Bibliographic descriptions hold structured data about items of intellectual or artistic creation. Cataloguing rules specify what data should be included in bibliographic descriptions, which can exist in various forms. Early catalogues in the form of lists and guard-books were followed by card catalogues, and more recently by machine-readable or electronic catalogues.
Electronic catalogues use formats that specify the data elements or attributes (author, title, etc,) required to describe items. Formats assign labels to each data element; the labels may be field tags or RDF metadata labels. Library catalogue systems use formats to structure data within a database, to retrieve items through searching specific data elements, and to display the held data to the searcher.
The MAchine Readable Cataloguing (MARC) format was developed in 1965-66 at the Library of Congress. National MARC variants were subsequently developed, as were other formats.