FOAF Vocabulary Specification

Class: foaf:Agent

The foaf:Agent class is the class of agents; things that do stuff. A well known sub-class is foaf:Person, representing people. Other kinds of agents include foaf:Organization and foaf:Group.

The foaf:Agent class is useful in a few places in FOAF where foaf:Person would have been overly specific. For example, the IM chat ID properties such as jabberID are typically associated with people, but sometimes belong to software bots.

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Class: foaf:Document

The foaf:Document class represents those things which are, broadly conceived, 'documents'.

The foaf:Image class is a sub-class of foaf:Document, since all images are documents.

We do not (currently) distinguish precisely between physical and electronic documents, or between copies of a work and the abstraction those copies embody. The relationship between documents and their byte-stream representation needs clarification (see foaf:sha1 for related issues).

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Class: foaf:Group

The foaf:Group class represents a collection of individual agents (and may itself play the role of a foaf:Agent, ie. something that can perform actions).

This concept is intentionally quite broad, covering informal and ad-hoc groups, long-lived communities, organizational groups within a workplace, etc. Some such groups may have associated characteristics which could be captured in RDF (perhaps a foaf:homepage, foaf:name, mailing list etc.).

While a foaf:Group has the characteristics of a foaf:Agent, it is also associated with a number of other foaf:Agents (typically people) who constitute the foaf:Group. FOAF provides a mechanism, the foaf:membershipClass property, which relates a foaf:Group to a sub-class of the class foaf:Agent who are members of the group. This is a little complicated, but allows us to make group membership rules explicit.

The markup (shown below) for defining a group is both complex and powerful. It allows group membership rules to match against any RDF-describable characteristics of the potential group members. As FOAF and similar vocabularies become more expressive in their ability to describe individuals, the foaf:Group mechanism for categorising them into groups also becomes more powerful.

While the formal description of membership criteria for a foaf:Group may be complex, the basic mechanism for saying that someone is in a foaf:Group is very simple. We simply use a foaf:member property of the foaf:Group to indicate the agents that are members of the group. For example:

 
<foaf:Group>
 <foaf:name>ILRT staff</foaf:name>
 <foaf:member>
  <foaf:Person>
   <foaf:name>Libby Miller</foaf:name>
   <foaf:homepage rdf:resource="http://ilrt.org/people/libby/"/>
   <foaf:workplaceHomepage rdf:resource="http://www.ilrt.bris.ac.uk/"/>
  </foaf:Person>
 </foaf:member>
</foaf:Group>

Behind the scenes, further RDF statements can be used to express the rules for being a member of this group. End-users of FOAF need not pay attention to these details.

Here is an example. We define a foaf:Group representing those people who are ILRT staff members. The foaf:membershipClass property connects the group (conceived of as a social entity and agent in its own right) with the class definition for those people who constitute it. In this case, the rule is that all group members are in the ILRTStaffPerson class, which is in turn populated by all those things that are a foaf:Person and which have a foaf:workplaceHomepage of http://www.ilrt.bris.ac.uk/. This is typical: FOAF groups are created by specifying a sub-class of foaf:Agent (in fact usually this will be a sub-class of foaf:Person), and giving criteria for which things fall in or out of the sub-class. For this, we use the owl:onProperty and owl:hasValue properties, indicating the property/value pairs which must be true of matching agents.

<!-- here we see a FOAF group described.
     each foaf group may be associated with an OWL definition 
     specifying the class of agents that constitute the group's membership -->
<foaf:Group>
 <foaf:name>ILRT staff</foaf:name>
 <foaf:membershipClass>
    <owl:Class rdf:about="http://ilrt.example.com/groups#ILRTStaffPerson">
     <rdfs:subClassOf rdf:resource="http://xmlns.com/foaf/0.1/Person"/>
     <rdfs:subClassOf>
       <owl:Restriction> 
         <owl:onProperty rdf:resource="http://xmlns.com/foaf/0.1/workplaceHomepage"/>
         <owl:hasValue rdf:resource="http://www.ilrt.bris.ac.uk/"/>
       </owl:Restriction> 
     </rdfs:subClassOf>
   </owl:Class>
 </foaf:membershipClass>
</foaf:Group>

Note that while these example OWL rules for being in the eg:ILRTStaffPerson class are based on a foaf:Person having a particular foaf:workplaceHomepage, this places no obligations on the authors of actual FOAF documents to include this information. If the information is included, then generic OWL tools may infer that some person is an eg:ILRTStaffPerson. To go the extra step and infer that some eg:ILRTStaffPerson is a foaf:member of the group whose foaf:name is "ILRT staff", tools will need some knowledge of the way FOAF deals with groups. In other words, generic OWL technology gets us most of the way, but the full foaf:Group machinery requires extra work for implimentors.

