <tt>dc:creator</tt>, 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
        <tt>http://www.example.org/index.html</tt> ) and the creator of
        the page (a distinct person, identified by
        <tt>http://www.example.org/staffid/85740</tt> ) is exactly the
        concept identified by
        <tt>http://purl.org/dc/elements/1.1/creator</tt> . Moreover,
        anyone else, or any program, that understands
        <tt>http://purl.org/dc/elements/1.1/creator</tt> will know
        exactly what is meant by this relationship.</p>

        <p>Of course, RDF's use of URIrefs doesn't solve all our
        problems because, for example, people can still use different
        URIrefs to refer to the same thing. However, the fact that
        these different URIrefs are used in the commonly-accessible
        "Web space" creates the opportunity both to identify
        equivalences among these different references, and to migrate
        toward the use of common references.</p>

        <p>The result of all this is that RDF provides a way to make
        statements that applications can more easily process. Now an
        application can't actually "understand" such statements, of
        course, but it can deal with them in a way that makes it seem
        like it does. 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
        we had to record about things were obviously in the form of the
        simple RDF statements we've illustrated so far. However, most
        real-world data involves structures that are more complicated
        than that, at least on the surface. For instance, in our
        original example, we recorded the date the Web page was created
        as a single <tt>exterms:creation-date</tt> property, with a
        plain literal as its value. However, suppose we wanted to show,
        as the value of the <tt>exterms:creation-date</tt> property,
        the month, day, and year as separate pieces of information? Or,
        in the case of John Smith's personal information, suppose we
        wanted to record his address. We might write the whole address
        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 we wanted to record John's address as a
        <em>structure</em> consisting of separate street, city, state,
        and Zip code values? How do we do this in RDF?</p>

        <p>We can represent such structured information in RDF by
        considering the aggregate thing we want to talk about (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, we
        create a new node to represent the concept of John Smith's
        address, and assign that concept a new URIref to identify it,
        say <tt>http://www.example.org/addressid/85740</tt> (which we
        will abbreviate as <tt>exaddressid:85740</tt>). We then write
        RDF statements (create additional arcs and nodes) 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="fig5dec16.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:Zip      "01730" .
</pre>
        </div>

        <p>Using this approach allows us to represent structured
        information in RDF, but it can involve generating numerous
        "intermediate" URIrefs 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>, we didn't
        really need the URIref we assigned to identify "John Smith's
        address", since we could just as easily have drawn the graph as
        in <a href="#figure6">Figure 6</a>:</p>

        <div class="figure">
          <img src="fig6dec16.png" alt="Using a Blank Node" /><br />
           <a id="figure6" name="figure6">Figure 6: Using a Blank
          Node</a>
        </div>

        <p>In <a href="#figure6">Figure 6</a>, which is a perfectly
        good RDF graph, we've used a node without a label to stand for
        the concept of "John Smith's address". This unlabeled node, or
        <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, we would need some
        form of explicit identifier for that node if we wanted to
        represent this graph as triples. To see this, we can try to
        write the triples corresponding to what is shown in <a
        href="#figure6">Figure 6</a>. What we would get would be
        something like:</p>

        <div class="exampleOuter exampleInner">
<pre>
exstaff:85740  exterms:address  ??? .
???            exterms:street   "1501 Grant Avenue" .
???            exterms:city     "Bedford" .
???            exterms:state    "Massachusetts" .
???            exterms:Zip      "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, we would also need a way to differentiate
        between these multiple blank nodes in a triples representation
        of the graph. To do this, we use <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-blank-node-id">blank
        node identifiers</a>, having the form <tt>_:name</tt>, to
        indicate the presence of blank nodes in triples. For instance,
        in this example we might use the blank node identifier
        <tt>_:johnaddress</tt> 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:Zip      "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.</p>

        <p>At the beginning of this section, we noted that we can
        represent aggregate structures, like John Smith's address, by
        considering the aggregate thing we want to talk about as a
        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. When we try to represent the
        relationship between John and the group of separate
        <em>components</em> of this address, we are dealing with an
        <em>n-ary</em> (n-way) relationship (in this case, n=5) between
        John and the street, city, state, and zip components. In order
        to represent such structures directly in RDF (e.g., considering
        the address as a group of street, city, state, and zip
        sub-components), we need to break this n-way relationship up
        into a group of separate binary relationships. Blank nodes give
        us one way to do this. Each time we have an n-ary relationship,
        we can choose one of the participants as the subject of the
        relationship (John in this case), and create a blank node to
        represent the rest of the relationship (John's address in this
        case). We can then represent the remaining participants in the
        relationship (such as the city in our example) as separate
        properties of the new resource represented by the blank
        node.</p>

        <p>Blank nodes also give us 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.,
        <tt>mailto:jane@example.org</tt>. However, this approach can
        cause problems. For example, we may want to record information
        about Jane's mailbox (e.g., the server it is on) as well as
        about Jane herself (e.g., her current address), and using a
        URIref for Jane based on her email address makes it difficult
        to know which thing we're talking about. The same problem
        exists when a company's Web page URL, say
        <tt>http://www.example.com/</tt>, is used as the URI of the
        company itself. Once again, we may need to record information
        about the Web page (e.g., who created it and when) as well as
        about the company, and using <tt>http://www.example.com/</tt>
        as an identifier for both makes it difficult to know which
        thing we're talking about.</p>

        <p>The fundamental problem is that using Jane's
        <em>mailbox</em> as a stand-in for <em>Jane</em> isn't really
        accurate: Jane and her mailbox are not the same thing, and
        hence their identifiers should be different. When Jane herself
        doesn't have a URI, a blank node gives us a more accurate way
        of modeling this situation. We can represent Jane by a blank
        node, and give the blank node an <tt>exterms:mailbox</tt>
        property having the URIref <tt>mailto:jane@example.org</tt> as
        its value. We can also assign the blank node an
        <tt>rdf:type</tt> property with a value of
        <tt>exterms:Person</tt> (we will discuss types in more detail
        in the following sections), an <tt>exterms:name</tt> property
        with a value of <tt>"Jane Smith"</tt>, and any other
        descriptive information we might want to provide, as shown in
        the following triples:</p>

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

        <p>This says, accurately, that "there is a resource of type
        <tt>exterms:Person</tt>, whose electronic mailbox is identified
        by <tt>mailto:jane@example.org</tt>, whose name is <tt>Jane
        Smith</tt>, 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 doesn't change the way we actually handle this kind of
        information very much. For example, if we know independently
        that an email address uniquely identifies someone at
        example.org (particularly if the address is unlikely to be
        reused), we can still use that fact to associate information
        about that person from multiple sources, even though the email
        address is not the person's URI. For example, if we were to
        find another piece of RDF on the web that described a book, and
        gives the author's contact information as
        <tt>mailto:jane@example.org</tt>, we might reasonably conclude
        that the author's name is Jane Smith. The point is that saying
        something like "the author of the book is
        <tt>mailto:jane@example.org</tt>" is typically a shorthand for
        "the author of the book is someone whose mailbox is
        <tt>mailto:jane@example.org</tt>". 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 unique identifiers. This is discussed further in
        <a href="#richerschemas">Section 5.5</a>.)</p>
      </div>

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

        <p>In the last section, we described how to handle situations
        in which we needed to take property values represented by plain
        literals, and break them up into structured values that
        identify the individual parts of those property values. Using
        this approach, instead of, say, recording the date a Web page
        was created as a single <tt>exterms:creation-date</tt>
        property, with a single plain literal as its value, we could
        represent the value as a structure consisting of the month,
        day, and year as separate pieces of information. However, so
        far, we've followed the practice of representing any constant
        values that serve as objects in RDF statements by these plain
        (untyped) literals, even when we probably intend for the value
        of the property to be a number (e.g., the value of a
        <tt>year</tt> or <tt>age</tt> property) or some other kind of
        more specialized value.</p>

        <p>For example, in <a href="#figure4">Figure 4</a> we
        illustrated an RDF graph recording information about John
        Smith. In that graph, we recorded the value of John Smith's
        <tt>exterms:age</tt> 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, our 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". However, an application reading that literal
        "27" would only know to do that if the application was
        explicitly given the information that the literal "27" was
        intended to represent a number, and knew which number the
        literal "27" was supposed to represent. The common practice in
        programming languages or database systems is to provide this
        kind of information by associating a <em>datatype</em> with the
        literal, in this case, a datatype like <tt>decimal</tt> or
        <tt>integer</tt>. 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
        <tt>integer</tt>, <tt>binary</tt>, or <tt>string</tt>. 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>Using a typed literal, we could describe John Smith's age 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 our 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="fig8dec16.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, we recorded the
        value of the page's <tt>exterms:creation-date</tt> property as
        the plain literal "August 16, 1999". However, using a typed
        literal, we could describe the creation date of the Web page 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="fig9dec16.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>As these examples illustrate, an RDF typed literal is formed
        by explicitly pairing a URIref identifying a particular
        datatype (in these examples, the datatypes <tt>integer</tt> and
        <tt>date</tt> from <a
        href="http://www.w3.org/TR/xmlschema-2/">XML Schema Part 2:
        Datatypes</a> <a href="#ref-xmlschema2">[XML-SCHEMA2]</a>) with
        a literal that the datatype uses to represent the intended
        value. In each case, this results in a single node in the RDF
        graph with the pair as its label.</p>

        <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. Instead, it
        relies on datatypes defined elsewhere that can be identified by
        a <a
        href="http://www.w3.org/TR/rdf-concepts/#dfn-datatype-URI">datatype
        URI</a>. RDF typed literals simply provide a way to explicitly
        indicate, for a given literal, what datatype should be used to
        interpret it. As far as RDF is concerned, you can write any
        pair of URIref and literal you want as a typed literal. This
        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 with different datatype
        systems, but RDF would impose no extra conversions into and out
        of a native set of RDF types.)</p>

        <p>The actual interpretation of a typed literal (determining
        the value it denotes) must be performed by an RDF processor
        that is programmed to "understand" that datatype. In
        particular, we've used XML Schema datatypes in the two examples
        we've just presented, and will be using XML Schema datatypes in
        most of our other examples as well (for one thing, XML Schema
        data types have URIrefs we can use to refer to them, specified
        in <a href="#ref-xmlschema2">[XML-SCHEMA2]</a>). XML Schema
        datatypes have a "first among equals" status in RDF. They 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 many RDF processors will be
        programmed to recognize these datatypes. However, RDF software
        could be programmed to process other sets of datatypes as
        well.</p>

        <p>RDF datatype concepts also borrow a conceptual framework
        from XML Schema datatypes <a
        href="#ref-xmlschema2">[XML-SCHEMA2]</a> to more precisely
        describe datatype requirements. RDF's use of this framework is
        defined in <a href="http://www.w3.org/TR/rdf-concepts/">RDF
        Concepts and Abstract Syntax</a> <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a>.</p>

        <p>The flexibility provided by RDF typed literals comes at a
        price. For one thing, RDF has no way of knowing whether or not
        a URIref 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. This validity can only be determined
        by software built to understand that datatype. For example, you
        could write 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="fig10dec16.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>The typed literal in <a href="#figure10">Figure 10</a> is
        valid RDF, but obviously an error as far as the
        <tt>xsd:integer</tt> datatype is concerned, since "pumpkin" is
        not defined as being a legal literal for
        <tt>xsd:integer</tt>.</p>

        <p>In general, RDF software may be called on to process RDF
        data that contains datatypes that it has not been programmed to
        understand, in which case there are some things the software
        will not be able to do. This includes recognizing whether or
        not a particular string represents a legal value for a
        particular datatype. In this case, RDF software not built to
        understand the <tt>xsd:integer</tt> datatype would not be able
        to recognize that "pumpkin" is not a valid
        <tt>xsd:integer</tt>.</p>
      </div>

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

        <p>Taken as a whole, RDF is 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 the <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. Together with the <a
        href="http://www.w3.org/TR/rdf-mt/">RDF Semantics</a> <a
        href="#ref-rdf-semantics">[RDF-SEMANTICS]</a> document, <a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a> provides the
        definition of the abstract syntax for RDF, together with its
        formal semantics (meaning).</p>

        <p>However, in addition to the basic techniques for
        representing RDF statements in diagrams (or triples) we've seen
        so far, it should be clear that we also need a way for people
        to define the <em>vocabularies</em> they intend to use in those
        statements, including:</p>

        <ul>
          <li>defining types of things (like <tt>ex:Person</tt>)</li>

          <li>defining properties (like <tt>ex:age</tt> and
          <tt>ex:creation-date</tt>), and</li>

          <li>defining 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 <tt>ex:age</tt>
          property should always be an <tt>xsd:integer</tt>).</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 we 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>We can illustrate the basic ideas behind the RDF/XML syntax
        using some of the examples we've presented already. Suppose we
        want to represent one of our initial statements:</p>

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

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

        <div class="figure">
          <img src="fig11dec16.png"
          alt="A Simple RDF Statement" /><br />
           <a id="figure11" name="figure11">Figure 11: A Simple RDF
          Statement</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><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 a
            Simple RDF Statement</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>(we have added line numbers to use in explaining the
        example).</p>

        <p>This seems like a lot of overhead. We can understand better
        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, <tt>&lt;?xml version="1.0"?&gt;</tt>, is the XML
        declaration, which indicates that the following content is XML,
        and what version of XML it is.</p>

        <p>Line 2 begins an <tt>rdf:RDF</tt> element. This indicates
        that the following XML content (starting here and ending with
        the <tt>&lt;/rdf:RDF&gt;</tt> in Line 7) is intended to
        represent RDF. Following the <tt>rdf:RDF</tt> on this same line
        is an XML namespace declaration, represented as an
        <tt>xmlns</tt> attribute of the <tt>rdf:RDF</tt> start-tag.
        This declaration specifies that all tags in this content
        prefixed with <tt>rdf:</tt> are part of the namespace
        identified by the URIref
        <tt>http://www.w3.org/1999/02/22-rdf-syntax-ns#</tt>. This
        namespace is the source for the RDF-specific terms used in
        RDF/XML.</p>

        <p>Line 3 specifies another XML namespace declaration, this
        time for the prefix <tt>exterms:</tt>. This is expressed as
        another <tt>xmlns</tt> attribute of the <tt>rdf:RDF</tt>
        element, and specifies that the namespace URIref
        <tt>http://www.example.org/terms/</tt> is to be associated with
        the <tt>exterms:</tt> prefix. This namespace is the source for
        the specific terms defined by our example organization,
        example.org. The "&gt;" at the end of line 3 indicates the end
        of the <tt>rdf:RDF</tt> start-tag. Lines 1-3 are general
        "housekeeping" necessary to indicate that we are defining
        RDF/XML content, and to identify the sources of the terms we
        are using.</p>

        <p>Lines 4-6 provide the RDF/XML for the specific statement
        we're representing. An obvious way to talk about any RDF
        statement is to say it's a <em>description</em>, and that it's
        <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 <tt>rdf:Description</tt>
        start-tag in Line 4 indicates that we're starting 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 <tt>rdf:about</tt> attribute to specify
        the URIref of the subject resource. Line 5 provides a
        <em>property element</em>, with the QName
        <tt>&lt;exterms:creation-date&gt;</tt> as its tag, to hold the
        plain literal <tt>August 19, 1999</tt> of the creation-date
        property of the statement. It is nested within the containing
        <tt>rdf:Description</tt> element, indicating that this property
        applies to the resource specified in the <tt>rdf:about</tt>
        attribute of the <tt>rdf:Description</tt> element. The URIref
        of the creation-date property corresponding to the QName
        <tt>&lt;exterms:creation-date&gt;</tt> is obtained by appending
        the name <tt>creation-date</tt> to the URIref of the
        <tt>exterms:</tt> prefix
        (<tt>http://www.example.org/terms/</tt>), giving
        <tt>http://www.example.org/terms/creation-date</tt>. Line 6
        indicates the end of this particular <tt>rdf:Description</tt>
        element.</p>

        <p>Finally, Line 7 indicates the end of the <tt>rdf:RDF</tt>
        element started on Line 2.</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 URIref
        labels for properties and object 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 are chosen so that concatenating them forms the
        URIref of the original node. The URIrefs of subject nodes are
        written as XML attribute values. The nodes labeled by literals
        (which are always object nodes) become element text content or
        attribute values. (All these options are described in <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>).</p>

        <p>We could represent an RDF graph consisting of multiple
        statements 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, if we wanted to write the following two
        statements:</p>

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

        <p>we could write the RDF/XML in <a href="#example3">Example
        3</a>:</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: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:Description rdf:about="http://www.example.org/index.html"&gt;
8.        &lt;exterms:language&gt;English&lt;/exterms:language&gt;
9.    &lt;/rdf:Description&gt;

10. &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 lines 7-9,
        a second <tt>rdf:Description</tt> element to represent the
        second statement. We could represent an arbitrary number of
        additional statements in the same way, using a separate
        <tt>rdf:Description</tt> 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 <tt>ex:index.html</tt> is the subject
        of several statements. To handle such cases, RDF/XML allows
        multiple property elements representing those properties to be
        nested within the <tt>rdf:Description</tt> element that
        identifies the subject resource. For example, if we wanted to
        represent the following group of statements about
        http://www.example.org/index.html:</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  exterms:language "English" .
</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="fig3dec16.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>we could write the RDF/XML as shown in <a
        href="#example4">Example 4</a>:</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;exterms:language&gt;English&lt;/exterms: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 namespace
        declaration (in Line 3), and an additional <tt>creator</tt>
        property element (in Line 8). In addition, we've nested the
        property elements for the three properties whose subject is
        <tt>http://www.example.org/index.html</tt> within a single
        <tt>rdf:Description</tt> element identifying that subject,
        rather than writing a separate <tt>rdf:Description</tt> element
        for each statement.</p>

        <p>Line 8 also introduces a new form of property element. (The
        element tag also uses a different namespace prefix, the new
        namespace prefix <tt>dc:</tt> we defined in Line 3.) The
        <tt>exterms:language</tt> element in Line 7 is similar to the
        <tt>exterms:creation-date</tt> element we defined in <a
        href="#example2">Example 2</a>. Both these elements represent
        properties with plain literals as property values, and such
        elements are specified by enclosing the literal within start-
        and end-tags corresponding to the property name. However, the
        <tt>dc:creator</tt> element on Line 8 represents a property
        whose value is <em>another resource</em>, rather than a
        literal. If we had written the URIref of this resource as a
        plain literal within start- and end-tags in the same way as we
        wrote the literal values of the other elements, we would be
        saying that the value of the <tt>dc:creator</tt> element was
        the <em>character string</em>
        <tt>http://www.example.org/staffid/85740</tt>, rather than the
        resource identified by that literal interpreted as a URIref. In
        order to indicate the difference, we've written the
        <tt>dc:creator</tt> element using what XML calls an
        <em>empty-element tag</em> (it has no separate end-tag), and
        defined the property value using an <tt>rdf:resource</tt>
        attribute within that empty element. The <tt>rdf:resource</tt>
        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
        that we write out the URIref, rather than abbreviating it as a
        QName, as we've done in writing element and attribute
        <em>names</em>.</p>

        <p>It is important to understand that the RDF/XML in the <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:</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;exterms:language&gt;English&lt;/exterms: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>We will describe a few additional RDF/XML abbreviations in
        the following sections. However, you should consult <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a> for a more thorough
        description of the abbreviations that are available.</p>

        <p>RDF/XML also allows us to represent graphs that include
        nodes that have no URIrefs, i.e., <em>blank nodes</em>. 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 we 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 blank nodes.
        These are described in <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>. The approach we will
        illustrate here, which is the most direct approach, is to
        assign a <em>blank node identifier</em> to the 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 <tt>rdf:nodeID</tt> attribute
        with a blank node identifier as its value in places where the
        URIref of a resource node would otherwise appear. Specifically,
        a statement with a blank node as its subject can be written in
        RDF/XML using an <tt>rdf:Description</tt> element which
        specifies an <tt>rdf:nodeID</tt> attribute instead of an
        <tt>rdf:about</tt> attribute. Similarly, a statement with a
        blank node as its object can be written using a property
        element with an <tt>rdf:nodeID</tt> attribute instead of an
        <tt>rdf:resource</tt> attribute. Using <tt>rdf:nodeID</tt>, <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 <tt>abc</tt> 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
        <tt>exterms:editor</tt> 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> we described in <a
        href="#typedliterals">Section 2.4</a> may be used as property
        values instead of the plain literals we have used in the
        examples so far. A typed literal is represented in RDF/XML by
        adding an <tt>rdf:datatype</tt> attribute specifying a datatype
        URIref to the property element containing the literal.</p>

        <p>For example, to change the statement from <a
        href="#example2">Example 2</a> to use a typed literal instead
        of a plain literal for the <tt>creation-date</tt> 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 <tt>ex:creation-date</tt>
        property element by adding an <tt>rdf:datatype</tt> 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 <tt>date</tt> datatype. Since this
        is an attribute value, the URIref must be written out, rather
        than using the QName abbreviation <tt>xsd:date</tt> that we
        used in the triple. A literal appropriate to this datatype is
        then written as the element content, in this case, the literal
        <tt>1999-08-16</tt>, which is the literal representation for
        August 16, 1999 in the XML Schema <tt>date</tt> datatype.</p>

        <p>For the most part, we will continue to use plain (untyped)
        literals in our examples. However, you should be aware that
        typed literals from appropriate datatypes, such as XML Schema
        datatypes, can always be used instead.</p>

        <p>Although additional abbreviated forms for writing RDF/XML
        are available, the facilities we have illustrated so far
        provide a simple but general way to serialize 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 serialization
        approaches described in <a
        href="#ref-rdf-syntax">[RDF-SYNTAX]</a>, this simple
        serialization 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, we've been describing resources that we imagine have
        been given URIrefs already. For instance, in our initial
        examples, we provided descriptive information about
        example.org's web page, whose URIref was
        http://www.example.org/index.html. We referred to this resource
        using an <tt>rdf:about</tt> attribute citing its full URIref.
        Although RDF doesn't specify or control how URIrefs are
        assigned to resources, sometimes we want to achieve the
        <em>effect</em> 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) <tt>http://www.example.com/2002/04/products</tt>. 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="#example8">Example 8</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example8" name="example8">Example 8: RDF/XML for
            example.com's Catalog</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.com/terms/"&gt;

4.     &lt;rdf:Description rdf:ID="item10245"&gt;
5.          &lt;exterms:model&gt;Overnighter&lt;/exterms:model&gt;
6.          &lt;exterms:sleeps&gt;2&lt;/exterms:sleeps&gt;
7.          &lt;exterms:weight&gt;2.4&lt;/exterms:weight&gt;
8.          &lt;exterms:packedSize&gt;14x56&lt;/exterms:packedSize&gt;
9.     &lt;/rdf:Description&gt;

  ...other product descriptions...

