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DDC's cleverness is in choosing decimals for its categories; this allows it to be both purely numerical while infinitely hierarchical. It also is a faceted classification, combining elements from different parts of the structure 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 class 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 system uses ten main classes, which are then further subdivided. It is a common misconception that all books in the DDS are non-fiction. However, the DDS has a number for all books, including those that generally become their own section of fiction. If DDS rules were strictly followed, fiction would be classified in 813. Most libraries create a separate fiction section because of the space that would be taken up in the 813s.

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 continentAfrica is the world's second-largest continent in both area and population, after Asia. 30,244,050 km2 (11,677,240 mi2) including the islands, it covers 20. 3% of the total land area on Earth, and with over 800 million human inhabitants it accounts for ar only 966–969. It is still more biased towards ChristianityChristian cross and its many variations are widely recognized as an ancient Christian symbol. Christianity is an Abrahamic religion based on the life and teachings of Jesus of Nazareth as described in the New Testament. Although Christians generally chara against other religionReligion sometimes used interchangeably with faith, is commonly defined as belief concerning the supernatural, sacred, or divine, and the practices and institutions associated with such belief. Borobudur, a Buddhist stupa built between 750 and 850 Adriaens, 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 JewThe word Jew is used in a wide number of ways, but generally refers to either a follower of the Jewish faith, a child of a Jewish mother, or a member of the Jewish culture or ethnicity. This article discusses the term as describing an ethnic group; for aish ones.

DDC's numbers formed the basis of the more expressive but complex Universal Decimal ClassificationLibrary and information science Knowledge representation The Universal Decimal Classification is a system of library classification developed by the Belgian bibliographers Paul Otlet and Henri la Fontaine at the end of the 19th century. It is based on the, 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 Library of Congress ClassificationThe 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. and several other countries), although most public libraries continue t developed shortly afterward.