The current design names the relationship as pointing from the group, to the member. This is convenient when writing XML/RDF that encloses the members within markup that describes the group. Alternate representations of the same content are allowed in RDF, so you can write claims about the Person and the Group without having to nest either description inside the other. For (brief) example:

<foaf:Group>
 <foaf:member rdf:nodeID="libby"/>
 <!-- more about the group here -->
</foaf:Group>
<foaf:Person rdf:nodeID="libby">
  <!-- more about libby here -->
</foaf:Person>

There is a FOAF issue tracker (8) associated with this FOAF term. A design goal is to make the most of W3C's OWL language for representing group-membership criteria, while also making it easy to leverage existing groups and datasets available online (eg. buddylists, mailing list membership lists etc). Feedback on the current design is solicited!

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Class: foaf:Image

The class foaf:Image is a sub-class of foaf:Document corresponding to those documents which are images.

Digital images (such as JPEG, PNG, GIF bitmaps, SVG diagrams etc.) are examples of foaf:Image.

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Class: foaf:OnlineAccount

A foaf:OnlineAccount represents the provision of some form of online service, by some party (indicated indirectly via a foaf:accountServiceHomepage) to some foaf:Agent. The foaf:holdsAccount property of the agent is used to indicate accounts that are associated with the agent.

See foaf:OnlineChatAccount for an example. Other sub-classes include foaf:OnlineEcommerceAccount and foaf:OnlineGamingAccount.

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Class: foaf:OnlineChatAccount

A foaf:OnlineChatAccount is a foaf:OnlineAccount devoted to chat / instant messaging.

This is a generalization of the FOAF Chat ID properties, foaf:jabberID, foaf:aimChatID, foaf:msnChatID, foaf:icqChatID and foaf:yahooChatID.

Unlike those simple properties, foaf:OnlineAccount and associated FOAF terms allows us to describe a great variety of online accounts, without having to anticipate them in the FOAF vocabulary.

For example, here is a description of an IRC chat account, specific to the Freenode IRC network:

<foaf:Person>
  <foaf:name>Dan Brickley</foaf:name>
  <foaf:holdsAccount>
    <foaf:OnlineAccount>
      <rdf:type rdf:resource="http://xmlns.com/foaf/0.1/OnlineChatAccount"/>
      <foaf:accountServiceHomepage rdf:resource="http://www.freenode.net/irc_servers.shtml"/>
      <foaf:accountName>danbri</foaf:accountName>
    </foaf:OnlineAccount>
  </foaf:holdsAccount>
</foaf:Person>

Note that it may be impolite to carelessly reveal someone else's chat identifier (which might also serve as an indicate of email address) As with email, there are privacy and anti-SPAM considerations. FOAF does not currently provide a way to represent an obfuscated chat ID (ie. there is no parallel to the foaf:mbox / foaf:mbox_sha1sum mapping).

In addition to the generic foaf:OnlineAccount and foaf:OnlineChatAccount mechanisms, FOAF also provides several convenience chat ID properties (foaf:jabberID, foaf:aimChatID, foaf:icqChatID, foaf:msnChatID,foaf:yahooChatID). These serve as as a shorthand for some common cases; their use may not always be appropriate.

We should specify some mappings between the abbreviated and full representations of Jabber, AIM, MSN, ICQ, Yahoo! and MSN chat accounts. This requires us to identify an appropriate foaf:accountServiceHomepage for each. If we wanted to make the foaf:OnlineAccount mechanism even more generic, we could invent a relationship that holds between a foaf:OnlineAccount instance and a convenience property. To continue the example above, we could describe how Freenode could define a property 'fn:freenodeChatID' corresponding to Freenode online accounts.

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Class: foaf:OnlineEcommerceAccount

A foaf:OnlineEcommerceAccount is a foaf:OnlineAccount devoted to buying and/or selling of goods, services etc. Examples include Amazon, eBay, PayPal, thinkgeek, etc.

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Class: foaf:OnlineGamingAccount

A foaf:OnlineGamingAccount is a foaf:OnlineAccount devoted to online gaming.

Examples might include EverQuest, Xbox live, Neverwinter Nights, etc., as well as older text-based systems (MOOs, MUDs and suchlike).

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Class: foaf:Organization

The foaf:Organization class represents a kind of foaf:Agent corresponding to social instititutions such as companies, societies etc.

This is a more 'solid' class than foaf:Group, which allows for more ad-hoc collections of individuals. These terms are rather roughly defined, and further work is needed to clearly specify their inter-relationships.

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Class: foaf:Person

The foaf:Person class represents people. Something is a foaf:Person if it is a person. We don't nitpic about whether they're alive, dead, real, or imaginary. The foaf:Person class is a sub-class of the foaf:Agent class, since all people are considered 'agents' in FOAF.

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Class: foaf:PersonalProfileDocument

No detailed documentation for this term.

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Class: foaf:Project

The foaf:Project class represents the class of things that are 'projects'. These may be formal or informal, collective or individual. It is often useful to indicate the foaf:homepage of a foaf:Project.

Further work is needed to specify the connections between this class and the FOAF properties foaf:currentProject and foaf:pastProject.

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Property: foaf:accountName

The foaf:accountName property of a foaf:OnlineAccount is a textual representation of the account name (unique ID) associated with that account.

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Property: foaf:accountServiceHomepage

The foaf:accountServiceHomepage property indicates a relationship between a foaf:OnlineAccount and the homepage of the supporting service provider.

