The large majority of materials in Sterling are housed in the bookstacks, which are contained in the building's tower. The bookstacks use two classification systems: the Yale Library system and the Library of Congress system. Adopted in the 1890s, the non-standard Yale system became cumbersome and inefficient for cataloging. Though replaced in 1970 by the Library of Congress system, many of the 5.7 million volumes held by the library at that time remain filed in the Yale system. The card catalogs in the nave once contained as many as 9.5 million cards, sorted in 8,700 trays.