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

        <p>(We've included the surrounding xml, RDF, and namespace
        information in lines 1 through 3, and line 10, but this
        information would only need to be defined once for the whole
        catalog, not repeated for each entry in the catalog).</p>

        <p><a href="#example8">Example 8</a> is similar to our previous
        examples in the way it represents the properties (model,
        sleeping capacity, weight) of the resource (the tent) being
        described. However, in line 4, the <tt>rdf:Description</tt>
        element has an <tt>rdf:ID</tt> attribute instead of an
        <tt>rdf:about</tt> attribute. Using <tt>rdf:ID</tt> indicates
        that we are using a <em>fragment identifier</em>, given by the
        value of the <tt>rdf:ID</tt> attribute (<tt>item10245</tt> in
        this case, which might be the catalog number assigned by
        example.com), as an abbreviation of the complete URIref of the
        resource we are describing. The fragment identifier
        <tt>item10245</tt> 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 <tt>#</tt> (to
        indicate that what follows is a fragment identifier) and then
        <tt>item10245</tt> to it, giving the absolute URIref
        <tt>http://www.example.com/2002/04/products#item10245</tt>.</p>

        <p>The <tt>rdf:ID</tt> attribute is somewhat similar to the ID
        attribute in XML and HTML, in that it defines a name which must
        be unique within the document (in this case, the catalog) in
        which it is defined. In this case, the <tt>rdf:ID</tt>
        attribute appears to be assigning a name (<tt>item10245</tt>)
        to this particular kind of tent. Any other RDF/XML within this
        catalog could refer to the tent by using the relative URIref
        <tt>#item10245</tt> in an <tt>rdf:about</tt> attribute. This
        would be understood as being a URIref defined relative to the
        base URIref of the catalog. Using a similar abbreviation, we
        could also have given the URIref of the tent by specifying
        <tt>rdf:about="#item10245"</tt> in the catalog entry (i.e., by
        specifying the relative URIref directly) instead of
        <tt>rdf:ID="item10245"</tt> . The two forms are essentially
        synonyms: the full URIref formed by RDF/XML is the same in
        either case:
        <tt>http://www.example.com/2002/04/products#item10245</tt>. In
        either case, 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, you can think of this use of a relative URIref as
        either 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 <tt>#item10245</tt> of the tent to the base URI
        of the catalog, forming the absolute URIref
        <tt>http://www.example.com/2002/04/products#item10245</tt>. 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="#example8">Example 8</a> might then be represented on
        exampleRatings.com's web site as shown in <a
        href="#example9">Example 9</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example9" name="example9">Example 9:
            exampleRatings.com's Rating of the Tent</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:sportex="http://www.exampleRatings.com/terms/"&gt;

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

        <p>In <a href="#example9">Example 9</a>, line 4 uses an
        <tt>rdf:Description</tt> element with an <tt>rdf:about</tt>
        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 doesn't 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 isn't in an entry in this catalog, it's 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, which is that anyone
        should be able say anything they want about existing resources
        <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 resources 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
        either 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, by creating a separate document, and providing the
        additional properties and values in <tt>rdf:Description</tt>
        elements that refer to the original resource via its URIref
        using <tt>rdf:about</tt>.</p>

        <p>The discussion above indicated that fragment identifiers
        such as <tt>#item10245</tt> 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 fragment identifier 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
        <tt>http://www.example.com/2002/04/products</tt>, example.org
        wanted to provide a duplicate catalog on a mirror site, say at
        <tt>http://mirror.example.com/2002/04/products</tt>. This could
        create a problem, since if the catalog was accessed from the
        mirror site, the URIref for our example tent would be generated
        from the URI of the containing document, forming
        <tt>http://mirror.example.com/2002/04/products#item10245</tt>,
        rather than
        <tt>http://www.example.com/2002/04/products#item10245</tt>, 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 without 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="#example10">Example 10</a> shows how
        we would define the catalog using XML Base:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example10" name="example10">Example 10: Using XML
            Base in example.com's Catalog</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.com/terms/"
4.               xml:base="http://www.example.com/2002/04/products"&gt;

5.     &lt;rdf:Description rdf:ID="item10245"&gt;
6.          &lt;exterms:model&gt;Overnighter&lt;/exterms:model&gt;
7.          &lt;exterms:sleeps&gt;2&lt;/exterms:sleeps&gt;
8.          &lt;exterms:weight&gt;2.4&lt;/exterms:weight&gt;
9.          &lt;exterms:packedSize&gt;14x56&lt;/exterms:packedSize&gt;
10.    &lt;/rdf:Description&gt;

  ...other product descriptions...

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

        <p>In <a href="#example10">Example 10</a>, the
        <tt>xml:base</tt> declaration in line 4 specifies that the base
        URI for the content within the <tt>rdf:RDF</tt> element (until
        another <tt>xml:base</tt> attribute is specified) is
        <tt>http://www.example.com/2002/04/products</tt>, 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 our tent,
        <tt>#item10245</tt>, will be interpreted as the same absolute
        URIref,
        <tt>http://www.example.com/2002/04/products#item10245</tt>, 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, we've been talking about 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, <tt>rdf:type</tt>. When an
        RDF resource is described with an <tt>rdf:type</tt> 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
        <tt>rdf:type</tt>, <a href="#example11">Example 11</a> shows
        how example.com might indicate that our product description is
        that of a tent:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example11" name="example11">Example 11: Describing a
            Tent with <tt>rdf:type</tt></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.com/terms/"
4.               xml:base="http://www.example.com/2002/04/products"&gt;

5.     &lt;rdf:Description rdf:ID="item10245"&gt;
6.          &lt;rdf:type rdf:resource="http://www.example.com/terms/Tent" /&gt;
7.          &lt;exterms:model&gt;Overnighter&lt;/exterms:model&gt;
8.          &lt;exterms:sleeps&gt;2&lt;/exterms:sleeps&gt;
9.          &lt;exterms:weight&gt;2.4&lt;/exterms:weight&gt;
10.         &lt;exterms:packedSize&gt;14x56&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
        <tt>rdf:type</tt> property in Line 6 indicates that the
        instance belongs to a class identified by the URIref
        <tt>http://www.example.com/terms/Tent</tt>. In this case, we
        imagine 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 <tt>exterms:weight</tt>), so we use the
        absolute URIref of the class to refer to it. If example.com had
        described these classes as part of the product catalog itself,
        we could have used the relative URIref <tt>#Tent</tt> to refer
        to it.</p>

        <p>RDF itself does not define a vocabulary for defining
        application-specific classes of things, such as <tt>Tent</tt>
        in this example. Instead, such classes would be described in an
        <em>RDF Schema</em>. The facilities provided by RDF for
        describing application-specific classes and their properties
        are 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>Since describing resources as instances of specific types or
        classes is fairly common, RDF/XML provides a special
        abbreviation for instances described as members of classes
        using the <tt>rdf:type</tt> property. In this abbreviation, the
        <tt>rdf:type</tt> property and its value are removed, and the
        <tt>rdf:Description</tt> element is replaced by an element
        whose name is the QName corresponding to the class URIref.
        Using this abbreviation, example.com's tent from <a
        href="#example11">Example 11</a> could also be described as
        shown in <a href="#example12">Example 12</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example12" name="example12">Example 12: Abbreviating
            the Tent's Type</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.com/terms/"
4.               xml:base="http://www.example.com/2002/04/products"&gt;

5.     &lt;exterms:Tent rdf:ID="item10245"&gt;
6.          &lt;exterms:model&gt;Overnighter&lt;/exterms:model&gt;
7.          &lt;exterms:sleeps&gt;2&lt;/exterms:sleeps&gt;
8.          &lt;exterms:weight&gt;2.4&lt;/exterms:weight&gt;
9.          &lt;exterms:packedSize&gt;14x56&lt;/exterms:packedSize&gt;
10.    &lt;/exterms:Tent&gt;

  ...other product descriptions...

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

        <p>Both <a href="#example11">Example 11</a> and <a
        href="#example12">Example 12</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 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 enable you to begin writing useful RDF/XML. For
        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, you should refer to the <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, including
      some built-in types and properties for representing groups of
      resources and RDF statements, and capabilities for deploying RDF
      information in the World Wide Web. 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, we might want 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 pre-defined
        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
        or literals. RDF defines three types of containers:</p>

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

          <li><tt>rdf:Seq</tt></li>

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

        <p>A <em>Bag</em> (a resource having type <tt>rdf:Bag</tt>) is
        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
        <tt>rdf:Seq</tt>) is 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 <tt>rdf:Alt</tt>) is 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, you give the resource an <tt>rdf:type</tt> property
        whose value is one of the pre-defined resources
        <tt>rdf:Bag</tt>, <tt>rdf:Seq</tt>, or <tt>rdf:Alt</tt>
        (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 <tt>rdf:_<em>n</em></tt>, where
        <em>n</em> is a decimal integer greater than zero, with no
        leading zeros, e.g., <tt>rdf:_1</tt>, <tt>rdf_2</tt>,
        <tt>rdf_3</tt>, 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
        <tt>rdf:type</tt> property.</p>

        <p>It is important to understand that while these types of
        containers are described using pre-defined 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
        <tt>rdf:Bag</tt> is than it has of what a resource of type
        <tt>ex:Tent</tt>, that we 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, Tim, John,
        Mary, and Sue", you could describe the course by giving it a
        <tt>s:students</tt> property whose value is a container of type
        <tt>rdf:Bag</tt> (the group of students) and then, using the
        container membership properties, describe the individual
        students as being members of that container, as in the RDF
        graph shown in <a href="#figure14">Figure 14</a>:</p>

        <div class="figure">
          <img src="fig14dec16.png"
          alt="A Simple Bag Container Description" /><br />
           <a id="figure14" name="figure14">Figure 14: A Simple Bag
          Container Description</a>
        </div>

        <p>Since the value of the <tt>s:students</tt> property in this
        example is described as a Bag, there is no intended
        significance in the order given for the URIrefs of each
        student, even though the properties in the graph have integers
        in their names. It is up to applications creating and
        processing graphs that include <tt>rdf:Bag</tt> 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="#example13">Example 13</a> describes the graph shown in
        <a href="#figure14">Figure 14</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example13" name="example13">Example 13: 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.edu/students/vocab#"&gt;

   &lt;rdf:Description rdf:about="http://example.edu/courses/6.001"&gt;
      &lt;s:students&gt;
         &lt;rdf:Bag&gt;
            &lt;rdf:li rdf:resource="http://example.edu/students/Amy"/&gt;
            &lt;rdf:li rdf:resource="http://example.edu/students/Tim"/&gt;
            &lt;rdf:li rdf:resource="http://example.edu/students/John"/&gt;
            &lt;rdf:li rdf:resource="http://example.edu/students/Mary"/&gt;
            &lt;rdf:li rdf:resource="http://example.edu/students/Sue"/&gt;
         &lt;/rdf:Bag&gt;
      &lt;/s:students&gt;
   &lt;/rdf:Description&gt;
&lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p><a href="#example13">Example 13</a> shows that RDF/XML
        provides <tt>li</tt> as a convenience element to avoid having
        to explicitly number each membership property. The numbered
        properties <tt>rdf:_1</tt>, <tt>rdf:_2</tt>, and so on are
        generated from the <tt>li</tt> elements in forming the
        corresponding graph. The element name <tt>li</tt> was chosen to
        be mnemonic with the term "list item" from HTML. Note also the
        use of a <tt>&lt;rdf:Bag&gt;</tt> element within the
        <tt>&lt;s:students&gt;</tt> property element. The
        <tt>&lt;rdf:Bag&gt;</tt> element is another example of the
        abbreviation we used in <a href="#example12">Example 12</a>
        that lets us replace both an <tt>rdf:Description</tt> element
        and an <tt>rdf:type</tt> element with a single element. Since
        no URIref is specified, the Bag is a blank node. Its nesting
        within the <tt>&lt;s:students&gt;</tt> 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 <tt>rdf:Seq</tt> container, and
        the corresponding RDF/XML, are similar to those for an
        <tt>rdf:Bag</tt> (the only difference is in the type,
        <tt>rdf:Seq</tt>). Once again, although an <tt>rdf:Seq</tt>
        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>As an illustration of an Alt container, the sentence "The
        source code for X11 may be found at ftp.example.org,
        ftp.example1.org, or ftp.example2.org" could be expressed in
        the RDF graph shown in <a href="#figure15">Figure 15</a>:</p>

        <div class="figure">
          <img src="fig15dec16.png"
          alt="A Simple Alt Container Description" /><br />
           <a id="figure15" name="figure15">Figure 15: A Simple Alt
          Container Description</a>
        </div>

        <p><a href="#example14">Example 14</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="example14" name="example14">Example 14: 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://ftp.example1.org"/&gt;
            &lt;rdf:li rdf:resource="ftp://ftp.example2.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 <tt>rdf:_1</tt>. This member is
        intended to be considered as the default or preferred value.
        Other than the member identified as <tt>rdf:_1</tt>, 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
        <tt>s:DistributionSite</tt> 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
        <tt>s:DistributionSite</tt> site property, or that
        <tt>ftp://ftp.example.org</tt> is the default or preferred
        value, must be built into an application's understanding of how
        an Alt is intended to behave, and/or into the meaning defined
        for the particular property (<tt>s:DistributionSite</tt> in
        this case), which also must be understood by the
        application.</p>

