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      Using labels to give semantics to tags - Design Issues
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    <address>
      Tim Berners-Lee<br />
      Date: 2006-11-23, last change: $Date: 2007/01/22 21:05:37
      $<br />
      Status: personal view only. Editing status: draft.
    </address>
    <p>
      <a href="./">Up to Design Issues</a>
    </p>
    <hr />
    <h1>
      Using labels to give semantics to tags
    </h1>
    <h3>
      Abstract
    </h3>
    <p>
      Existing user interfaces for managing, for example, mail,
      photos, contacts and songs allow searching using both
      user-generated 'tags' and also well-defined properties such
      as the date-time of a photograph, or the values of headers in
      an email message. Users have on many web sites provided many
      tags, but their re-use by others has been limited due to the
      fact that the same tag word has quite different meaning when
      used by another person or used on another site.
    </p>
    <p>
      Other data, such 'geotagging' of places, declaration of
      friends and colleagues, by contrast, have a well-defined
      meaning and allow query across data from many sites. This
      article discusses how the user interface metaphor of a
      luggage label cam used to associate metadata from
      well-defined ontology with tags from a particular context.
    </p>
    <p>
      The article discuses ways of encoding the labels in RDF.
    </p>
    <h3>
      Introduction
    </h3>
    <p>
      There are a mixed feelings about the passion for tagging
      which typifies the Web 2.0 wave. On the one hand, there is
      excitement about the fact that users are, as a large number,
      adding re-usable information to the information space,
      allowing sites such as del.icio.us and flickr to sort,
      cluster and query masses of otherwise amorphous photos and
      web content. On the other hand, there is the sinking feeling
      that tags are headed the same way as keywords of Information
      Retrieval in the 1980s: initial hope, and then being stranded
      between the unbearable constraints of a controlled vocabulary
      and the hopeless ambiguity of uncontrolled user-generated
      keywords. Tom Gruber, writer of books on ontology who runs a
      Web 2.0 site himself. gave a talk at ISWC 2006 which touched
      on bringing the gap, and taking the passion to organize and
      express, and using it to make re-usable data.
    </p>
    <p>
      There is currently a tension in the tagging world as to
      whether tags are regarded as global in meaning, or whether
      there meaning really depends on the tagger. In del.icio.us,
      one can query for thinks tagged with a certain word by a
      certain person. (I heard of one online community which was
      considering making a system to allow one formally to state
      when one has committed to use a given tag in the same way as
      another person, or growing mesh of people. That would be a
      very interesting feature, as it would allow a useful
      definition to gain growing acceptance, to progressively move
      from being a private idea to being a group global standard.)
    </p>
    <p>
      Meanwhile, other sites get users to provide semantic web data
      with well-defined global ontologies. The locations of people,
      events and photos, relationships between people, authorship
      of publications, things and people an image depicts, and so
      on, is done using well-defined identifiers (under the covers)
      for everything involved, including the relationships and
      properties. The resulting data is extremely re-usable. The
      problem is that it isn't as quick as tagging with a single
      word off the top of one's head.
    </p>
    <h3>
      Example: Soccer folders
    </h3>
    <p>
      I face these problems day to day, and like many geeks, am
      driven by the urge to make the boring things in life happen
      automatically, with the computer helping more effectively.
      There are lots of things I can do with N3 rules -- but I'd
      like to have a nice user interface to it which hides as much
      technology beneath the surface as possible. I'd like as many
      non-geeks as possible to be able to use the same tools.
    </p>
    <p>
      Let's take one example. I took a bunch of photos of a local
      soccer team, once when they played Wayland, and once when
      they played Arlington. I loaded them all into iPhoto. I
      wanted to burn a CD for the team of the best of the bunch. I
      also want to be able to find them later.
    </p>
    <p>
      On the first day, I didn't take any other photos, so the
      simplest thing was to make a 'smart folder' (actually 'smart
      Album' in iPhoto) , which had in it by definition the photos
      taken on that day. The smart folder allows you to specify a
      combination (<em>and</em> or <em>or</em>) a number of
      constraints such as time, keyword, text and rating. I called
      this one <em>Soccer vs Wayland</em>.
    </p>
    <p>
      On the second day, I took other photos as well, so the smart
      folder was going to be more complicated. So instead, I just
      found all the photos, selected them, and dumped them in a new
      plain folder <em>Soccer vs Arlington</em>.
    </p>
    <p>
      These of course one would represent in RDF as classes. - but
      we'll get into that later.
    </p>
    <p>
      Ok, so here's where we get into wish-list territory.
    </p>
    <p>
      1) At that point, I wanted to be able to make a virtual
      folder <em>Soccer</em>, and make the two folders subfolders.
