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Subject cataloging involves determining the "aboutness" of a particular work in hand. It usually involves assigning one or more subject headings from a specified subject thesaurus, such as the Library of Congress Subject Headings (LCSH). The first subject heading assigned is usually called the Primary subject heading, and any additional headings are called Secondary subject headings. These are all text-searchable controlled vocabulary elements used to collocate similar works together in the library catalog. Good Subject cataloging also involves the creation of "see also" references directing users from non-authorized terms to the correct term used in the designated subject thesaurus in the local catalog.
Subject cataloging also involves the assignment of a unique classification number from a predetermined classification schedule such as Library of Congress Classification (LCC), the Dewey Decimal System (DDC22), or the Universal Decimal Classfication (UDC) scheme. The proper use of LCC is also governed by the Subject Cataloging Manual (SCM) published by Library of Congress, which includes rules for Shelflisting.
The unique classification number provides a logical way to shelve books on similar subjects in the same area of the library and facilitates browsing and serendipitous discovery.
Subject cataloging takes many years to learn to do well, but is viewed by many catalogers to be the most intellectually satisfying aspect of cataloging work.