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Property: foaf:aimChatID

The foaf:aimChatID property relates a foaf:Agent to a textual identifier ('screenname') assigned to them in the AOL Instant Messanger (AIM) system. See AOL's AIM site for more details of AIM and AIM screennames. The iChat tools from Apple also make use of AIM identifiers.

See foaf:OnlineChatAccount (and foaf:OnlineAccount) for a more general (and verbose) mechanism for describing IM and chat accounts.

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Property: foaf:based_near

The foaf:based_near relationship relates two "spatial things" (anything that can be somewhere), the latter typically described using the geo:lat / geo:long geo-positioning vocabulary (See GeoInfo in the W3C semweb wiki for details). This allows us to say describe the typical latitute and longitude of, say, a Person (people are spatial things - they can be places) without implying that a precise location has been given.

We do not say much about what 'near' means in this context; it is a 'rough and ready' concept. For a more precise treatment, see GeoOnion vocab design discussions, which are aiming to produce a more sophisticated vocabulary for such purposes.

FOAF files often make use of the contact:nearestAirport property. This illustrates the distinction between FOAF documents (which may make claims using any RDF vocabulary) and the core FOAF vocabulary defined by this specification. For further reading on the use of nearestAirport see UsingContactNearestAirport in the FOAF wiki.

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Property: foaf:birthday

The foaf:birthday property is a relationship between a foaf:Agent and a string representing the month and day in which they were born (Gregorian calendar). See BirthdayIssue for details of related properties that can be used to describe such things in more flexible ways.

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Property: foaf:currentProject

A foaf:currentProject relates a foaf:Person to a foaf:Document indicating some collaborative or individual undertaking. This relationship indicates that the foaf:Person has some active role in the project, such as development, coordination, or support.

When a foaf:Person is no longer involved with a project, or perhaps is inactive for some time, the relationship becomes a foaf:pastProject.

If the foaf:Person has stopped working on a project because it has been completed (successfully or otherwise), foaf:pastProject is applicable. In general, foaf:currentProject is used to indicate someone's current efforts (and implied interests, concerns etc.), while foaf:pastProject describes what they've previously been doing.

Note that this property requires further work. There has been confusion about whether it points to a thing (eg. something you've made; a homepage for a project, ie. a foaf:Document or to instances of the class foaf:Project, which might themselves have a foaf:homepage. In practice, it seems to have been used in a similar way to foaf:interest, referencing homepages of ongoing projects.

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Property: foaf:depiction

The foaf:depiction property is a relationship between a thing and an foaf:Image that depicts it. As such it is an inverse of the foaf:depicts relationship.

A common use of foaf:depiction (and foaf:depicts) is to indicate the contents of a digital image, for example the people or objects represented in an online photo gallery.

Extensions to this basic idea include 'Co-Depiction' (social networks as evidenced in photos), as well as richer photo metadata through the mechanism of using SVG paths to indicate the regions of an image which depict some particular thing. See 'Annotating Images With SVG' for tools and details.

The basic notion of 'depiction' could also be extended to deal with multimedia content (video clips, audio), or refined to deal with corner cases, such as pictures of pictures etc.

The foaf:depiction property is a super-property of the more specific property foaf:img, which is used more sparingly. You stand in a foaf:depiction relation to any foaf:Image that depicts you, whereas foaf:img is typically used to indicate a few images that are particularly representative.

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Property: foaf:depicts

The foaf:depicts property is a relationship between a foaf:Image and something that the image depicts. As such it is an inverse of the foaf:depiction relationship. See foaf:depiction for further notes.

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Property: foaf:dnaChecksum

The foaf:dnaChecksum property is mostly a joke, but also a reminder that there will be lots of different identifying properties for people, some of which we might find disturbing.

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Property: foaf:family_name

A number of naming constructs are under development to provide naming substructure; draft properties include foaf:firstName, foaf:givenname, and foaf:surname. These are not currently stable or consistent; see the issue tracker for design discussions, status and ongoing work on rationalising the FOAF naming machinery.

There is also a simple foaf:name property.

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Property: foaf:firstName

A number of naming constructs are under development to provide naming substructure; draft properties include foaf:firstName, foaf:givenname, and foaf:surname. These are not currently stable or consistent; see the issue tracker for design discussions, status and ongoing work on rationalising the FOAF naming machinery.

There is also a simple foaf:name property.

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Property: foaf:fundedBy

The foaf:fundedBy property relates something to something else that has provided funding for it.

This property is under-specified, experimental, and should be considered liable to change.

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Property: foaf:geekcode

The foaf:geekCode property is used to represent a 'Geek Code' for some foaf:Person.

See the Code of the Geeks specification for details of the code, which provides a somewhat frivolous and willfully obscure mechanism for characterising technical expertise, interests and habits. The foaf:geekCode property is not bound to any particular version of the code. The last published version of the code was v3.12 in March 1996.

As the Geek Code website notes, the code played a small (but amusing) part in the history of the Internet. The foaf:geekCode property exists in acknowledgement of this history. It'll never be 1996 again.