      (There used to be a photo processing tool called
      <em>Retriever</em> which would handle hierarchical
      classifications well, but that I lost track of.) This would
      indicate that anything in either of the two Soccer subfolders
      was a member of the Soccer folder -- or was tagged 'soccer'
      if you like.
    </p>
    <p>
      In fact, you can make a smart folder <em>Soccer</em>
      consisting of all the things which are either in <em>Soccer
      vs Wayland</em> or <em>Soccer vs Arlington</em>. You have to
      make it as a smart folder, which is not as intuitive, but
      woks fine. It doesn't give me the nice hierarchical user
      interface.
    </p>
    <h3>
      Labels
    </h3>
    <p>
      Actually I now want to associate some exportable re-usable
      data. The folder names are essentially my local tags.
      Exporting them doesn't help much.
    </p>
    <p>
      Suppose, for example, I want to geotag the photos, so that I
      can find them on a map, or people interested in sports at the
      given field could find them. The current user interface
      allows me to select all the photos in one folder and apply
      keywords and apply metadata to them, as a batch operation. It
      is actually useful that the data is carefully stored in each
      photo, but it is sad that the fact that the metadata (such as
      a comment about the game) was applied to everything in the
      folder.
    </p>
    <p>
      I'd like to be able to associate the random tag name I just
      made up with properties to be applied to each of the things
      tagged. Suppose at the user interface we introduce a
      <dfn>label</dfn>. A label is a set of common metadata that I
      want to apply to things at once.
    </p>
    <p>
      The user interface could really milk the <em>label</em>
      metaphor, by representing a label as a box with a hole in the
      end with a bit of string. It clashes perhaps with the folder
      metaphor. If we use both, then I'd like to be able to drop a
      label on a folder, and let all the things in the folder
      inherit the labeled properties.
    </p>
    <p>
      I'd like to see for each photo firstly what properties it
      has, but secondarily which labels and hence folder the
      properties came from.
    </p>
    <p>
      The essential thing about a label is that as I build it, I am
      prompted to use shared ontologies. They could be group
      ontologies which others have exported, they could be globally
      understood ontologies like time and place, and email address
      of a person depicted. As I create the label from an
      (extendable) set of options in menus, and using drag and drop
      and other user interface tricks for noting relationships, I
      am creating data which will be much more useful than the tag.
      The tag then I can slap on very easily.
    </p>
    <p>
      The hope is then that by making label creation something
      which is low cost, because I have to do it only once and can
      apply it many times, the incentive for me @@
    </p>
    <h3>
      Expressing labels
    </h3>
    <p>
      In this section we leave the user interaction and discuss the
      way in which labels can be exchanged in RDF under the covers.
      This of course is important for interoperability. A label can
      be expressed in many ways. in bits on the wire. The label
      describes a set of things, which in RDF is a class<a href=
      "#L753">*</a>. Information about the class and the things in
      it -- the things labeled -- can be given in various ways.
    </p>
    <h4>
      As a rule
    </h4>
    <p>
      As a rule, it could look like
    </p>
    <pre>
   { ?x a soc:SoccerWaylandPhoto }
=&gt; { ?x geo:approxLocation [ geo:lat 47. geo:long 78 ];
        foaf:depicts soc:ourTeam.
   }
</pre>
    <h4>
      In OWL
    </h4>
    <p>
      A label is a fairly direct use of OWL restrictions:
    </p>
    <pre>
SoccerWaylandPhoto rdfs:subClassOf [
    [ a owl:Restriction; owl:owl:onProperty geo:approxLocation;
      owl:hasValue  [ geo:lat 47. geo:long 78 ]],
    [ a owl:Restriction; owl:onPredicate foaf:depicts;
     owl:allValuesFrom soc:ourTeam].

</pre>
    <p>
      (Let's not discuss the modeling of depiction here, rather
      elsewhere.) This is very much the sort of thing OWL is
      designed for.
    </p>
    <h4>
      How not to
    </h4>
    <p>
      There is one trap which one must beware of. Remember that the
      label is a concept. It is a class. It isn't a photo. The
      label may have been created by someone, at a particular time,
      but that person and that time have nothing to do with the
      creator and time of a photo which is so labeled. You can
      <strong>not</strong> write
    </p>
    <pre>
soc:SoccerWaylandPhoto
        geo:approxLocation [ geo:lat 47. geo:long 78 ];
        foaf:depicts soc:ourTeam.
