The Dewey Decimal Classification (DDC, also called the Dewey Decimal System) is a proprietary system of library classification developed by Melvil Dewey in 1876, and has since then been greatly modified and expanded through twenty-two major revisions, the most recent in 2004. The system is a method for placing books on library shelves in a specific and repeatable order that makes it easier to find any specific book or to return it to its proper place.
The DDC attempts to organize all knowledge into ten main classes. The ten main classes are then further subdivided. Each main class has ten divisions, and each division has ten sections. Hence the system can be summarized in 10 main classes, 100 divisions and 1000 sections. DDC's advantage in choosing decimals for its categories allows it to be both purely numerical and infinitely hierarchical. It also uses some aspects of a faceted classification scheme, 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 no less than three digits but otherwise of indeterminate length with a decimal point before the fourth digit, where present (e.g. 330 for economics + .9 for geographic treatment + .04 for Europe = 330.94 European economy; 973 for United States + .05 form division for periodicals = 973.05, periodicals concerning the United States generally).
Books are placed on the shelf in increasing numerical order; the whole number to the left of the decimal is in counting order, while the digits to the right of the decimal are compared one digit at a time, with a blank coming before zero. (Example: 050, 220, 330.973, 331 etc.) When two books have the same subject, and therefore the same classification number, the second line of the call number, which usually has the first letter or first several letters of the author's last name (or the title if there is no identifiable author), is placed in alphabetical order.
It is a common misconception that all books in the DDC are non-fiction. The DDC has a number for all books, including those that generally become their own section of fiction. If DDC rules are strictly followed, American fiction is classified in 813. Most libraries create a separate fiction section to allow shelving in a more generalized fashion than Dewey provides for, or to avoid the space that would be taken up in the 800s.
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.). Besides its frequent revision, DDC's main advantage over its chief American rival—the Library of Congress Classification system developed shortly afterward—is its simplicity. Thanks to the use of pure notation, a mnemonics system and a hierarchical decimal place system, it is generally easier to use.
DDC and UDC are also more flexible than Library of Congress Classification because of greater use of facets (via auxiliary tables) while Library of Congress Classification is almost totally enumerative.
On the flip side, DDC's decimal system means that it is less hospitable to the addition of new subjects, as opposed to Library of Congress Classification, which has 21 classes at the top level. Another side effect of this is that DDC notations can be much longer compared to the equivalent class in other classification systems.
Another disadvantage of DDC is that it was developed in the 19th century, by essentially one man, and was built on a top-down approach to classify all human knowledge which made it difficult to adapt to changing fields of knowledge. In contrast, the Library of Congress Classification system was developed based mainly on the idea of literary warrant; classes were added (by individual experts in each area) only when needed for works owned by the Library of Congress. As a result, while the Library of Congress Classification system was able to incorporate changes and additions of new branches of knowledge, particularly in the fields of engineering and computer science (the greater hospitability of the Library of Congress Classification was also a factor), DDC has been criticized for being inadequate for covering those areas. As a result, most major academic libraries in the US do not use the DDC because the classification of works in those areas is not specific enough.
The Library of Congress Classification system is not without problems. Because each area is developed by an expert according to demands of cataloging, there is little consistency. It is also highly US-centric (more so than DDC) because of the nature of the system, and compared to DDC and UDC it has been translated into far fewer languages.
The Online Computer Library Center of Dublin, Ohio, acquired the trademark and any copyrights associated with the Dewey Decimal System when it bought Forest Press in 1988. OCLC maintains the classification system and publishes new editions of the system. The work of assigning a DDC number to each newly published book is performed by a division of the Library of Congress, whose recommended assignments are either accepted or rejected by the OCLC after review by an advisory board; to date all have been accepted.
In September 2003, the OCLC sued the Library Hotel for trademark infringement. The settlement was that the OCLC would allow the Library Hotel to use the system in its hotel and marketing. In exchange, the Hotel would acknowledge the Center's ownership of the trademark and make a donation to a nonprofit organization promoting reading and literacy among children.