Note that the Geek Code is a densely packed collections of claims about the person it applies to; to express these claims explicitly in RDF/XML would be incredibly verbose. The syntax of the Geek Code allows for '<' and '>' characters, which have special meaning in RDF/XML. Consequently these should be carefully escaped in markup.

An example Geek Code:

GED/J d-- s:++>: a-- C++(++++) ULU++ P+ L++ E---- W+(-) N+++ o+ K+++ w--- O- M+ V-- PS++>$ PE++>$ Y++ PGP++ t- 5+++ X++ R+++>$ tv+ b+ DI+++ D+++ G++++ e++ h r-- y++**

...would be written in FOAF RDF/XML as follows:

<foaf:geekCode> GED/J d-- s:++&gt;: a-- C++(++++) ULU++ P+ L++ E---- W+(-) N+++ o+ K+++ w--- O- M+ V-- PS++&gt;$ PE++&gt;$ Y++ PGP++ t- 5+++ X++ R+++&gt;$ tv+ b+ DI+++ D+++ G++++ e++ h r-- y++** </foaf:geekCode>

See also the geek code entry in everything2, which tells us that the geek code originated in 1993; it was inspired (according to the inventor) by previous "bear", "smurf" and "twink" style-and-sexual-preference codes from lesbian and gay newsgroups. There is also a Geek Code Decoder Page and a form-based generator.

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Property: foaf:gender

The foaf:gender property relates a foaf:Agent (typically a foaf:Person) to a string representing its gender. In most cases the value will be the string 'female' or 'male' (in lowercase without surrounding quotes or spaces). Like all FOAF properties, there is in general no requirement to use foaf:gender in any particular document or description. Values other than 'male' and 'female' may be used, but are not enumerated here. The foaf:gender mechanism is not intended to capture the full variety of biological, social and sexual concepts associated with the word 'gender'.

Anything that has a foaf:gender property will be some kind of foaf:Agent. However there are kinds of foaf:Agent to which the concept of gender isn't applicable (eg. a foaf:Group). FOAF does not currently include a class corresponding directly to "the type of thing that has a gender". At any point in time, a foaf:Agent has at most one value for foaf:gender. FOAF does not treat foaf:gender as a static property; the same individual may have different values for this property at different times.

Note that FOAF's notion of gender isn't defined biologically or anatomically - this would be tricky since we have a broad notion that applies to all foaf:Agents (including robots - eg. Bender from Futurama is 'male'). As stressed above, FOAF's notion of gender doesn't attempt to encompass the full range of concepts associated with human gender, biology and sexuality. As such it is a (perhaps awkward) compromise between the clinical and the social/psychological. In general, a person will be the best authority on their foaf:gender. Feedback on this design is particularly welcome (via the FOAF mailing list, rdfweb-dev). We have tried to be respectful of diversity without attempting to catalogue or enumerate that diversity.

This may also be a good point for a periodic reminder: as with all FOAF properties, documents that use 'foaf:gender' will on occassion be innacurate, misleading or outright false. FOAF, like all open means of communication, supports lying. Application authors using FOAF data should always be cautious in their presentation of unverified information, but be particularly sensitive to issues and risks surrounding sex and gender (including privacy and personal safety concerns). Designers of FOAF-based user interfaces should be careful to allow users to omit foaf:gender when describing themselves and others, and to allow at least for values other than 'male' and 'female' as options. Users of information conveyed via FOAF (as via information conveyed through mobile phone text messages, email, Internet chat, HTML pages etc.) should be skeptical of unverified information.

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Property: foaf:givenname

A number of naming constructs are under development to provide naming substructure; draft properties include foaf:firstName, foaf:givenname, and foaf:surname. These are not currently stable or consistent; see the issue tracker for design discussions, status and ongoing work on rationalising the FOAF naming machinery.

There is also a simple foaf:name property.

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Property: foaf:holdsAccount

The foaf:holdsAccount property relates a foaf:Agent to an foaf:OnlineAccount for which they are the sole account holder. See foaf:OnlineAccount for usage details.

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Property: foaf:homepage

The foaf:homepage property relates something to a homepage about it.

Many kinds of things have homepages. FOAF allows a thing to have multiple homepages, but constrains foaf:homepage so that there can be only one thing that has any particular homepage.

A 'homepage' in this sense is a public Web document, typically but not necessarily available in HTML format. The page has as a foaf:topic the thing whose homepage it is. The homepage is usually controlled, edited or published by the thing whose homepage it is; as such one might look to a homepage for information on its owner from its owner. This works for people, companies, organisations etc.

The foaf:homepage property is a sub-property of the more general foaf:page property for relating a thing to a page about that thing. See also foaf:topic, the inverse of the foaf:page property.

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Property: foaf:icqChatID

The foaf:icqChatID property relates a foaf:Agent to a textual identifier assigned to them in the ICQ Chat system. See the icq chat site for more details of the 'icq' service. Their "What is ICQ?" document provides a basic overview, while their "About Us page notes that ICQ has been acquired by AOL. Despite the relationship with AOL, ICQ is at the time of writing maintained as a separate identity from the AIM brand (see foaf:aimChatID).

