RDF-star

RDF-star (originally spelled RDF*, and covering Turtle-star, TriG-star, and SPARQL-star) is an extension of the RDF data model that lets a triple itself serve as the subject or object of another triple, so one can make statements about statements without the verbose four-triple pattern of classical RDF reification. It was proposed in 2014 by Olaf Hartig and Bryan Thompson, refined from 2019 onward within a subgroup of the W3C RDF-DEV Community Group, and ultimately fed into the W3C RDF-star / RDF 1.2 Working Group chartered in August 2022. It is not a standalone serialization but a coordinated set of syntactic and semantic extensions layered onto existing RDF concrete syntaxes such as Turtle, TriG, N-Triples, N-Quads, and RDF/XML.

In the community-report form, an embedded triple is written as a quoted triple using double angle brackets, for example << :employee38 :jobTitle "Assistant Designer" >> :accordingTo :employee22 ., and Turtle-star adds an annotation shorthand {| ... |} to attach metadata to a triple asserted on the same line. As the work matured into RDF 1.2, the model was reworked around triple terms (an RDF term written <<( s p o )>>) combined with an explicit reifier joined via the rdf:reifies predicate; the older << >> form becomes syntactic sugar for a reifying triple. A significant semantic shift accompanied this: RDF 1.2 triple terms are referentially transparent (an IRI inside a triple term denotes the same resource as elsewhere), whereas the earlier RDF-star quoted triples were often treated as opaque.

The primary appeal is compact, machine-friendly edge metadata: provenance, certainty, timestamps, and weights on individual assertions, a pattern central to knowledge graphs and closely aligned with the labeled edges of property graphs. This makes RDF-star a common interoperability bridge between RDF and property-graph systems. SPARQL-star correspondingly extends SPARQL so quoted triples can appear in triple patterns and be bound to variables.

Support is broad across the RDF ecosystem, including Jena/RIOT, RDFLib (Python), and dotNetRDF, plus commercial triple stores. Limitations are worth noting honestly: the RDF-star and RDF 1.2 designs are not identical, so data written under the older opaque quoted-triple semantics may not round-trip cleanly, and mixing the abbreviated annotation syntax with the underlying reification vocabulary can confuse newcomers.