        <p>Alt containers are frequently used in conjunction with
        language tagging. For example, a work whose title has been
        translated into several languages might have its <tt>Title</tt>
        property pointing to an Alt container holding 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 of 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 ways to describe groups of
        resources, rather than using the ones described here. 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 not
        the same as 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. We might have 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 as a whole approved the resolution;
        it does not necessarily state that each committee member
        individually voted in favor of the resolution. In this case, it
        would be potentially misleading to model this sentence as three
        separate <tt>exterms:approvedBy</tt> 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 <tt>exterms:approvedBy</tt> 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>Finally, when using these RDF containers, it is important to
        understand that you are not <em>constructing</em> containers,
        as you would a programming language data structure; instead,
        you are <em>describing</em> containers (groups of things) that
        actually exist. For instance, in the Rules Committee example
        just given, the Rules Committee is an unordered group of
        people, whether you describe it in RDF that way or not. When
        you give the Rules Committee resource an <tt>rdf:type</tt>
        property whose value is <tt>rdf:Bag</tt>, you are simply
        describing the Rules Committee as having whatever
        characteristics you associate with things of type
        <tt>rdf:Bag</tt>, not constructing a particular data structure
        to hold the members of the group (you could indicate that the
        Rules Committee was a Bag without describing any members at
        all). Similarly, when you use the container membership
        properties, you are simply describing a container resource as
        having certain things as members. You are not necessarily
        saying that the things that you describe 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 Bag, not that they are the
        only members of the Bag.</p>
      </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". This is because, 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
        <tt>rdf:List</tt>, the predefined properties <tt>rdf:first</tt>
        and <tt>rdf:rest</tt>, and the predefined resource
        <tt>rdf:nil</tt>.</p>

        <p>To illustrate this, you could represent the sentence "The
        students in course 6.001 are Amy, Tim, and John" using the
        graph shown in <a href="#figure16">Figure 16</a>:</p>

        <div class="figure">
          <img src="fig16dec16.png"
          alt="An RDF Collection (list structure)" /><br />
           <a id="figure16" name="figure16">Figure 16: An RDF
          Collection (list structure)</a>
        </div>

        <p>For each member of the collection, such as <tt>s:Amy</tt>,
        there is a corresponding resource of type <tt>rdf:List</tt>.
        This list resource is linked to the collection member by an
        <tt>rdf:first</tt> property, and to the rest of the list by an
        <tt>rdf:rest</tt> property. The end of the list is indicated by
        an <tt>rdf:rest</tt> property being the resource
        <tt>rdf:nil</tt>. This structure will be familiar to those who
        know the Lisp programming language. As in Lisp, the
        <tt>rdf:first</tt> and <tt>rdf:rest</tt> properties allow
        applications to traverse the structure.</p>

        <p>RDF/XML provides a special notation to make it easier to
        describe collections. In RDF/XML, a collection is described by
        a property element that has the attribute
        <tt>rdf:parseType="Collection"</tt>, and that contains a group
        of nested elements representing the members of the collection.
        The <tt>rdf:parseType="Collection"</tt> attribute indicates
        that the enclosed elements are to be used to create the
        corresponding list structure in the RDF graph.</p>

        <p>To illustrate how this works, the RDF/XML from <a
        href="#example15">Example 15</a> would result in the RDF graph
        shown in <a href="#figure16">Figure 16</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example15" name="example15">Example 15: 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.edu/students/vocab#"&gt;

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

        <p>The use of <tt>rdf:parseType="Collection"</tt> always
        indicates the construction of a list structure like the one
        shown in <a href="#figure16">Figure 16</a>, which defines a
        fixed finite list of items with a given length and terminated
        by <tt>rdf:nil</tt>, and uses "new" blank nodes which 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.
        For example, it is not illegal to assert that a given node has
        two distinct values of the <tt>rdf:first</tt> property, or to
        simply omit part of the description of a collection. In
        general, therefore, RDF applications which require collections
        to be well-structured should be written to check that the
        collection vocabulary is being used appropriately, in order to
        be fully robust.</p>
      </div>

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

        <p>RDF applications sometimes need to make <em>statements about
        statements</em>, for instance, to record information about when
        a statement was made, who made it, or other similar
        information. For example, consider a statement about the tent
        we discussed in <a href="#newresources">Section 3.2</a>:</p>

        <p class="ptriple"><tt><u>product item10245</u> has a
        <u>weight</u> whose value is <u>2.4</u></tt></p>

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

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

        <p>Now, suppose we wanted to say in RDF that this statement was
        made by John Smith. Since in RDF we can only make statements
        about <em>resources</em>, what we would like to be able to do
        is write something like:</p>

        <div class="exampleOuter exampleInner">
<pre>
 <strong>[[</strong>exproducts:item10245  exterms:weight  "2.4" .<strong>]]</strong> dc:creator  exstaff:85740 .
</pre>
        </div>

        <p>That is, we want to be able to turn the original statement
        into a resource, so that we can make it the subject of another
        RDF statement that talks about it. RDF provides a built-in
        vocabulary for modeling statements as resources. This modeling
        is called <em>reification</em> in RDF, and a model of a
        statement is called the <em>reification</em> of the
        statement.</p>

        <p>The RDF reification vocabulary consists of the type
        <tt>rdf:Statement</tt>, and the properties
        <tt>rdf:subject</tt>, <tt>rdf:predicate</tt>, and
        <tt>rdf:object</tt>. In this vocabulary, a triple of the
        form:</p>

        <div class="exampleOuter exampleInner">
<pre>
foo  rdf:type  rdf:Statement .
</pre>
        </div>

        <p>is a statement that the resource <tt>foo</tt> is an RDF
        triple in some RDF document. The three properties
        <tt>rdf:subject</tt>, <tt>rdf:predicate</tt>, and
        <tt>rdf:object</tt>, when applied to <tt>foo</tt>, then specify
        the subject, predicate, and object components of that triple
        <tt>foo</tt>.</p>

        <p>Using this vocabulary, a <em>reification</em> of our
        original triple:</p>

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

        <p>is given by the graph:</p>

        <div class="exampleOuter exampleInner">
<pre>
_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject exproducts:item10245 .
_:xxx rdf:predicate exterms:weight . 
_:xxx rdf:object "2.4" .
</pre>
        </div>

        <p>(The node that is intended to refer to the first triple, the
        blank node <tt>_:xxx</tt> in the reification, could be either a
        blank node or a URIref.)</p>

        <p>The <em>intended</em> interpretation of a reification like
        this is that <tt>_:xxx</tt> should be understood as referring
        to the original triple (as a whole), which is described by the
        subject, predicate, and object triples in the reification. So,
        using the reification, we would express the fact that the
        original statement was made by John Smith using the graph:</p>

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

        <p>Note that the intended interpretation is that the triple
        that <tt>_:xxx</tt> refers to is 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. There could be several such triples that have the same
        subject, predicate and object properties. Although a graph is
        defined as a set of triples, several instances with the same
        triple structure might occur in different documents. Thus,
        without this understanding, it would be meaningful to claim
        that <tt>_:xxx</tt> does not refer to the triple in the first
        graph, but to some other triple with the same structure. This
        particular interpretation of reification is used because
        reification is intended to be used to express properties such
        as dates of composition and source information, as in our
        example, and these properties need to be applied to specific
        instances of triples.</p>

        <p>Note also that the assertion of the reified statement is not
        the same as the assertion of the original statement, and
        neither implies the other. That is, when someone asserts that
        John said foo, they are not asserting foo themselves, just that
        John said it. Conversely, when someone asserts foo, they are
        not also asserting its reification, since by asserting foo they
        are not also saying that there are such things as statements
        that they intend to talk about.</p>

        <p>We have referred to the <em>intended</em> interpretation of
        reification in the discussion above because, while this may be
        the interpretation that is generally intended when reification
        is used, RDF reification does not actually capture all this
        meaning. Specifically, RDF syntax by itself provides no way to
        "connect" an RDF triple to its reification. All that the
        graph:</p>

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

        <p>actually says is, "there is a statement that has a subject
        <tt>exproducts:item10245</tt>, a predicate
        <tt>exterms:weight</tt>, and an object <tt>2.4</tt>, and John
        made it". It does <em>not</em> say that that statement
        (referred to by <tt>_:xxx</tt>) is the same as some particular
        statement in some particular RDF document.</p>

        <p>To see this, given the original triple:</p>

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

        <p>and the following reification of it, together with an
        additional triple that associates John with the
        reification:</p>

        <div class="exampleOuter exampleInner">
<pre>
_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject exproducts:item10245 .
_:xxx rdf:predicate exterms:weight . 
_:xxx rdf:object "2.4" .

_:xxx dc:creator exstaff:85740 .
</pre>
        </div>

        <p>note that there is nothing that explicitly associates
        <tt>_:xxx</tt> with the original triple, and hence would allow
        you to say that John created it.</p>

        <p>This does not mean that such "provenance" information 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, you
        could make statements about the resource identified by that URI
        and, based on some application-dependent understanding of how
        those statements should be interpreted, 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 you could certainly make statements about those individual
        statements, using their URIs to identify them. In these cases,
        you would not need to use the reification vocabulary at
        all.</p>

        <p>To see this, if our original triple had a URI, say
        <tt>ex:statementfoo</tt>, then you could attribute that
        statement to John simply by the statement:</p>

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

        <p>with no use of the reification vocabulary.</p>

        <p>In addition, you could use the reification vocabulary
        directly according to the intended interpretation described
        above, and have 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>Finally, since the relation between triples and reifications
        of triples in any RDF graph or graphs need not be one-to-one,
        asserting a property about some resource described by a
        reification does not necessarily mean that the same property
        holds of another such resource, even if it has the same
        components. For example, given the following graph:</p>

        <div class="exampleOuter exampleInner">
<pre>
_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject exproducts:item10245 . 
_:xxx rdf:predicate exterms:weight . 
_:xxx rdf:object "2.4" .

_:yyy rdf:type rdf:Statement .
_:yyy rdf:subject exproducts:item10245 . 
_:yyy rdf:predicate exterms:weight . 
_:yyy rdf:object "2.4" .

_:xxx dc:creator exstaff:85740 .
</pre>
        </div>

        <p>it does not follow that:</p>

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

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

        <p>In <a href="#structuredproperties">Section 2.3</a>, we noted
        that the RDF data 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 "manager" holds between two
        employees (presumably one manages the other).</p>

        <p>However, in some cases we need to be able to represent
        information involving higher arity relations (relations between
        more than two resources) in RDF. We discussed one example of
        this in <a href="#structuredproperties">Section 2.3</a>, 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
        Zip. If we had tried to write this as a relation, we'd have
        seen that address was 5-ary relation of the form:</p>

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

        <p>We indicated that we can represent such structured
        information in RDF by considering the aggregate thing we want
        to talk about (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:Zip      "01730" .
</pre>
        </div>

        <p>(where <tt>_:johnaddress</tt> 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: you select one of the participants (John in this case) to
        serve as the subject of the original relation (<tt>address</tt>
        in this case). You then specify an intermediate resource to
        represent the rest of the relation (either with or without
        assigning it a URI), and 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 <tt>exterms:address</tt> 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 example, in our tent example in <a
        href="#newresources">Section 3.2</a>, we gave the weight of the
        particular tent we were describing as the plain literal "2.4",
        i.e.,</p>

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

        <p>In fact, a more complete description of the weight would
        have been "2.4 kilograms" rather than just "2.4". To state
        this, the value of the <tt>exterms:weight</tt> property would
        need to have two components, the literal "2.4" and an
        indication of the unit of measure (kilograms). In this
        situation the literal "2.4" could be considered the "main"
        value of the <tt>exterms:weight</tt> property, because
        frequently the value would be recorded simply as the value
        "2.4" (as we did 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 is
        considered as simply another kind of structured value. To
        represent this, we use a separate resource to represent the
        structured value as a whole (the weight, in this case), and to
        serve as the object of the original statement. We then give
        that resource properties representing the individual parts of
        the structured value. In this case, we need a property for the
        literal "2.4", and a property for the unit "kilograms". RDF
        provides a pre-defined <tt>rdf:value</tt> property to describe
        the main value (if there is one) of a structured value. So in
        this case, we would give the literal "2.4" as the value of the
        <tt>rdf:value</tt> property, and give the resource
        <tt>exunits:kilograms</tt> as the value of an
        <tt>exterms:units</tt> property (assuming the resource
        <tt>exunits:kilograms</tt> is defined in a example.org schema
        with the URIref
        <tt>http://www.example.org/units/kilograms</tt>). The resulting
        triples would be:</p>

