A library catalog (or library catalogue in British English) is a register of all bibliographic items found in a library or group of libraries, such as a network of libraries at several locations. A bibliographic item can be any information entity (e.g., books, computer files, graphics, realia, cartographic materials, etc.) that is considered library material (e.g., a single novel in an anthology), or a group of library materials (e.g., a trilogy), or linked from the catalog (e.g., a webpage) as far as it is relevant to the catalog and to the users (patrons) of the library.
In November 1789, during the dechristianization of France during the French Revolution, the process of collecting all books from religious houses was initiated. Using these books in a new system of public libraries included an inventory of all books. The backs of the playing cards contained the bibliographic information for each book and this inventory became known as the "French Cataloging Code of 1791".
Melvil Dewey saw well beyond the importance of standardized cards and sought to outfit virtually all facets of library operations. To the end he established a Supplies Department as part of the ALA, later to become a stand-alone company renamed the Library Bureau. In one of its early distribution catalogs, the bureau pointed out that “no other business had been organized with the definite purpose of supplying libraries”. With a focus on machine-cut index cards and the trays and cabinets to contain them, the Library Bureau became a veritable furniture store, selling tables, chairs, shelves and display cases, as well as date stamps, newspaper holders, hole punchers, paper weights, and virtually anything else a library could possibly need. With this one-stop shopping service, Dewey left an enduring mark on libraries across the country. Uniformity spread from library to library.
During the early modern period, libraries were organized through the direction of the librarian in charge. There was no universal method, so some books were organized by language or book material, for example, but most scholarly libraries had recognizable categories (like philosophy, saints, mathematics). The first library to list titles alphabetically under each subject was the Sorbonne library in Paris. Library catalogs originated as manuscript lists, arranged by format (folio, quarto, etc.) or in a rough alphabetical arrangement by author. Before printing, librarians had to enter new acquisitions into the margins of the catalog list until a new one was created. Because of the nature of creating texts at this time, most catalogs were not able to keep up with new acquisitions.
For some works, even the title can be standardized. The technical term for this is uniform title. For example, translations and re-editions are sometimes sorted under their original title. In many catalogs, parts of the Bible are sorted under the standard name of the book(s) they contain. The plays of William Shakespeare are another frequently cited example of the role played by a uniform title in the library catalog.
In a subject catalog, one has to decide on which classification system to use. The cataloger will select appropriate subject headings for the bibliographic item and a unique classification number (sometimes known as a "call number") which is used not only for identification but also for the purposes of shelving, placing items with similar subjects near one another, which aids in browsing by library users, who are thus often able to take advantage of serendipity in their search process.
Rules governing the creation of MARC catalog records include not only formal cataloging rules such as Anglo-American Cataloguing Rules, second edition (AACR2), Resource Description and Access (RDA) but also rules specific to MARC, available from both the U.S. Library of Congress and the OCLC, the Online Computer Library Center global cooperative which builds and maintains WorldCat.
MARC was originally used to automate the creation of physical catalog cards, but its use evolved into direct access to the MARC computer files during the search process.