</pre>
    <h4>
      Special vocabulary
    </h4>
    <p>
      It is possible to make a special label terms which are only
      used only for labels:
    </p>
    <pre>
soc:SoccerWaylandPhoto
        LAB:approxLocation [ geo:lat 47. geo:long 78 ];
        LAB:depicts soc:ourTeam.
</pre>
    <p>
      and have some metadata like
    </p>
    <pre>
foaf:depicts  ex:labelPredicate LAB:depicts.
geo:approxLocation ex:labelPredicate LAB:approxLocation.
</pre>
    <p>
      and a general rule like
    </p>
    <pre>
    { ?x a ?lab. ?lab ?p ?z. ?p ex:labelPredicate ?q }
 =&gt; { ?x ?q ?z }.
</pre>
    <p>
      or
    </p>
    <pre>
    { ?lab ?p ?z. ?p ex:labelPredicate ?q }
 =&gt; { ?lab rdfs:subClassOf [ a owl:Restriction;
       owl:onProperty ?q; owl:hasValue ?z] }.
</pre>
    <p>
      These methods are more or less inter-convertible. There are
      various communities which understand OWL and N3 rules, which
      may find those forms most convenient.
    </p>
    <h3>
      Sharing tags and labels
    </h3>
    <p>
      The architecture of this system then is that tags are
      initially local to the user. Anyone can use any word to to
      tag anything they want. Labels are used to associate meaning
      with them, but the tag itself is local.
    </p>
    <p>
      Mapped into RDF, tags are classes in a local namespace. They
      can of course be shared. Tagging things with other people's
      tags attributes to them the properties associated with those
      tags, if any. Some people may define tags with rather loosely
      defined meaning, and no RDF labels, in which case others will
      be less inclined to use those tags.
    </p>
    <h3>
      Smart Labels and one-variable rules
    </h3>
    <p>
      When one combines a selection expression of a 'smart folder'
      with a label, then the result is a form of rule which is
      restricted to one variable. This can be expressed in OWL as a
      subclass relationship between restrictions.
    </p>
    <p>
      A lot of information can be expressed as rules, but finding
      an intuitive user interface to allow lay users to express
      their needs with rules has been a stumbling block. These
      smart folder and label metaphors, combined, could be a route
      to solving this problem<a href="#L842">*</a>.
    </p>
    <h2>
      Work in the area
    </h2>
    <p>
      There are many systems which use selection rules to define
      virtual sets of things. There probably lots which use an
      abstraction equivalent to labels.
    </p>
    <p>
      One system which effectively uses labels is (I think)
      described as 'semantic folders' (@@link Lassila and Deepali),
      to be published
    </p>
    <p>
      There is a language for labels being defined, as it happens,
      by the Web Content Labeling (WCL) Incubator Group at W3C. The
      final form of expression has not been decided.
    </p>
    <h2>
      Conclusion
    </h2>
    <p>
      The concept of a label as a preset set of data which is
      applied to things and classes of things provides an intuitive
      user interface for a operation which should be simple for
      untrained users.
    </p>
    <hr />
    <h3>
      See also
    </h3>
    <p>
      Newman.R., <a href=
      "http://www.holygoat.co.uk/projects/tags/">Tag ontology
      design&gt;</a>, 2005-03-29.
    </p>
    <p>
      Stefano Mazzochi,<a href=
      "http://www.betaversion.org/~stefano/linotype/news/85/">Folksologies:
      de-idealizing ontologies</a>, 2005-05-05
    </p>
    <p>
      Tom Gruber, <em>Where the Social Web Meets the Semantic
      Web</em>, Keynote, ISWC 2006. ( <a href=
      "http://seminars.ijs.si/iswc2006/video.asp?video_id=1831">video</a>)
    </p>
    <p>
      <a href="http://www.w3.org/2005/Incubator/wcl/">W3C Content
      Label Incubator Group</a>
    </p>
    <p>
      Dan Connolly, <a href=
      "http://www.w3.org/2000/10/swap/test/powder4.n3">Some test
      cases from WCL/POWDER work in N3.</a>
    </p>
    <h2>
      Footnotes
    </h2>
    <p>
      <a name="L753" id="L753">*</a>we do not here discuss the
      difference between rdfs:Class and owl:Class
    </p>
    <p>
      <a name="L842" id="L842">How</a> could other variables be
      added? Other variables can be expressed as paths from the
      base variable, and paths can be selected from a menu-like
      tree, and so on. The tabulator has a user interface for
      selecting a subgraph for a query. The smart folder selection
      panel could have the option for adding another similar panel
      for an item connected by a search path.
    </p>
    <hr />
    <p>
      <a href="Overview.html">Up to Design Issues</a>
    </p>
    <p>
      <a href="../People/Berners-Lee">Tim BL</a>
    </p>
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