To arrange the books on our shelves, we use the Library of Congress Classification System. This system organizes material according to twenty-one areas of knowledge. The 21 categories (labeled A-Z except I,O,W,X and Y) are further divided by adding one or two additional letters and a set of numbers. This system is in use at the Library of Congress and at many other academic and research libraries in the United States.
Understanding Library of Congress Classification System Call Numbers A brief introduction and overview of call numbers used in the Library of Congress Classification System by Matt Rosenberg.
About the U.S. Superintendent of Documents Classification System (SUDOC) (for government publications).