See foaf:OnlineChatAccount (and foaf:OnlineAccount) for a more general (and verbose) mechanism for describing IM and chat accounts.

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Property: foaf:img

The foaf:img property relates a foaf:Person to a foaf:Image that represents them. Unlike its super-property foaf:depiction, we only use foaf:img when an image is particularly representative of some person. The analogy is with the image(s) that might appear on someone's homepage, rather than happen to appear somewhere in their photo album.

Unlike the more general foaf:depiction property (and its inverse, foaf:depicts), the foaf:img property is only used with representations of people (ie. instances of foaf:Person). So you can't use it to find pictures of cats, dogs etc. The basic idea is to have a term whose use is more restricted than foaf:depiction so we can have a useful way of picking out a reasonable image to represent someone. FOAF defines foaf:img as a sub-property of foaf:depiction, which means that the latter relationship is implied whenever two things are related by the former.

Note that foaf:img does not have any restrictions on the dimensions, colour depth, format etc of the foaf:Image it references.

Terminology: note that foaf:img is a property (ie. relationship), and that code:Image is a similarly named class (ie. category, a type of thing). It might have been more helpful to call foaf:img 'mugshot' or similar; instead it is named by analogy to the HTML IMG element.

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Property: foaf:interest

The foaf:interest property represents an interest of a foaf:Agent, through indicating a foaf:Document whose foaf:topic(s) broadly characterises that interest.

For example, we might claim that a person or group has an interest in RDF by saying they stand in a foaf:interest relationship to the RDF home page. Loosly, such RDF would be saying "this agent is interested in the topic of this page".

Uses of foaf:interest include a variety of filtering and resource discovery applications. It could be used, for example, to help find answers to questions such as "Find me members of this organisation with an interest in XML who have also contributed to CPAN)".

This approach to characterising interests is intended to compliment other mechanisms (such as the use of controlled vocabulary). It allows us to use a widely known set of unique identifiers (Web page URIs) with minimal pre-coordination. Since URIs have a controlled syntax, this makes data merging much easier than the use of free-text characterisations of interest.

Note that interest does not imply expertise, and that this FOAF term provides no support for characterising levels of interest: passing fads and lifelong quests are both examples of someone's foaf:interest. Describing interests in full is a complex undertaking; foaf:interest provides one basic component of FOAF's approach to these problems.

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Property: foaf:jabberID

The foaf:jabberID property relates a foaf:Agent to a textual identifier assigned to them in the Jabber messaging system. See the Jabber site for more information about the Jabber protocols and tools.

Jabber, unlike several other online messaging systems, is based on an open, publically documented protocol specification, and has a variety of open source implementations. Jabber IDs can be assigned to a variety of kinds of thing, including software 'bots', chat rooms etc. For the purposes of FOAF, these are all considered to be kinds of foaf:Agent (ie. things that do stuff). The uses of Jabber go beyond simple IM chat applications. The foaf:jabberID property is provided as a basic hook to help support RDF description of Jabber users and services.

See foaf:OnlineChatAccount (and foaf:OnlineAccount) for a more general (and verbose) mechanism for describing IM and chat accounts.

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Property: foaf:knows

The foaf:knows property relates a foaf:Person to another foaf:Person that he or she knows.

We take a broad view of 'knows', but do require some form of reciprocated interaction (ie. stalkers need not apply). Since social attitudes and conventions on this topic vary greatly between communities, counties and cultures, it is not appropriate for FOAF to be overly-specific here.

If someone foaf:knows a person, it would be usual for the relation to be reciprocated. However this doesn't mean that there is any obligation for either party to publish FOAF describing this relationship. A foaf:knows relationship does not imply friendship, endorsement, or that a face-to-face meeting has taken place: phone, fax, email, and smoke signals are all perfectly acceptable ways of communicating with people you know.

You probably know hundreds of people, yet might only list a few in your public FOAF file. That's OK. Or you might list them all. It is perfectly fine to have a FOAF file and not list anyone else in it at all. This illustrates the Semantic Web principle of partial description: RDF documents rarely describe the entire picture. There is always more to be said, more information living elsewhere in the Web (or in our heads...).

Since foaf:knows is vague by design, it may be suprising that it has uses. Typically these involve combining other RDF properties. For example, an application might look at properties of each foaf:weblog that was foaf:made by someone you "foaf:knows". Or check the newsfeed of the online photo archive for each of these people, to show you recent photos taken by people you know.

To provide additional levels of representation beyond mere 'knows', FOAF applications can do several things.

They can use more precise relationships than foaf:knows to relate people to people. The original FOAF design included two of these ('knowsWell','friend') which we removed because they were somewhat awkward to actually use, bringing an inappopriate air of precision to an intrinsically vague concept. Other extensions have been proposed, including Eric Vitiello's Relationship module for FOAF.

