Materials are given call numbers, which serve to identify shelf locations. For example, the call number QK668.B7 has been given to a book about plant anatomy written by an author named Branford. That book would be shelved with other QK books.
If there are several volumes in a set, they all have the same call number, except for the last line, which indicates the volume number.
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U.S. government publications are an exception. The U.S. Superintendent of Documents assigns special numbers to these documents; government publications are usually not classified by the Library of Congress system.