Library of Congress reading room
The Library of Congress Classification (LCC) is a system of library classification developed by the Library of Congress. It is used by most research and university libraries in the U.S. and several other countries - most public libraries continue to use the Dewey Decimal Classification (DDC).
The classification was originally developed by Herbert Putnam with the advice of Charles Ammi Cutter in 1897 before he assumed the librarianship of Congress. It was influenced by Cutter Expansive Classification, DDC, and was designed for the use by the Library of Congress. The new system replaced a fixed location system developed by Thomas Jefferson. By the time of Putnam's departure from his post in 1939 all the classes except K (Law) and parts of B (Philosophy and Religion) were well developed. It has been criticized as lacking a sound theoretical basis; many of the classification decisions were driven by the particular practical needs of that library, rather than considerations of epistemological elegance.
Although it divides subjects into broad categories, it is essentially enumerative in nature.
The National Library of Medicine classification system (NLM) uses unused letters W and late Qs. Some libraries use NLM in conjuction with LCC, not using LCC's R (Medicine).
Letter classes I, O, W, X and Y are not standardly used.
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