In addition to using more specialised inter-personal relationship types (eg rel:acquaintanceOf etc) it is often just as good to use RDF descriptions of the states of affairs which imply particular kinds of relationship. So for example, two people who have the same value for their foaf:workplaceHomepage property are typically colleagues. We don't (currently) clutter FOAF up with these extra relationships, but the facts can be written in FOAF nevertheless. Similarly, if there exists a foaf:Document that has two people listed as its foaf:makers, then they are probably collaborators of some kind. Or if two people appear in 100s of digital photos together, there's a good chance they're friends and/or colleagues.

So FOAF is quite pluralistic in its approach to representing relationships between people. FOAF is built on top of a general purpose machine language for representing relationships (ie. RDF), so is quite capable of representing any kinds of relationship we care to add. The problems are generally social rather than technical; deciding on appropriate ways of describing these interconnections is a subtle art.

Perhaps the most important use of foaf:knows is, alongside the rdfs:seeAlso property, to connect FOAF files together. Taken alone, a FOAF file is somewhat dull. But linked in with 1000s of other FOAF files it becomes more interesting, with each FOAF file saying a little more about people, places, documents, things... By mentioning other people (via foaf:knows or other relationships), and by providing an rdfs:seeAlso link to their FOAF file, you can make it easy for FOAF indexing tools ('scutters') to find your FOAF and the FOAF of the people you've mentioned. And the FOAF of the people they mention, and so on. This makes it possible to build FOAF aggregators without the need for a centrally managed directory of FOAF files...

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Property: foaf:logo

The foaf:logo property is used to indicate a graphical logo of some kind. It is probably underspecified...

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Property: foaf:made

The foaf:made property relates a foaf:Agent to something foaf:made by it. As such it is an inverse of the foaf:maker property, which relates a thing to something that made it. See foaf:made for more details on the relationship between these FOAF terms and related Dublin Core vocabulary.

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Property: foaf:maker

The foaf:maker property relates something to a foaf:Agent that foaf:made it. As such it is an inverse of the foaf:made property.

The foaf:name (or other rdfs:label) of the foaf:maker of something can be described as the dc:creator of that thing.

For example, if the thing named by the URI http://rdfweb.org/people/danbri/ has a foaf:maker that is a foaf:Person whose foaf:name is 'Dan Brickley', we can conclude that http://rdfweb.org/people/danbri/ has a dc:creator of 'Dan Brickley'.

FOAF descriptions are encouraged to use dc:creator only for simple textual names, and to use foaf:maker to indicate creators, rather than risk confusing creators with their names. This follows most Dublin Core usage. See UsingDublinCoreCreator for details.

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Property: foaf:mbox

The foaf:mbox property is a relationship between the owner of a mailbox and a mailbox. These are typically identified using the mailto: URI scheme (see RFC 2368).

Note that there are many mailboxes (eg. shared ones) which are not the foaf:mbox of anyone. Furthermore, a person can have multiple foaf:mbox properties.

In FOAF, we often see foaf:mbox used as an indirect way of identifying its owner. This works even if the mailbox is itself out of service (eg. 10 years old), since the property is defined in terms of its primary owner, and doesn't require the mailbox to actually be being used for anything.

Many people are wary of sharing information about their mailbox addresses in public. To address such concerns whilst continuing the FOAF convention of indirectly identifying people by referring to widely known properties, FOAF also provides the foaf:mbox_sha1sum mechanism, which is a relationship between a person and the value you get from passing a mailbox URI to the SHA1 mathematical function.

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Property: foaf:mbox_sha1sum

A foaf:mbox_sha1sum of a foaf:Person is a textual representation of the result of applying the SHA1 mathematical functional to a 'mailto:' identifier (URI) for an Internet mailbox that they stand in a foaf:mbox relationship to.

In other words, if you have a mailbox (foaf:mbox) but don't want to reveal its address, you can take that address and generate a foaf:mbox_sha1sum representation of it. Just as a foaf:mbox can be used as an indirect identifier for its owner, we can do the same with foaf:mbox_sha1sum since there is only one foaf:Person with any particular value for that property.

Many FOAF tools use foaf:mbox_sha1sum in preference to exposing mailbox information. This is usually for privacy and SPAM-avoidance reasons. Other relevant techniques include the use of PGP encryption (see Edd Dumbill's documentation) and the use of FOAF-based whitelists for mail filtering.

Code examples for SHA1 in C#, Java, PHP, Perl and Python can be found in Sam Ruby's weblog entry. Remember to include the 'mailto:' prefix, but no trailing whitespace, when computing a foaf:mbox_sha1sum property.

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Property: foaf:member

The foaf:member property relates a foaf:Group to a foaf:Agent that is a member of that group.

See foaf:Group for details and examples.

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Property: foaf:membershipClass

The foaf:membershipClass property relates a foaf:Group to an RDF class representing a sub-class of foaf:Agent whose instances are all the agents that are a foaf:member of the foaf:Group.

See foaf:Group for details and examples.

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Property: foaf:msnChatID

The foaf:msnChatID property relates a foaf:Agent to a textual identifier assigned to them in the MSN online Chat system. See Microsoft's the MSN chat site for more details (or for a message saying "MSN Chat is not currently compatible with your Internet browser and/or computer operating system" if your computing platform is deemed unsuitable).

