The Dewey Decimal Classification scheme for library materials was designed in 1876 by Melville Dewey; it is the most commonly used classification system for libraries. It divides knowledge into ten main classes and further subdivides them, accompanying each division and subdivision of knowledge with a decimal notation called a "call number." Each letter or number in the Dewey system indicates a piece of information about the book including its physical size, subject matter, author and specific works by that author. Books in this scheme are arranged strictly hierarchically.
The Library of Congress classification scheme was developed in 1897 in the Library of Congress. It divides knowledge into 21 classes. This scheme uses one letter to denote the class of material and two- or three-letter combinations to represent its specific subclass. Some books are then given a decimal notation with a letter and a number to represent even more specific topics, like location, time periods or bibliographic forms. Books in the LOC scheme are in alphabetical or hierarchical lists.
This classification scheme (also called SuDocs) is specially designed for the arrangement of publications by the federal government. It is arranged essentially by the issuing agency. Each document is assigned a letter, which designates the particular government agency that issued the publication, and numbers that identify subordinate bureaus within that agency as well as that bureau's specific publications.