The Library of Congress Subject Headings (LCSH) comprise a thesaurus (in the information science sense, a controlled vocabulary) of subject headings, maintained by the United States Library of Congress, for use in bibliographic records. LC Subject Headings are an integral part of bibliographic control, which is the function by which libraries collect, organize and disseminate documents. LCSHs are applied to every item within a library's collection, and facilitate a user's access to items in the catalogue that pertain to similar subject matter. If users could only locate items by 'title' or other descriptive fields, such as 'author' or 'publisher', they would have to expend an enormous amount of time searching for items of related subject matter, and undoubtedly miss locating many items because of the ineffective and inefficient search capability.
Subject heading classification is a human and intellectual endeavor, where trained professionals apply topic descriptions to items in their collections. Naturally, every library may choose to categorize the subject matter of their items differently, without a uniform consentaneous standard. The widespread use and acceptance of the Library of Congress Subject Headings facilitates the uniform access and retrieval of items in any library in the world using the same search strategy and LCSH thesaurus, if the correct headings have been applied to the item by the library. Thus, LCSH decisions involve a great amount of debate and even controversy in the library community.
Despite LCSH's wide-ranging and comprehensive scope, there are libraries where the use of LCSH is not ideal or effective. To deal with these types of collections and user communities, other subject headings may be required. The United States National Library of Medicine developed Medical Subject Headings (MeSH) to use on its many health science databases and collection. Many university libraries may not apply both LCSH and MeSH headings to items. In Canada, the National Library of Canada worked with LCSH representatives to create a complementary set of Canadian Subject Headings (CSH) to access and express the topic content of documents on Canada and Canadian topics.
Historically, issues have revolved around the terms employed to describe racial or ethnic groups. Notable has been the terms used to describe African-Americans. Until the 1990s, the LCSH administrators had a strict policy of not changing terms for a subject category. This was enforced to tighten and eliminate the duplication or confusion that might arise if subject headings were changed. Therefore, one term to describe African-American topics in LCSH was 'Afro-American' long after that term lost currency and acceptance in the population. LCSH decided to allow some alteration of terms in 1996 to better reflect the needs and access of library users. Nevertheless, many common terms, or 'natural language' terms are not used in LCSH, and may in effect limit the ability for users to locate items. There is a growing tradition of research in Library and Information Science faculties about the cultural and gender biases that affect the terms used in LCSH, which in turn may limit or deprive library users access to information stored and disseminated in collections. A notable American Library Science scholar on this subject is Sanford Berman.
Criticism has also arisen regarding the biased organization and description of materials on sexuality. Works about heterosexuality are scarcely labeled as such in LCSH, giving users the impression that only queer sexuality deserves examination because heterosexuality is normative.
The Subject Headings are published in large red volumes (currently ten), which are typically displayed in the reference sections of research libraries. They may also be searched online in the Library of Congress Classification Web, a subscription service, or free of charge (as individual records) at Library of Congress Authorities. The Library of Congress issues weekly updates. The data is published for a fee by the Cataloging Distribution Service.
A web service, lcsh.info, was set up by Ed Summers, a Library of Congress employee, circa April 2008, using SKOS to allow for simple browsing of the subject headings. lcsh.info was shut down by the Library of Congress's order on December 18, 2008. This announcement was met with great dismay from the library science and semantic web communities, e.g. Tim Berners-Lee and Tim Spalding of LibraryThing. After some delay, the Library did set up its own web service for LCSH browsing at id.loc.gov in April 2009.
Increasingly, the use of hyperlinked, web-based Online Public Access Catalogues, or OPACs, allow users to hyperlink to a list of similar items displayed by LCSH once one item of interest is located. However, because LCSH are not necessarily expressed in natural language, many users may choose to search OPACs by keywords. Moreover, users unfamiliar with OPAC searching and LCSH, may incorrectly assume their library has no items on their desired topic, if they chose to search by 'subject' field, and the terms they entered do not strictly conform to a LCSH. For example, 'body temperature regulation' is used in place of 'thermoregulation'. Thus the easiest way to find and use LCSH is to start with a 'keyword' search and then look at the Subject Headings of a relevant item to locate other related material.
In April, 2009 the Beta version of ID.LOC.GOV with the LCSH vocabulary went live.
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