Call number, also called call number, is the code used by library book shelving and the main basis for literature borrowing and collection inventory.

Generally, it consists of several groups of numbers arranged by branches, which are often printed on the upper left corner of the catalog card, on the book label below the spine, and above the title page or back cover of the book. A call number can only represent one kind of book. Because the shelving methods of library books can basically be divided into two categories: classified shelving and formal shelving. Therefore, call numbers can basically be divided into two categories: classified call numbers and formal call numbers.