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

        <p>which can be exchanged using the RDF/XML shown in <a
        href="#example16">Example 16</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example16" name="example16">Example 16: RDF/XML
            using <tt>rdf:value</tt></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: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&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>(<a href="#example16">Example 16</a> uses some additional
        RDF/XML abbreviations that were not discussed in <a
        href="#rdfxml">Section 3</a>, but are described 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
        <tt>rdf:value</tt> 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>You need not use <tt>rdf:value</tt> for these purposes
        (e.g., you could assign your own property name, such as
        <tt>ex:amount</tt>, in the example above), and RDF does not
        associate any particular meaning with it. <tt>rdf:value</tt> is
        simply provided as a convenience for use in these
        commonly-occurring situations.</p>
      </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 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 our examples in <a
      href="#newresources">Section 3.2</a> would want to describe
      classes such as <tt>exterms:Tent</tt>, and use properties such as
      <tt>exterms:model</tt>, <tt>exterms:weightInKg</tt>, and
      <tt>exterms:packedSize</tt> to describe them (we use QNames with
      various "example" namespace prefixes 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 <tt>ex2:Book</tt> or
      <tt>ex2:MagazineArticle</tt>, and use properties such as
      <tt>ex2:author</tt>, <tt>ex2:title</tt>, and <tt>ex2:subject</tt>
      to describe them. Other applications might need to describe
      classes such as <tt>ex3:Person</tt> and <tt>ex3:Company</tt>, and
      properties such as <tt>ex3:age</tt>, <tt>ex3:jobTitle</tt>,
      <tt>ex3:stockSymbol</tt>, and <tt>ex3:numberOfEmployees</tt>. RDF
      itself provides no vocabulary for specifying these things.
      Instead, such classes and properties are described in an RDF
      <em>vocabulary</em>. The facilities for describing RDF
      vocabularies are specified in <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>.</p>

      <p>RDF Schema does not provide a specific vocabulary of
      application-oriented classes like <tt>exterms:Tent</tt>,
      <tt>ex2:Book</tt>, or <tt>ex3:Person</tt>, and properties like
      <tt>exterms:weightInKg</tt>, <tt>ex2:author</tt> or
      <tt>ex3:JobTitle</tt>. Instead, it provides the mechanisms needed
      to <em>specify</em> such classes and properties as part of a
      vocabulary, and to indicate which classes and properties are
      expected to be used together (for example, you might expect the
      property <tt>ex3:jobTitle</tt> to be used in describing a
      <tt>ex3:Person</tt>). 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 <tt>ex:Dog</tt>
      might be defined as a subclass of <tt>ex:Mammal</tt> which is a
      subclass of <tt>ex:Animal</tt>, meaning that any resource which
      is in class <tt>ex:Dog</tt> is also considered to be in class
      <tt>ex:Animal</tt>. 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. We will say
      more about this point in <a href="#interpretingschema">Section
      5.3</a>.</p>

      <p>RDF Schema uses RDF itself to specify the RDF type system, by
      providing a set of pre-defined RDF resources and properties,
      together with their meanings, that can be used to describe
      user-specific classes and properties. These additional RDF Schema
      resources extend RDF to include a larger reserved vocabulary with
      additional meaning. The RDF Schema (RDFS) vocabulary is defined
      in a namespace identified by the URI reference
      <tt>http://www.w3.org/2000/01/rdf-schema#"</tt> (in the examples,
      we will use the prefix <tt>rdfs:</tt> to refer to this
      namespace). We will illustrate RDF Schema's basic resources and
      properties in the following sections.</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
        RDFS-defined resources <tt>rdfs:Class</tt> and
        <tt>rdfs:Resource</tt>, and the properties <tt>rdf:type</tt>
        and <tt>rdfs:subClassOf</tt>.</p>

        <p>For example, suppose we wanted to use RDF to provide
        information about different kinds of motor vehicles. In RDF
        Schema, we 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, we
        intend for the instances of our class to be resources that are
        motor vehicles.</p>

        <p>In RDF Schema, a <em>class</em> is any resource having an
        <tt>rdf:type</tt> property whose value is the RDFS-defined
        resource <tt>rdfs:Class</tt>. So our motor vehicle class would
        be described by assigning the class a URIref, say
        <tt>ex:MotorVehicle</tt> (using <tt>ex:</tt> to stand for the
        namespace URIref
        <tt>http://www.example.org/schemas/vehicles</tt>, we will use
        in this example) and describing that resource with an
        <tt>rdf:type</tt> property whose value is the RDFS-defined
        resource <tt>rdfs:Class</tt>. That is, we would write the RDF
        statement:</p>

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

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

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

        <p>(We are using a frequently-used 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 RDFS)</p>

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

        <p>After describing class <tt>ex:MotorVehicle</tt>, we might
        want to describe additional classes representing various
        specialized kinds of motor vehicle, e.g., passenger vehicles,
        vans, minivans, and so on. We can describe these classes in the
        same way as we described class <tt>ex:MotorVehicle</tt>, 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, we want to do more than just describe
        the individual classes; we also want to indicate their special
        relationship to class <tt>ex:MotorVehicle</tt>, i.e., that they
        are specialized <em>kinds</em> of MotorVehicle. To do this, we
        use the RDFS concept of <em>subclass</em>.</p>

        <p>An RDF subclass represents a subset/superset relationship
        between two classes. We describe this relationship using the
        pre-defined <tt>rdfs:subClassOf</tt> property to relate the two
        classes. For example, to state that <tt>ex:Van</tt> is a
        subclass of <tt>ex:MotorVehicle</tt>, we would write the RDF
        statement:</p>

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

        <p>The meaning of the <tt>rdfs:subClassOf</tt> relationship is
        that if resource <tt>ex:companyVan</tt> is an instance of
        <tt>ex:Van</tt>, then <tt>ex:companyVan</tt> is also implicitly
        considered an instance of <tt>ex:Motorvehicle</tt> (that is,
        you can "infer" or act as if <tt>ex:companyVan</tt> is an
        instance of <tt>ex:MotorVehicle</tt> even if this is not
        explicitly stated).</p>

        <p>The <tt>rdfs:subClassOf</tt> property is
        <em>transitive</em>. This means, for example, that if we have
        the RDF statements:</p>

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

        <p>then <tt>ex:MiniVan</tt> is also implicitly a subclass of
        <tt>ex:Motorvehicle</tt>. As a result, resources that are
        instances of class <tt>ex:MiniVan</tt> are also considered
        instances of class <tt>ex:Motorvehicle</tt> (as well as of
        class <tt>ex:Van</tt>). A class may be a subclass of more than
        one class (for example, <tt>ex:MiniVan</tt> may be a subclass
        of both <tt>ex:Van</tt> and <tt>ex:PassengerVehicle</tt>). All
        classes are implicitly subclasses of class
        <tt>rdfs:Resource</tt> (since the instances belonging to all
        classes are resources).</p>

        <p><a href="#figure17">Figure 17</a> shows the full class
        hierarchy we have been discussing.</p>

        <div class="figure">
          <img src="fig17dec16.png"
          alt="A Simple Class Hierarchy" /><br />
           <a id="figure17" name="figure17">Figure 17: A Simple Class
          Hierarchy</a>
        </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="#example1">Example 17</a> shows how this schema
        could be written in RDF/XML.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example17" name="example17">Example 17: The Vehicle
            Class Hierarchy in RDF/XML</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:rdfs="http://www.w3.org/2000/01/rdf-schema#"&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>

        <p>This RDF/XML introduces names, such as
        <tt>MotorVehicle</tt>, for the resources (classes) that it
        describes using <tt>rdf:ID</tt>, to give the effect of
        "assigning" URIrefs relative to the schema document as we
        described in <a href="#newresources">Section 3.2</a>. Relative
        URIrefs based on these names can then be used in other class
        definitions within the same schema (e.g., as we used
        <tt>#MotorVehicle</tt> in the description of the other
        classes). The full URIref of this class, assuming that the
        schema itself was the resource
        <tt>http://example.org/schemas/vehicles</tt>, would be
        <tt>http://example.org/schemas/vehicles#MotorVehicle</tt> (as
        shown in <a href="#figure17">Figure 17</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
        <tt>xml:base="http://example.org/schemas/vehicles"</tt>
        declaration.</p>

        <p>To refer to these classes in RDF instance data (e.g., data
        describing individual vehicles of these classes) located
        elsewhere, we would need to use the full URIrefs to identify
        the classes. For example, to describe the resource
        <tt>ex2:companyCar</tt> as an instance of the class
        <tt>ex:MotorVehicle</tt> described in this schema, we could use
        the RDF/XML shown in <a href="#example18">Example 18</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example18" name="example18">Example 18: An Instance
            of <tt>ex:MotorVehicle</tt></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"&gt;

   &lt;rdf:Description rdf:ID="companyCar"&gt;
     &lt;rdf:type rdf:resource="http://example.org/schemas/vehicles#MotorVehicle"/&gt;
   &lt;/rdf:Description&gt;

&lt;/rdf:RDF&gt;
</pre>
          </div>
        </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 <tt>rearSeatLegRoom</tt> to
        describe a passenger vehicle). In RDF Schema, properties are
        described using the RDF-defined class <tt>rdf:Property</tt>,
        and the RDFS-defined properties <tt>rdfs:domain</tt>,
        <tt>rdfs:range</tt>, and <tt>rdfs:subPropertyOf</tt>.</p>

        <p>All properties in RDF are described as instances of class
        <tt>rdf:Property</tt>. So a new property, such as
        <tt>exterms:weightInKg</tt>, is described by assigning the
        property a URIref, and describing that resource with an
        <tt>rdf:type</tt> property whose value is the resource
        <tt>rdf:Property</tt>. That is, we would write 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 RDFS-defined properties <tt>rdfs:range</tt> and
        <tt>rdfs:domain</tt> to further describe application-specific
        properties.</p>

        <p>The <tt>rdfs:range</tt> property is used to indicate that
        the values of a particular property are instances of a
        designated class. For example, if we wanted to indicate that
        the property <tt>ex:author</tt> had values that are instances
        of class <tt>ex:Person</tt>, we 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 <tt>ex:Person</tt> is a
        class, <tt>ex:author</tt> is a property, and that RDF
        statements using the <tt>ex:author</tt> property have instances
        of <tt>ex:Person</tt> as objects.</p>

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

        <p>The <tt>rdfs:range</tt> 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 we wanted to indicate that the
        property <tt>ex:age</tt> had values from the XML Schema
        datatype <tt>xsd:integer</tt>, we would write the RDF
        statement:</p>

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

        <p>The datatype <tt>xsd:integer</tt> is identified by its
        URIref (the full URIref being
        <tt>http://www.w3.org/2001/XMLSchema#integer</tt>). This URIref
        can be used without explicitly stating in the RDF 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 RDFS-defined class
        <tt>rdfs:Datatype</tt>. To state that <tt>xsd:integer</tt> is a
        datatype, we would write the RDF statement:</p>

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

        <p>This statement says that <tt>xsd:integer</tt> 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 we are defining a new
        datatype. There is no way to define datatypes in RDFS. As noted
        in <a href="#typedliterals">Section 2.4</a>, datatypes are
        defined externally to RDFS, and referred to in RDF statements
        by their URIrefs. What this statement does is document the
        existence of the datatype, and indicates explicitly that it is
        being used in this schema.</p>

