Dewey Decimal cataloging works by dividing all areas of human knowledge into 10 broad categories. The Dewey Decimal System then subdivides these broad categories into more specific categories in a numerical system that results in books with similar subject matter being placed in close physical proximity on the shelves.
Named after its creator, the Dewey Decimal System is a proprietary library classification system that Melville Dewey created while working as the head librarian of Amherst College in 1876. More libraries around the world use the Dewey Decimal System to organize their books than any other classification system.
The Dewey Decimal System provides 10 broad categories for subject matter, which the first digit of the book's call letter represents. The second digit of the call number represents which of the 10 subdivisions of those categories the book belongs in. The subdivision of categories continues to become more specific with a maximum of seven digits in each call number.
For example, a book on Finsler geometry would have a call letter of 516.375. This call letter shows that the book is in the top level 500 category of natural sciences, the 510 division of mathematics, the 516 division of geometry, the 516.3 division of analytic geometries, the 516.37 division of metric differential geometries and the 516.375 division of Finsler geometry.