It is not currently clear how MSN chat IDs relate to the more general Microsoft Passport identifiers.

See foaf:OnlineChatAccount (and foaf:OnlineAccount) for a more general (and verbose) mechanism for describing IM and chat accounts.

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Property: foaf:myersBriggs

The foaf:myersBriggs property represents the Myers Briggs (MBTI) approach to personality taxonomy. It is included in FOAF as an example of a property that takes certain constrained values, and to give some additional detail to the FOAF files of those who choose to include it. The foaf:myersBriggs property applies only to the foaf:Person class; wherever you see it, you can infer it is being applied to a person.

The foaf:myersBriggs property is interesting in that it illustrates how FOAF can serve as a carrier for various kinds of information, without necessarily being commited to any associated worldview. Not everyone will find myersBriggs (or star signs, or blood types, or the four humours) a useful perspective on human behaviour and personality. The inclusion of a Myers Briggs property doesn't indicate that FOAF endorses the underlying theory, any more than the existence of foaf:weblog is an endorsement of soapboxes.

The values for foaf:myersBriggs are the following 16 4-letter textual codes: ESTJ, INFP, ESFP, INTJ, ESFJ, INTP, ENFP, ISTJ, ESTP, INFJ, ENFJ, ISTP, ENTJ, ISFP, ENTP, ISFJ. If multiple of these properties are applicable, they are represented by applying multiple properties to a person.

For further reading on MBTI, see various online sources (eg. this article). There are various online sites which offer quiz-based tools for determining a person's MBTI classification. The owners of the MBTI trademark have probably not approved of these.

This FOAF property suggests some interesting uses, some of which could perhaps be used to test the claims made by proponents of the MBTI (eg. an analysis of weblog postings filtered by MBTI type). However it should be noted that MBTI FOAF descriptions are self-selecting; MBTI categories may not be uniformly appealing to the people they describe. Further, there is probably a degree of cultural specificity implicit in the assumptions made by many questionaire-based MBTI tools; the MBTI system may not make sense in cultural settings beyond those it was created for.

See also Cory Caplinger's summary table or the RDFWeb article, FOAF Myers Briggs addition for further background and examples.

Note: Myers Briggs Type Indicator and MBTI are registered trademarks of Consulting Psychologists Press Inc. Oxford Psycholgists Press Ltd has exclusive rights to the trademark in the UK.

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Property: foaf:name

The foaf:name of something is a simple textual string.

XML language tagging may be used to indicate the language of the name. For example:

FOAF provides some other naming constructs. These are under development. While foaf:name does not explicitly represent name substructure (family vs given etc.) it does provide a basic level of interoperability. See the RDFWeb/FOAF issue tracker for status of work on this issue.

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Property: foaf:nick

The foaf:nick property relates a foaf:Person to a short (often abbreviated) nickname, such as those use in IRC chat, online accounts, and computer logins.

This property is necessarily vague, because it does not indicate any particular naming control authority, and so cannot distinguish a person's login from their (possibly various) IRC nicknames or other similar identifiers. However it has some utility, since many people use the same string (or slight variants) across a variety of such environments.

For specific controlled sets of names (relating primarily to Instant Messanger accounts), FOAF provides some convenience properties: foaf:jabberID, foaf:aimChatID, foaf:msnChatID and foaf:icqChatID. Beyond this, the problem of representing such accounts is not peculiar to Instant Messanging, and it is not scaleable to attempt to enumerate each naming database as a distinct FOAF property. The foaf:OnlineAccount term (and supporting vocabulary) are provided as a more verbose and more expressive generalisation of these properties.

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Property: foaf:page

The foaf:page property relates a thing to a document about that thing.

As such it is an inverse of the foaf:topic property, which relates a document to a thing that the document is about.

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Property: foaf:pastProject

After a foaf:Person is no longer involved with a foaf:currentProject, or has been inactive for some time, a foaf:pastProject relationship can be used. This indicates that the foaf:Person was involved with the described project at one point.

If the foaf:Person has stopped working on a project because it has been completed (successfully or otherwise), foaf:pastProject is applicable. In general, foaf:currentProject is used to indicate someone's current efforts (and implied interests, concerns etc.), while foaf:pastProject describes what they've previously been doing.

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Property: foaf:phone

The foaf:phone of something is a phone, typically identified using the tel: URI scheme.

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Property: foaf:plan

The foaf:plan property provides a space for a foaf:Person to hold some arbitrary content that would appear in a traditional '.plan' file. The plan file was stored in a user's home directory on a UNIX machine, and displayed to people when the user was queried with the finger utility.

A plan file could contain anything. Typical uses included brief comments, thoughts, or remarks on what a person had been doing lately. Plan files were also prone to being witty or simply osbscure. Others may be more creative, writing any number of seemingly random compositions in their plan file for people to stumble upon.

See History of the Finger Protocol by Rajiv Shah for more on this piece of Internet history. The foaf:geekCode property may also be of interest.

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Property: foaf:primaryTopic

No detailed documentation for this term.

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Property: foaf:publications

The foaf:publications property indicates a foaf:Document listing (primarily in human-readable form) some publications associated with the foaf:Person. Such documents are typically published alongside one's foaf:homepage.

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Property: foaf:schoolHomepage

The schoolHomepage property relates a foaf:Person to a foaf:Document that is the foaf:homepage of a School that the person attended.

FOAF does not (currently) define a class for 'School' (if it did, it would probably be as a sub-class of foaf:Organization). The original application area for foaf:schoolHomepage was for 'schools' in the British-English sense; however American-English usage has dominated, and it is now perfectly reasonable to describe Universities, Colleges and post-graduate study using foaf:schoolHomepage.

This very basic facility provides a basis for a low-cost, decentralised approach to classmate-reunion and suchlike. Instead of requiring a central database, we can use FOAF to express claims such as 'I studied here' simply by mentioning a school's homepage within FOAF files. Given the homepage of a school, it is easy for FOAF aggregators to lookup this property in search of people who attended that school.

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Property: foaf:sha1

The foaf:sha1 property relates a foaf:Document to the textual form of a SHA1 hash of (some representation of) its contents.

The design for this property is neither complete nor coherent. The foaf:Document class is currently used in a way that allows multiple instances at different URIs to have the 'same' contents (and hence hash). If foaf:sha1 is an owl:InverseFunctionalProperty, we could deduce that several such documents were the self-same thing. A more careful design is needed, which distinguishes documents in a broad sense from byte sequences.

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Property: foaf:surname

A number of naming constructs are under development to provide naming substructure; draft properties include foaf:firstName, foaf:givenname, and foaf:surname. These are not currently stable or consistent; see the issue tracker for design discussions, status and ongoing work on rationalising the FOAF naming machinery.

There is also a simple foaf:name property.

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Property: foaf:theme

The foaf:theme property is rarely used and under-specified. The intention was to use it to characterise interest / themes associated with projects and groups. Further work is needed to meet these goals.

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Property: foaf:thumbnail

The foaf:thumbnail property is a relationship between a full-size foaf:Image and a smaller, representative foaf:Image that has been derrived from it.

It is typical in FOAF to express foaf:img and foaf:depiction relationships in terms of the larger, 'main' (in some sense) image, rather than its thumbnail(s). A foaf:thumbnail might be clipped or otherwise reduced such that it does not depict everything that the full image depicts. Therefore FOAF does not specify that a thumbnail foaf:depicts everything that the image it is derrived from depicts. However, FOAF does expect that anything depicted in the thumbnail will also be depicted in the source image.

A foaf:thumbnail is typically small enough that it can be loaded and viewed quickly before a viewer decides to download the larger version. They are often used in online photo gallery applications.

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Property: foaf:tipjar

No detailed documentation for this term.

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Property: foaf:title

The approriate values for foaf:title are not formally constrained, and will vary across community and context. Values such as 'Mr', 'Mrs', 'Ms', 'Dr' etc. are expected.

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Property: foaf:topic

The foaf:topic property relates a document to a thing that the document is about.

As such it is an inverse of the foaf:page property, which relates a thing to a document about that thing.

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Property: foaf:topic_interest

The foaf:topic_interest property is generally found to be confusing and ill-defined and is a candidate for removal. The goal was to be link a person to some thing that is a topic of their interests (rather than, per foaf:interest to a page that is about such a topic).

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Property: foaf:weblog

The foaf:weblog property relates a foaf:Agent to a weblog of that agent.

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Property: foaf:workInfoHomepage

The foaf:workInfoHomepage of a foaf:Person is a foaf:Document that describes their work. It is generally (but not necessarily) a different document from their foaf:homepage, and from any foaf:workplaceHomepage(s) they may have.

The purpose of this property is to distinguish those pages you often see, which describe someone's professional role within an organisation or project. These aren't really homepages, although they share some characterstics.

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Property: foaf:workplaceHomepage

The foaf:workplaceHomepage of a foaf:Person is a foaf:Document that is the foaf:homepage of a foaf:Organization that they work for.

By directly relating people to the homepages of their workplace, we have a simple convention that takes advantage of a set of widely known identifiers, while taking care not to confuse the things those identifiers identify (ie. organizational homepages) with the actual organizations those homepages describe.

<foaf:Person>
 <foaf:name>Dan Brickley</foaf:name>
 <foaf:workplaceHomepage rdf:resource="http://www.w3.org/"/>
</foaf:Person>

Note that several other FOAF properties work this way; foaf:schoolHomepage is the most similar. In general, FOAF often indirectly identifies things via Web page identifiers where possible, since these identifiers are widely used and known. FOAF does not currently have a term for the name of the relation (eg. "workplace") that holds between a foaf:Person and an foaf:Organization that they work for.

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Property: foaf:yahooChatID

The foaf:yahooChatID property relates a foaf:Agent to a textual identifier assigned to them in the Yahoo online Chat system. See Yahoo's the Yahoo! Chat site for more details of their service. Yahoo chat IDs are also used across several other Yahoo services, including email and Yahoo! Groups.

See foaf:OnlineChatAccount (and foaf:OnlineAccount) for a more general (and verbose) mechanism for describing IM and chat accounts.

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