        <p>The <tt>rdfs:domain</tt> property is used to indicate that a
        particular property applies to a designated class. For example,
        if we wanted to indicate that the property <tt>ex:author</tt>
        applies to instances of class <tt>ex:Book</tt>, we 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 <tt>ex:Person</tt> is a
        class, <tt>ex:author</tt> is a property, and that RDF
        statements using the <tt>ex:author</tt> property have instances
        of <tt>ex:Book</tt> as subjects.</p>

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

        <p>We can illustrate the use of these range and domain
        descriptions by extending our vehicle schema, adding two
        properties <tt>ex:registeredTo</tt> and
        <tt>ex:rearSeatLegRoom</tt>, a new class <tt>ex:Person</tt>,
        and explicitly describing the datatype <tt>xsd:integer</tt> as
        a datatype. The <tt>ex:registeredTo</tt> property applies to
        any <tt>ex:MotorVehicle</tt> and its value is a
        <tt>ex:Person</tt>. For the sake of this example,
        <tt>ex:rearSeatLegRoom</tt> applies only to instances of class
        <tt>ex:PassengerVehicle</tt>. The value is an
        <tt>xsd:integer</tt> giving the number of centimeters of rear
        seat legroom. These descriptions are shown in <a
        href="#example19">Example 19</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example19" name="example19">Example 19: Some
            Property Descriptions for the Vehicle Schema</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdf:Description rdf:ID="registeredTo"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/&gt;
  &lt;rdfs:domain rdf:resource="#MotorVehicle"/&gt;
  &lt;rdfs:range rdf:resource="#Person"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:ID="rearSeatLegRoom"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/&gt;
  &lt;rdfs:domain rdf:resource="#PassengerVehicle"/&gt; 
  &lt;rdfs:range rdf:resource="http://www.w3.org/2001/XMLSchema#integer"/&gt;
&lt;/rdf:Description&gt;

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

&lt;rdf:Description rdf:about="http://www.w3.org/2001/XMLSchema#integer"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Datatype"/&gt;
&lt;/rdf:Description&gt;
</pre>
          </div>
        </div>

        <p>Note that we have not used an <tt>&lt;rdf:RDF&gt;</tt>
        element in <a href="#example19">Example 19</a> because we have
        assumed we are adding this RDF/XML to the vehicle schema we
        described earlier, identified by
        <tt>http://example.org/schemas/vehicles</tt>. This same
        assumption also allows us to use relative URIrefs like
        <tt>#MotorVehicle</tt> to refer to other classes from that
        schema.</p>

        <p>RDF Schema provides a way to specialize <em>properties</em>
        as well as classes. We describe this specialization
        relationship between two properties using the pre-defined
        <tt>rdfs:subPropertyOf</tt> property. For example, if
        <tt>ex:primaryDriver</tt> and <tt>ex:driver</tt> are both
        properties, we can describe these properties, and the fact that
        <tt>ex:primaryDriver</tt> is a specialization of
        <tt>ex:driver</tt>, 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 the <tt>rdfs:subPropertyOf</tt> relationship
        is that if an instance <tt>ex:fred</tt> is an
        <tt>ex:primaryDriver</tt> of the instance
        <tt>ex:companyVan</tt>, then <tt>ex:fred</tt> is implicitly
        considered to also be an <tt>ex:primaryDriver</tt> of
        <tt>ex:companyVan</tt>. The RDF/XML describing these properties
        (assuming again that we are adding this to the vehicle schema
        we described earlier) is shown in <a href="#example20">Example
        20</a>.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example20" name="example20">Example 20: More
            Properties for the Vehicle Schema</a>
          </div>

          <div class="exampleInner">
<pre>
&lt;rdf:Description rdf:ID="driver"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/&gt;
  &lt;rdfs:domain rdf:resource="#MotorVehicle"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:ID="primaryDriver"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/&gt;
  &lt;rdfs:subPropertyOf rdf:resource="#driver"/&gt;
&lt;/rdf:Description&gt;
</pre>
          </div>
        </div>

        <p>A property may be a subproperty of zero, one or more
        properties. All RDF <tt>rdfs:range</tt> and
        <tt>rdfs:domain</tt> properties that apply to an RDF property
        also apply to each of its subproperties. So in the above
        example <tt>ex:primaryDriver</tt>, because of its subproperty
        relationship to <tt>ex:driver</tt>, implicitly also has an
        <tt>rdfs:domain</tt> of <tt>ex:MotorVehicle</tt>.</p>

        <p><a href="#example1">Example 21</a> shows the RDF/XML for the
        full vehicle schema, containing all the descriptions we've
        given so far:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example21" name="example21">Example 21: The Full
            Vehicle Schema</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: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:Description rdf:ID="Person"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Class"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:about="http://www.w3.org/2001/XMLSchema#integer"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/2000/01/rdf-schema#Datatype"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:ID="registeredTo"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/&gt;
  &lt;rdfs:domain rdf:resource="#MotorVehicle"/&gt;
  &lt;rdfs:range rdf:resource="#Person"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:ID="rearSeatLegRoom"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/&gt;
  &lt;rdfs:domain rdf:resource="#PassengerVehicle"/&gt; 
  &lt;rdfs:range rdf:resource="http://www.w3.org/2001/XMLSchema#integer"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:ID="driver"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/&gt;
  &lt;rdfs:domain rdf:resource="#MotorVehicle"/&gt;
&lt;/rdf:Description&gt;

&lt;rdf:Description rdf:ID="primaryDriver"&gt;
  &lt;rdf:type rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Property"/&gt;
  &lt;rdfs:subPropertyOf rdf:resource="#driver"/&gt;
&lt;/rdf:Description&gt;

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

        <p>Now that we've shown how to describe classes and properties
        using RDF Schema, we can see what instances corresponding to
        those descriptions might look like. For example, <a
        href="#example22">Example 22</a> describes an instance of the
        <tt>ex:PassengerVehicle</tt> class we described above, together
        with some hypothetical values for its properties.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example22" name="example22">Example 22: An Instance
            of <tt>ex:PassengerVehicle</tt></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"&gt;

    &lt;rdf:Description rdf:ID="johnSmithsCar"&gt;
         &lt;rdf:type rdf:resource="http://example.org/schemas/vehicles#PassengerVehicle"/&gt;
         &lt;ex:registeredTo rdf:resource="http://www.example.org/staffid/85740"/&gt;
         &lt;ex:rearSeatLegRoom 
             rdf:datatype="http://www.w3.org/2001/XMLSchema#integer"&gt;127&lt;/ex:rearSeatLegRoom&gt;
         &lt;ex:primaryDriver rdf:resource="http://www.example.org/staffid/85740"/&gt;
    &lt;/rdf:Description&gt;
  &lt;/rdf:RDF&gt;
</pre>
          </div>
        </div>

        <p>We are assuming that this instance is described in a
        separate document from the schema and, as before, that the
        schema is the resource
        <tt>http://example.org/schemas/vehicles</tt>. So we provide the
        namespace declaration
        <tt>xmlns:ex="http://example.org/schemas/vehicles"</tt> to
        refer to the schema, which allows the instance data to use
        abbreviations such as <tt>ex:registeredTo</tt> to refer
        unambiguously to properties described in that schema. However,
        when we use the <tt>rdf:type</tt> property to indicate the
        class membership of the instance, we must use the full URIref
        of the class to refer to it (since we cannot use a QName
        referencing the <tt>ex:</tt> namespace as a value of the
        <tt>rdf:resource</tt> attribute.</p>

        <p>Note that we can use an <tt>ex:registeredTo</tt> property in
        describing this instance of <tt>ex:PassengerVehicle</tt>,
        because <tt>ex:PassengerVehicle</tt> is a subclass of
        <tt>ex:MotorVehicle</tt>. Note also that we use a typed literal
        for the value of the <tt>ex:rearSetLegRoom</tt> property in our
        instance, rather than a plain literal (i.e., we didn't say
        <tt>&lt;ex:rearSeatLegRoom&gt;127&lt;/ex:rearSeatLegRoom&gt;</tt>).
        Because the schema describes the range of this property as an
        <tt>xsd:integer</tt>, the value of the property must be a typed
        literal of that datatype in order to match the range
        description (i.e., the range declaration does not "assign" a
        datatype to a plain literal).</p>

        <p>As we discussed in <a href="#newresources">Section 3.2</a>,
        the RDF/XML syntax provides an abbreviation for instances
        defined as members of classes using the <tt>rdf:type</tt>
        property. Using this abbreviation, we could describe this same
        instance as shown in <a href="#example23">Example 23</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example23" name="example23">Example 23: An
            Abbreviation of the Instance from Example 22</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"&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="http://www.w3.org/2001/XMLSchema#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>In <a href="#example23">Example 23</a> the class is
        identified by an element name (<tt>ex:PasssengerVehicle</tt>)
        rather than as the value of an <tt>rdf:resource</tt> attribute,
        so we can abbreviate it with the QName
        <tt>ex:PasssengerVehicle</tt> rather than writing it as a full
        URIref as we did in the earlier form.</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
        resource, using <em>domain</em> and <em>range</em> properties.
        For example, a typical object-oriented programming language
        might define a class <tt>Book</tt> with an attribute called
        <tt>author</tt> having values of type <tt>Person</tt>. A
        corresponding RDF schema would describe a class
        <tt>ex:Book</tt>, and, in a separate description, a property
        <tt>ex:author</tt> having a domain of <tt>ex:Book</tt> and a
        range of <tt>ex:Person</tt>.</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
        <tt>author</tt> is part of the description of class
        <tt>Book</tt>, and applies <em>only</em> to instances of class
        <tt>Book</tt>. Another class (say, <tt>softwareModule</tt>)
        might also have an attribute called <tt>author</tt>, 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>So, for example, an RDF schema could describe a property
        <tt>exterms:weight</tt> 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. (Of course,
        this is a "benefit" which must be used with care, to insure
        that properties are not mis-applied in inappropriate
        situations.)</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
        <tt>Book</tt> with an <tt>author</tt> attribute having values
        of type <tt>Person</tt>, this is usually interpreted as a group
        of <em>constraints</em>. The language will not allow the
        creation of an instance of <tt>Book</tt> without an
        <tt>author</tt> attribute, and it will not allow an instance of
        <tt>Book</tt> with an <tt>author</tt> attribute that does not
        have a <tt>Person</tt> as its value. Moreover, if
        <tt>author</tt> is the <em>only</em> attribute defined for
        class <tt>Book</tt>, the language will not allow an instance of
        <tt>Book</tt> 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
        <tt>ex:author</tt> property has an <tt>rdfs:range</tt> of class
        <tt>ex:Person</tt>. This is simply an RDF statement that RDF
        statements containing <tt>ex:author</tt> properties have
        instances of <tt>ex:Person</tt> 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 <tt>ex:author</tt> property has a
        value of the indicated (<tt>ex:Person</tt>) 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 <tt>ex:author</tt>
        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 <tt>ex:Person</tt>. A
        third application might receive some RDF data that includes an
        <tt>ex:author</tt> property whose value is a resource of class
        <tt>ex:Corporation</tt>, 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., you might have an instance
        of <tt>ex:Book</tt> without an <tt>ex:author</tt> property,
        even if <tt>ex:author</tt> is described as having a domain of
        <tt>ex:Book</tt>), or with <em>additional</em> properties (you
        might describe an instance of <tt>ex:Book</tt> with an
        <tt>ex:technicalEditor</tt> property, even though you haven't
        described such a property in your particular schema.)</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 also provides a number of other properties, which
        can be used to provide documentation and other information
        about an RDF schema or about instances. For example the
        <tt>rdfs:comment</tt> property can be used to provide a
        human-readable description of a resource. The
        <tt>rdfs:label</tt> property can be used to provide a more
        human-readable version of a resource's name. The
        <tt>rdfs:seeAlso</tt> property can be used to indicate a
        resource that might provide additional information about the
        subject resource. The <tt>rdfs:isDefinedBy</tt> property is a
        subproperty of <tt>rdfs:seeAlso</tt>, 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. For further discussion of these properties,
        you should consult <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>.</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. 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
          <tt>hasAncestor</tt>) is <em>transitive</em>, e.g., that if A
          <tt>hasAncestor</tt> B, and B <tt>hasAncestor</tt> C, then A
          <tt>hasAncestor</tt> 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 concept.</li>

