Libraries use classification systems to organize books and other materials by subject so that they can be found quickly by library users. Each item has a unique "address," or call number, to help you locate it in the library. Call numbers are placed on the spines of books and are included in library catalog records. Books are arranged on the shelves in order by call number.

American University Library uses Library of Congress Classification, which has become the system of choice for many large research libraries. This system uses a combination of letters and numbers to arrange materials by subject area. This makes it easier to browse the shelves for materials that are similar.

The following table lists the major classes of the Library of Congress Classification system: