The Dewey Decimal Classification (DDC) is a system of library classification developed by Melvil Dewey (1851-1931) in 1876, and since greatly modified and expanded in the course of twenty major revisions.
Strictly numerical in its basic arrangement, DDC is a broadly faceted rather then purely enumerative classification, in that it combines elements from different parts of the schedule to construct a number representing the subject content (often combining two subject elements with linking numbers and geographical and temporal elements) and form of an item rather than drawing upon a list containing each classmark and its meaning.
Except for general works and fiction, works are classified principally by subject, with extensions for subject relationships, place, time or type of material, producing classification numbers of not less than three digits but otherwise of indeterminate length with a decimal point before the fourth digit, where present (e.g. 330 for economy + 94 for Europe = 330.94 European economy; 973 for United States + 005 form division for periodicals - 973.005, periodicals concerning the United States generally); classmarks are to be read as numbers, in the order: 050, 220, 330.973, 331 etc. Any letter should be read as preceding any number that might have occupied the same character position, so "330.94 A" would come before 330.943.
The ten main classes are:
|000||Information science and general reference|
|100||Philosophy and psychology|
|700||Arts and recreation|
|900||History and geography|
DDC is commonly used in public and school libraries throughout the world, and especially the U.S. The schedule contains marked geographical biases derived from its 19th-century origins: Northern Africa for instance occupies all of 961-965, the rest of the continent only 966-969. It is still more biased towards Christianity against other religions, the former covering all of 200-289, while all others get only 290-299 to share: recent versions permit another religion to be placed in 200-289, with Christianity relegated to 290-299, but this is mainly used by libraries operated by non-Christian religious groups, especially Jewish ones.
DDC's numbers formed the basis of the more expressive but complex Universal Decimal Classification, which combines the basic Dewey numbers with selected punctuation marks (comma, colon, parentheses etc.). Despite its frequent revision, DDC is widely considered theoretically inferior to other more modern systems which make freer use of alphabetical characters to produce shorter classmarks for concepts of equal complexity, though it continues to offer a more expressive format than the simpler enumerative alphanumeric Library of Congress classification developed shortly afterward.