          <li>specifying that two different instances (having different
          URIrefs) actually represent the same individual.</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>In the previous sections, we have described the general
      capabilities of RDF and RDF Schema. While we have used examples
      within those sections to illustrate those capabilities, and some
      of those examples may have suggested potential RDF applications,
      we have not yet discussed any <em>real</em> ones. In this
      section, we 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 (we have already used the Dublin
        Core <tt>creator</tt> element in earlier examples). The current
        elements of the Dublin Core are defined in the <a
        href="http://dublincore.org/documents/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 Meta
        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="#example24">Example 24</a>,
        describes a Web site home page using Dublin Core
        properties:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example24" name="example24">Example 24: 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 lower case). Even if the initial letter were
        identically uppercase, the XML namespace mechanism enables us
        to distinguish between these two elements (one is
        <tt>rdf:Description</tt>, and the other is
        <tt>dc:description</tt>). Also, as a matter of interest, if you
        access <a
        href="http://purl.org/dc/elements/1.1/">http://purl.org/dc/elements/1.1/</a>
        in a Web browser (as of the current writing), you will get an
        RDF Schema declaration for <a
        href="#ref-dublin-core">[DC]</a>.</p>

        <p>Our second example, <a href="#example25">Example 25</a>,
        describes a published magazine:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example25" name="example25">Example 25: 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>In <a href="#example25">Example 25</a>, we've used (in the
        third line from the bottom) the Dublin Core <em>qualifier</em>
        <tt>isPartOf</tt> (from a separate namespace) to indicate that
        this magazine is "part of" the previously-described web
        site.</p>

        <p>The third example, <a href="#example26">Example 26</a>,
        describes a specific article in the magazine described in <a
        href="#example25">Example 25</a>.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example26" name="example26">Example 26: 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 @ 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>In <a href="#example26">Example 26</a>, we've also used the
        qualifier <tt>isPartOf</tt>, this time to indicate that this
        article is "part of" the previously-described magazine.</p>
      </div>

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

        <p><a
        href="http://www.prismstandard.org/techdev/prismspec11.asp">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 <a
        href="http://www.prismstandard.org/">PRISM Working Group</a> 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 LexisNexis 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 which 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 namespace (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="#example27">Example 27</a> describes a photograph, giving
        basic information on its title, photographer, format, etc.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example27" name="example27">Example 27: 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://wanderlust.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 in three new
        namespaces, generally cited using the prefixes <tt>prism:</tt>,
        <tt>pcv:</tt>, and <tt>prl:</tt>.</p>

        <p><tt>prism:</tt> This prefix refers to the main PRISM
        namespace, whose URI is
        <tt>http://prismstandard.org/namespaces/basic/1.0/</tt>. Most
        of its properties are more specific versions of properties from
        the Dublin Core. For example, more specific versions of
        <tt>dc:date</tt> are provided by properties like
        <tt>prism:publicationTime</tt>, <tt>prism:releaseTime</tt>,
        <tt>prism:expirationTime</tt>, etc.</p>

        <p><tt>pcv:</tt> This prefix refers to the PRISM Controlled
        Vocabulary namespace, whose URI is
        <tt>http://prismstandard.org/namespaces/pcv/1.0/</tt>.
        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 PRISM Controlled
        Vocabulary (pcv) namespace provides properties for specifying
        terms in a vocabulary, the relations between terms, and
        alternate names for the terms.</p>

        <p><tt>prl:</tt> This prefix refers to the PRISM Rights
        Language namespace, whose URI is
        <tt>http://prismstandard.org/namespaces/prl/1.0/</tt>. 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
        can't be "used", depending on conditions of time, geography,
        and industry. This is believed to be an 80/20 tradeoff 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 <tt>coverage</tt> 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) 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;Grece&lt;/pcv:label&gt;
  &lt;/pcv:Descriptor&gt;
&lt;/dc:coverage&gt;
</pre>
        </div>

        <p>(using a structured value).</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>We will close this section with two final examples. <a
        href="#example28">Example 28</a> says that the image
        (<tt>.../Corfu.jpg</tt>) cannot be used (<tt>#none</tt>) in the
        tobacco industry (code 21 in SIC, the Standard Industrial
        Classifications).</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example28" name="example28">Example 28: 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://wanderlust.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="#example29">Example 29</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="example29" name="example29">Example 29: Additional
            Information about the Image from Example 28</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://wanderlust.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>

        <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 stylesheets 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 two RDF
        vocabularies: one for general packaging descriptions, and
        another specifically for describing XML resources. The XPackage
        framework also allows customization through extension and/or
        restriction.</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
        stylesheets 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="#example30">Example 30</a> (with namespace declarations
        removed for simplicity):</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example30" name="example30">Example 30: 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, stylesheets, and
        images) are described within this package description document.
        The XHTML document resource itself is described using an RDF
        resource description element <tt>&lt;xpackage:resource&gt;</tt>
        from the XPackage <em>ontology</em> (the term XPackage uses for
        a vocabulary). Each resource description element may include
        RDF properties from various ontologies. In <a
        href="#example31">Example 31</a>, the document's MIME content
        type ("application/xhtml+xml") is defined using a standard
        XPackage property from the XPackage ontology,
        <tt>xpackage:contentType</tt>. Another property, the document's
        author (in this case, "Garret Wilson"), is described using a
        property from the Dublin Core (which is considered a <em>custom
        ontology</em> in XPackage), resulting in a <tt>dc:creator</tt>
        property. XPackage itself specifies an extension property set
        specifically for XML-based resources, the XML ontology,
        including specifying XML namespaces and stylesheets used with
        the <tt>xmlprop:namespace</tt> and <tt>xmlprop:style</tt>
        properties, respectively.</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example31" name="example31">Example 31: A
            Description of an XHTML Document</a>
          </div>

          <div class="exampleInner">
<pre>
    &lt;!--doc.html--&gt;
    &lt;xpackage:resource rdf:about="urn:examples:xhtmldocument-doc"&gt;
      &lt;rdfs:comment&gt;The XHTML document.&lt;/rdfs:comment&gt;
      &lt;xpackage:location xlink:href="doc.html"/&gt;
      &lt;xpackage:contentType&gt;application/xhtml+xml&lt;/xpackage:contentType&gt;
      &lt;xmlprop:namespace rdf:resource="http://www.w3.org/1999/xhtml"/&gt;
      &lt;xmlprop:style rdf:resource="urn:examples:xhtmldocument-stylesheet"/&gt;
      &lt;xmlprop:annotation rdf:resource="urn:examples:xhtmldocument-annotation"/&gt;
      &lt;dc:creator&gt;Garret Wilson&lt;/dc:creator&gt;
      &lt;xpackage:manifest&gt;
        &lt;rdf:Bag&gt;
          &lt;rdf:li rdf:resource="urn:examples:xhtmldocument-stylesheet"/&gt;
          &lt;rdf:li rdf:resource="urn:examples:xhtmldocument-image"/&gt;
        &lt;/rdf:Bag&gt;
      &lt;/xpackage:manifest&gt;
    &lt;/xpackage:resource&gt;
</pre>
          </div>
        </div>

        <p>The <tt>xpackage:manifest</tt> property indicates that both
        the stylesheet and image resources are necessary for
        processing; those resources are described separately within the
        package description document. The example stylesheet resource
        description in <a href="#example32">Example 32</a> lists its
        location ("stylesheet.css") using the XPackage ontology
        <tt>xpackage:location</tt> property (which is compatible with
        XLink), and shows through use of the XPackage ontology
        <tt>xpackage:contentType</tt> property that it is a CSS
        stylesheet ("text/css").</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example32" name="example32">Example 32: A Stylesheet
            Resource Description</a>
          </div>

          <div class="exampleInner">
<pre>
    &lt;!--stylesheet.css--&gt;
    &lt;xpackage:resource rdf:about="urn:examples:xhtmldocument-css"&gt;
      &lt;rdfs:comment&gt;The document stylesheet.&lt;/rdfs:comment&gt;
      &lt;xpackage:location xlink:href="stylesheet.css"/&gt;
      &lt;xpackage:contentType&gt;text/css&lt;/xpackage:contentType&gt;
    &lt;/xpackage:resource&gt;
</pre>
          </div>
        </div>

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

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

        <p>When you consider all of the information that you access on
        the Web on a day-to-day basis: schedules, to-do lists, news
        headlines, search results, "What's New", etc., it becomes
        increasingly difficult to manage this information and integrate
        it into a coherent whole as the sources and diversity of the
        information increase. <a href="http://purl.org/rss/1.0">RSS
        1.0</a> ("RDF Site Summary") is an RDF vocabulary that provides
        a lightweight multipurpose extensible metadata description and
        syndication format. In short, RSS 1.0 is a powerful and
        extensible way of describing, managing and making available to
        broad audiences relevant and timely information. It allows this
        information to be made available in a rich and reusable way,
        and 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>, shown in <a
        href="#figure18">Figure 18</a>, is a primary point of contact
        with the public and serves in part to disseminate information
        about the deliverables of the Consortium. 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 repurpose 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="figure18" name="figure18">Figure 18: The W3C Home
          Page</a>
        </div>

        <p>Numerous sites all over the Web provide RSS 1.0 feeds. <a
        href="#example33">Example 33</a> is an example of the <a
        href="http://www.w3.org/2000/08/w3c-synd/home.rss">W3C
        feed</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example33" name="example33">Example 33: 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="#example33">Example 33</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>Online 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="#figure19">Figure 19</a> (each
          mirroring W3C's column of news). These gather feeds from
          thousands of sources, and separate each of the
          <tt>&lt;item&gt;</tt>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/software/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="figure19" name="figure19">Figure 19: 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" is 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 <a href="http://standards.ces.com/cim/">Common
        Information Model</a> (CIM). 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 Distributed Management Task Force for
        defining management information for distributed software,
        network, and enterprise environments. The DMTF CIM also has an
        XML representation, but does not use RDF.]</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
        Unified Modeling Language (UML). The base class of the CIM is
        the <tt>PowerSystemResource</tt> class, with other more
        specialized classes such as <tt>Substation</tt>,
        <tt>Switch</tt>, and <tt>Breaker</tt> 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="#example34">Example 34</a>
        shows examples of CIM/XML class and property definitions:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example34" name="example34">Example 34: 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 an 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 serialization of models. In addition,
        CIM/XML implements some extensions to the RDF Schema vocabulary
        (defined in the <tt>cims:</tt> namespace) to support inverse
        roles and multiplicity (cardinality) constraints describing how
        many instances of a given property are allowed for a given
        resource from the CIM UML diagrams (allowable values for a
        multiplicity declaration are zero-or-one, exactly-one,
        zero-or-more, one-or-more). The properties in <a
        href="#example35">Example 35</a> illustrate these
        extensions:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example35" name="example35">Example 35: 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 RDFS 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 DAML+OIL) and UML (and its XML
        representations).</p>

        <p>The need for additional declarative power illustrated by the
        need to add cardinality constraints to CIM/XML shows the type
        of requirement leading to the development of more powerful
        RDF-based schema/ontology languages such as DAML+OIL or 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 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> 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 molecular
        function, biological process and cellular component. 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 (actually, RDF/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. RDF was chosen for use in the XML versions of
        the ontologies because of its flexibility in representing these
        graph structures, as well as its widespread tool support.</p>

