Library of Congress Subject Headings (LCSH) – Library of Congress Subject Headings (LCSH) is the list of headings produced from the subject authority file maintained by the United States Library of Congress for use in bibliographic records. It is popularly known by its abbreviation as LCSH and is sometimes used interchangeably with the phrase subject authority file. LCSH is a multidisciplinary vocabulary that includes headings in all subjects, from science to religion, to history, social science, education, literature, and philosophy. It also includes headings for geographic features, ethnic groups, historical events, building names, etc. Library of Congress Subject Headings (LCSH) is the most widely used subject vocabulary in the world. It is the model for many other vocabularies in English and other languages and has been translated into numerous languages.

The strongest aspect of LCSH is that it represents subject headings of the Library of Congress, the national library of the United States, one of the richest of national libraries of the world. The administrative and managerial machinery of LC has made it possible for LCSH to stand out as an undisputed leader. LCSH is also used as indexing vocabulary in a number of published bibliographies. LCSH comprise a thesaurus or a controlled vocabulary of subject headings which is used by a cataloger or an indexer to assign subject headings to a bibliographic record to represent the subject of a work he/she is cataloging. LCSH contain the preferred subject access terms (controlled vocabulary) that are assigned as an added entry in the bibliographic record which works as an access point and enables the work to be searched and retrieved by subject from the library catalog database. The controlled vocabulary identifies synonym terms and selects one preferred term among them to be used as the subject heading. For homonyms, it explicitly identifies the multiple concepts expressed by that word or phrase. Cross-references are used with headings to direct the user from terms not used as headings to the term that is used, and from broader and related topics to the one chosen to represent a given subject.

The fortieth edition of Library of Congress Subject Headings (LCSH 40) contains headings established by the Library through January 2018. The headings included in this list were obtained by creating a file consisting of all subject heading and subdivision records in verified status in the subject authority file at the Library of Congress. There were 342,947 authority records in the file then. The subject authority database from which the headings in this edition were drawn indicates that the file contains approximately 24,390 personal name headings of which 23,272 represent family names, 10,034 corporate headings, 6 meeting or conference headings, 481 uniform titles, 242,511 topical subject headings, and 61,885 geographic subject headings. There are 764 general USE references, 4,351 general see also references, 299,751 references from one usable heading to another, and 362,646 references from unused terms to used headings. The creation and revision of subject headings is a continuous process. Approximately 5,000 new headings, including headings with subdivisions, are added to LCSH each year. Proposals for new headings and revisions to existing ones are submitted by catalogers at the Library of Congress and by participants in the Subject Authority Cooperative Program (SACO). More information on SACO may be found at http://www.loc.gov/aba/pcc.

Approved proposals become part of the online authority file of subject headings at the Library of Congress, from which various publications are created. Five services provide information about new and revised headings. First, a distribution service supplies the subject headings in the MARC 21 authorities format via Internet FTP on a weekly basis to supplement the master database file of subject authority records. Second, L.C. Subject Headings Monthly Lists are a timely source of information about new and changed subject headings, class numbers, references and scope notes. The lists are posted monthly to the World Wide Web at http://www.loc.gov/aba/cataloging/subject/weeklylists. Third, Classification Web provides World Wide Web access to Library of Congress Subject Headings and Library of Congress Classification to subscribers. Fourth, subject authority records are included in the Library’s Web authorities service and may be searched and viewed at http://authorities.loc.gov. Fifth, subject authorities are freely available for searching and download through the Library’s Linked Data Service at http://id.loc.gov.