In RDF, a blank node or anonymous resource or bnode) is a resource, or node in an RDF graph, which is not identified by a URI. A blank node can be used as subject or object in an RDF triple.

Example Edit

"John has a friend born the 21st of April" can be written with two triples linked by a blank node representing the anonymous friend of John.

ex:John   foaf:knows       _:p1
_:p1      foaf:birthDate   04-21

The first triple reads "John knows p1". The second triple reads "p1 is born on April 21st"

ex:John is a named resource, which means this resource is absolutely identified by the URI obtained by replacing the ex: prefix by the XML namespace it stands for, such as

_:p1 represents John's anonymous friend, not identified by a URI. One can know by the semantics declared in the FOAF vocabulary that the class of _:p1 is foaf:Person.

RDF-XML Notation Edit

In RDF-XML syntax a blank node can be represented by nested elements, such as the following.

<foaf:Person rdf:about="">
  <foaf:Person foaf:birthDate="04-21"/>

If the same blank node is used more that once in the same RDF graph, it can be identified by a rdf:nodeID attribute. This identification is limited to the local graph. For example to express that John and Mary have a common friend, one can write.

<foaf:Person rdf:about="">
  <foaf:Person rdf:nodeID="b1"/>
<foaf:Person rdf:about="">
  <foaf:Person rdf:nodeID="b1"/>

Classical use casesEdit

Representation of complex data Edit

A blank node can be used to indirectly attach to a resource a consistent set of properties which together represent a complex data, such as a postal address. The different fields of the complex data are represented as properties attached to the blank node. For example in the RDF VCard vocabulary one will write.

   <rdf:Description rdf:about = "" >
     <vCard:ADR rdf:parseType="Resource">
       <vCard:Street>111 Lake Drive </vCard:Street>
       <vCard:Locality>WonderCity </vCard:Locality>

Anonymous classes in OWLEdit

The ontology language OWL uses blank nodes to represent anonymous classes such as unions or intersections of classes, or classes called restrictions, defined by a constraint on a property.

For example to express that a person has at most one birth date, one will define the class "Person" as a subclass of an anonymous class of type "owl:Restriction". This anonymous class is defined by two attributes specifying the constrained property and the constraint itself (cardinality > 1)

<owl:Class rdf:about="">
       <owl:onProperty rdf:resource=""/>
</owl:Class>fr:Ressource anonyme

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