        <p><a href="#example36">Example 36</a> shows some sample GO
        information from the <a
        href="http://www.geneontology.org/doc/GO.doc.html">GO
        documentation</a>:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example36" name="example36">Example 36: 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="#example36">Example 36</a> illustrates that
        <tt>go:term</tt> is the basic element. The GO has added its own
        extensions to the RDF vocabulary (they do not use RDFS). For
        example, term <tt>GO:0016209</tt> has the element
        <tt>&lt;go:isa
        rdf:resource="http://www.geneontology.org/go#GO:0003674"
        /&gt;</tt>. This tag represents the relationship
        "<tt>GO:0016209</tt> isa <tt>GO:0003674</tt>", or, in English,
        "Antioxidant is a molecular function." Another specialized
        relationship is <tt>go:part-of</tt>. For example,
        <tt>GO:0003674</tt> has the element <tt>&lt;go:part-of
        rdf:resource="http://www.geneontology.org/go#GO:0003673"
        /&gt;</tt>. 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 <tt>go:dbxref</tt> element represents the term in an
        external database, and <tt>go:association</tt> represents the
        gene associations of each term. <tt>go:association</tt> can
        have both <tt>go:evidence</tt>, which holds a
        <tt>go:dbxref</tt> to the evidence supporting the association,
        and a <tt>go:gene_product</tt>, which contains the gene symbol
        and <tt>go:dbxref</tt>.</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 a graph or network
        structure, rather than being a strict hierarchy. The GO is also
        another example in which the 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
        is RDF/XML. In addition, 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.</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="#example37">Example 37</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="example37" name="example37">Example 37: 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="#example38">Example 38</a> shows a fragment of UAProf's
        <em>HardwarePlatform</em> component:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example38" name="example38">Example 38: 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="#example39">Example 39</a> is
        shows such a profile:</p>

        <div class="exampleOuter">
          <div class="c1">
            <a id="example39" name="example39">Example 39: 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>In <a href="#intro">Section 1</a>, we 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>We have already discussed the subjects of the first three 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>). In this section, we briefly
      describe the remaining documents, 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 we've seen 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
        broad sense may depend on many factors, including social
        conventions, comments in natural language, or links to other
        content-bearing documents (<a
        href="#ref-rdf-concepts">[RDF-CONCEPTS]</a> provides further
        discussion of the various notions of meaning in RDF). 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 an RDF graph.
        The <a href="http://www.w3.org/TR/rdf-mt/">RDF Semantics</a> <a
        href="#ref-rdf-semantics">[RDF-SEMANTICS]</a> defines this
        formal meaning, using a technique called <em>model theory</em>
        for specifying the semantics of a formal language. In other
        words, the RDF model theory provides the formal underpinnings
        for all of the concepts we have described. 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/2002/WD-rdf-testcases-20021112/#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 normative
        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><i><a
          href="http://www.w3.org/TR/2002/WD-rdf-concepts-20021108/">Resource
          Description Framework (RDF): Concepts and Abstract
          Syntax</a></i>, Klyne G., Carroll J. (Editors), World Wide
          Web Consortium, 08 November 2002 (work in progress). <a
          href="http://www.w3.org/TR/2002/WD-rdf-concepts-20021108/">This
          version</a> is
          http://www.w3.org/TR/2002/WD-rdf-concepts-20021108/. 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><i><a
          href="http://www.ietf.org/internet-drafts/draft-swartz-rdfcore-rdfxml-mediatype-01.txt">
          Application/rdf+xml Media Type Registration</a></i>, Swartz
          A., IETF Internet Draft, August 2002 (work in progress).
          Version available at <a
          href="http://www.ietf.org/internet-drafts/draft-swartz-rdfcore-rdfxml-mediatype-01.txt">
          http://www.ietf.org/internet-drafts/draft-swartz-rdfcore-rdfxml-mediatype-01.txt</a>.</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/2002/WD-rdf-mt-20021112/">RDF
          Semantics</a></cite>, Hayes P. (Editor), World Wide Web
          Consortium, 12 November 2002 (work in progress). <a
          href="http://www.w3.org/TR/2002/WD-rdf-mt-20021112/">This
          version</a> is http://www.w3.org/TR/2002/WD-rdf-mt-20021112/.
          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><i><a
          href="http://www.w3.org/TR/2002/WD-rdf-syntax-grammar-20021108/">
          RDF/XML Syntax Specification (Revised)</a></i>, Beckett D.
          (Editor), World Wide Web Consortium, 8 November 2002 (work in
          progress). <a
          href="http://www.w3.org/TR/2002/WD-rdf-syntax-grammar-20021108/">
          This version</a> is
          http://www.w3.org/TR/2002/WD-rdf-syntax-grammar-20021108/.
          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/2002/WD-rdf-testcases-20021112/">RDF
          Test Cases</a></cite>, Grant J., Beckett D. (Editors), World
          Wide Web Consortium, 12 November 2002 (work in progress). <a
          href="http://www.w3.org/TR/2002/WD-rdf-testcases-20021112/">This
          version</a> is
          http://www.w3.org/TR/2002/WD-rdf-testcases-20021112/. 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><i><a
          href="http://www.w3.org/TR/2002/WD-rdf-schema-20021112/">RDF
          Vocabulary Description Language 1.0: RDF Schema</a></i>,
          Brickley D., Guha R.V. (Editors), World Wide Web Consortium,
          12 November 2002 (work in progress). <a
          href="http://www.w3.org/TR/2002/WD-rdf-schema-20021112/">This
          version</a> is
          http://www.w3.org/TR/2002/WD-rdf-schema-20021112/. 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-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. This document is
          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>
        </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/2001/WD-CCPP-struct-vocab-20010315/">
          Composite Capability/Preference Profiles (CC/PP): Structure
          and Vocabularies</a></cite>, Klyne G., Reynolds F., Woodrow
          C., Ohto H., Butler, M., World Wide Web Consortium, 08
          November 2002 (work in progress). <a
          href="http://www.w3.org/TR/2002/WD-CCPP-struct-vocab-20021108/">
          This version</a> is
          http://www.w3.org/TR/2002/WD-CCPP-struct-vocab-20021108/. 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-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://cgi.omg.org/docs/formal/01-06-01.pdf">Utility
          Management System (UMS) Data Access Facility</a></cite>,
          Object Management Group, OMG document formal/01-06-01, June
          2001. <a
          href="http://cgi.omg.org/docs/formal/01-06-01.pdf">This
          document</a> is
          http://cgi.omg.org/docs/formal/01-06-01.pdf.</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/dces/">Dublin Core
          Metadata Element Set, Version 1.1: Reference
          Description</a></cite>, 02 July 1999. <a
          href="http://dublincore.org/documents/dces/">This
          document</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-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-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-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/2002/WD-owl-ref-20021112/">OWL Web
          Ontology Language 1.0 Reference</a></cite>, Dean M., Connolly
          D., van Harmelen F., Hendler J., Horrocks I., McGuinness
          D.L., Patel-Schneider P.F., Stein L.A. (Editors), World Wide
          Web Consortium, 12 November 2002 (work in progress). <a
          href="http://www.w3.org/TR/2002/WD-owl-ref-20021112/">This
          version</a> is
          http://www.w3.org/TR/2002/WD-owl-ref-20021112/. 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/techdev/prismspec11.asp">PRISM:
          Publishing Requirements for Industry Standard
          Metadata</a></cite>, Version 1.1, 19 February 2002. <a
          href="http://www.prismstandard.org/techdev/prismspec11.asp">This
          document</a> is
          http://www.prismstandard.org/techdev/prismspec11.asp.</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-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-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-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-20021023.html">
          XML Package (XPackage) 1.0</a></cite> , Wilson G., Open eBook
          Forum Editor's Working Draft, 23 October 2002. <a
          href="http://www.xpackage.org/specification/xpackage-draft-20021023.html">
          This version</a> is
          http://www.xpackage.org/specification/xpackage-draft-20021023.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>
    </div>
    <hr />

    <div class="section">
      <h2 id="uri"><a id="identifiers" name="identifiers"></a>Appendix
      A: More on Uniform Resource Identifiers (URIs)</h2>

      <p>As we saw 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><tt>http:</tt> (Hypertext Transfer Protocol, for Web
        pages)</li>

        <li><tt>mailto:</tt> (email addresses), e.g.,
        mailto:em@w3.org</li>

        <li><tt>ftp:</tt> (File Transfer Protocol)</li>

        <li><tt>urn:</tt> (Uniform Resource Names, intended to be
        persistent location-independent resource identifiers), e.g.,
        <tt>urn:isbn:0-520-02356-0</tt> (for a book)</li>
      </ul>

      <p>URIs are defined in <a
      href="http://www.isi.edu/in-notes/rfc2396.txt">RFC 2396</a> <a
      href="#ref-uri">[URIS]</a>. Some additional discussion of URIs
      can be found in <a href="http://www.w3.org/Addressing/">Naming
      and Addressing: URIs, URLs, ...</a> <a
      href="#ref-nameaddress">[NAMEADDRESS]</a>. 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 you may have,
      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
      <tt>http:</tt>, depend on centralized systems such as DNS, other
      schemes, such as <tt>freenet:</tt>, are completely decentralized.
      This means that, as with any other kind of name, you don't need
      special authority or permission to create a URI for something.
      Also, you can create URIs for things you don't own, just as in
      ordinary language you can use whatever name you like for things
      you don't own.</p>

      <p>As we also saw 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
      <tt>http://www.example.org/index.html#section2</tt> consists of
      the URI <tt>http://www.example.org/index.html</tt> and (separated
      by the "#" character) the fragment identifier
      <tt>Section2</tt>.</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
      <tt>http://www.example.org/index.html</tt>. 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
      <tt>otherpage.html</tt>, when appearing in a resource
      <tt>http://www.example.org/index.html</tt>, would be filled out
      to the absolute URIref
      <tt>http://www.example.org/otherpage.html</tt>. 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 <tt>http://www.example.org/index.html</tt>, if
      <tt>#section2</tt> appeared as a URIref, it would be considered
      equivalent to the absolute URIref
      <tt>http://www.example.org/index.html#section2</tt>.</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. You should consult
      <a href="#ref-rdf-syntax">[RDF-SYNTAX]</a> 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 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><tt>http://www.example.org/index.html</tt><br />
       <tt>http://www.example.org/index.html#Section2</tt></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
      doesn't mean that the HTML-defined containment relationship might
      not exist, just that RDF doesn't assume that a relationship
      exists based only on the fact that the URI parts of the URI
      references are the same.)</p>
    </div>

    <div class="section">
      <h2 id="xml"><a id="documents" name="documents"></a>Appendix B:
      More on the Extensible Markup Language (XML)</h2>

      <p>The <a
      href="http://www.w3.org/TR/1998/REC-xml-19980210.html">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>). 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. 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 (<tt>&lt;sentence&gt;</tt>,
      <tt>&lt;person&gt;</tt>, 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
      <tt>&lt;animal&gt;dog&lt;/animal&gt;</tt>. This consists of the
      <em>start-tag</em> <tt>&lt;animal&gt;</tt>, the element
      <em>content</em>, and a matching <em>end-tag</em>
      <tt>&lt;/animal&gt;</tt>. This <tt>animal</tt> element, together
      with the <tt>person</tt> element, are nested as part of the
      content of the <tt>sentence</tt> 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
      <tt>&lt;animal&gt;&lt;/animal&gt;</tt>, or by using a shorthand
      form of tag called an <em>empty-element tag</em>, as in
      <tt>&lt;animal/&gt;</tt>.</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
      <tt>&lt;person&gt;</tt> element contains the attribute
      <tt>webid="http://example.com/#johnsmith"</tt> (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 <tt>&lt;person&gt;</tt> 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 useful practice 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. The following example illustrates the
      use of an XML namespace.</p>

      <div class="exampleOuter exampleInner">
<pre>
&lt;my:sentence xmlns:my="http://example.com/xml/documents/"&gt;
   &lt;my:person my:webid="http://example.com/#johnsmith"&gt;I&lt;/my:person&gt; 
just got a new pet &lt;my:animal&gt;dog&lt;/my:animal&gt;.
&lt;/my:sentence&gt;
</pre>
      </div>

      <p>In this example, the attribute
      <tt>xmlns:my="http://example.com/xml/documents/"</tt> declares a
      namespace for use in this piece of XML. It maps the
      <em>prefix</em> <tt>my</tt> to the namespace URI
      <tt>http://example.com/xml/documents/</tt>. The XML content can
      then use <em>qualified names</em> (or <em>QNames</em>) like
      <tt>my:person</tt> 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, we don't have 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>As noted in <a href="#basicconcepts">Section 2.1</a>, RDF
      defines a particular XML markup language, called RDF/XML, which
      is described in more detail in <a href="#rdfxml">Section
      3</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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