Most libraries use some sort of call number scheme to organize their books and make it easier for patrons to find materials.
The two most common call number schemes are the Dewey Decimal Classification scheme, which is used by a majority of public and K-12 school libraries, and the Library of Congress Classification scheme, which is used primarily by local government and academic libraries. There are other schemes that are used by special libraries, like medical and federal government libraries.
Original page source: North Dakota State Library