Rdf Primer / quote 50 of 55
Reification and the reification quad
RDF applications sometimes need to describe other RDF statements using RDF, for instance, to record information about when statements were made, who made them, or other similar information (this is sometimes referred to as "provenance" information). [...] RDF provides a built-in vocabulary intended for describing RDF statements. A description of a statement using this vocabulary is called a reification of the statement. The RDF reification vocabulary consists of the type rdf:Statement, and the properties rdf:subject, rdf:predicate, and rdf:object. However, while RDF provides this reification vocabulary, care is needed in using it, because it is easy to imagine that the vocabulary defines some things that are not actually defined. [...] The conventional use of the RDF reification vocabulary always involves describing a statement using four statements in this pattern; the four statements are sometimes referred to as a "reification quad" for this reason.