As LCSH is the official list of Library of Congress, not only does it give an authority to the standard vocabulary of LC, but also maintains its utility, updating the list at regular intervals, revising/modifying the headings according to changing needs and usage. The twentieth edition of LCSH contains an extensive introduction that includes lists of pattern headings and sub-divisions with instructions for use. You should read the introduction of LCSH which will give you understanding of the list. LCSH is said to be used with several auxiliary aids. Most important of these is the "Subject Cataloguing Manual: Subject Headings" published in 1996. Five services provide information about new and changed headings. First, a computer tape distribution service supplies the subject headings in the USMARC format on a weekly basis to supplement the master database file of subject authority records. Second, "LC Subject Headings Weekly lists" are a timely source of information about new and changed subject headings, class numbers, references and scope notes. Third, the quarterly LCSH in microfiche provides the equivalent of new edition of LCSH each quarter. Fourth, the complete subject authority file is distributed four times a year with LC classification on CD-ROM under the title "Classification plus". Fifth, "Cataloguing Service Bulletin" gives information on selected new, changed, and cancelled headings. With its inbuilt updating mechanism and the range of subject headings in a wide variety of subjects, LCSH is a tool of undisputed authority.