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Introduction
Most bibliographic metadata on the Web, such as data describing a book, article, or image, follows the implicit model of a single entity (a "resource") with attributes (properties). This model is reflected, for example, in the widely used Dublin-Core-based XML format of the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH). Over the past two decades, however, the library world has developed more differentiated models of bibliographic resources. These models do not see a book as just a book, but as a set of entities variously reflecting the meaning, expression, and physicality of a resource.
Readers with a background in modern library science will recognize Figure 1 as depicting the four "Group 1" entities - Work, Expression, Manifestation, and Item (WEMI) - defined in the specification Functional Requirements for Bibliographic Records (FRBR). The WEMI entities are connected in a daisy chain cascading from the (abstract) work to a (concrete) information artifact, the item, and vice-versa. Since its creation by a working group of the International Federation of Library Associations (IFLA) in the 1990s, the FRBR model has been incorporated into Resource Description and Access (RDA), designated successor to the standard Anglo-American Cataloging Rules (AACR). In parallel to RDA, the Bibliographic Framework Initiative of the US Library of Congress (BIBFRAME) has adopted a FRBR-like model with just two entities as the basis for its draft successor to the Machine Readable Cataloging (MARC) format: an (abstract) work is instantiated in a (concrete) instance.
The emergence since 2000 of the Semantic Web idea, and since 2006 of the Linked Data cloud, have led the maintainers of these multi-entity bibliographic models to publish the models as vocabularies expressed in Resource Description Framework (RDF) and Web Ontology Language (OWL), the standard Semantic Web languages of the World Wide Web Consortium (W3C). (This paper does not distinguish OWL "ontologies" from other RDF vocabularies.) FRBR and RDA were translated into RDF from source specifications that use non-RDF formalisms, while the BIBFRAME vocabulary was born RDF.
The objective of this paper is to explain, in plain English, what the RDF vocabularies for FRBR, RDA, and BIBFRAME say about the nature of their bibliographic entities. The analysis compares how the bibliographic entities are defined as RDF classes with particular attention to how those...