Classification is the process of assigning a number to an item so as to be able to shelve the item with other items on the same subject. In the United States there are two commonly used classification schemes: the Dewey Decimal Classification and the Library of Congress Classification. Both are used widely and actively updated.
The Library of Congress Classification (LCC) was initially designed at the beginning of the 20th century for the collection of the Library of Congress (LC). Since then, many other large American academic and research libraries have adopted it. The system divides all knowledge into 21 basic classes, each beginning with a single letter of the alphabet. There are further breakdowns with two- or three-letter subclasses, as well as hierarchies and subtopics defined by spans of numbers.
The Dewey Decimal Classification (DDC) system, devised by library pioneer Melvil Dewey in 1873 and first published in 1876, and owned by OCLC since 1988, provides a dynamic structure for the organization of library collections. It follows Dewey's division of knowledge into ten broad categories, which in turn are further divided into ten sections. Libraries in more than 135 countries use DDC to organize their collections. The 23rd edition of the full DDC is available through WebDewey; the 15th edtion of the Abridged DDC is available to WebDewey subscribers in PDF form.
The Universal Decimal Classification (UDC) was developed by Belgian bibliographers Paul Otlet and Henri La Fontaine at the end of the 19th century. It is a multilingual classification scheme, now managed by the UDC Consortium.
Shelflisting is the process of subarranging books on the same subject for the shelf. The arrangement is typically alphabetic by author's name, unless the classification schedule dictates otherwise.
There are two methods in common use, both referred to as "Cuttering," after the 19th century librarian who devised the scheme. One uses 2- or 3-digit numbers taking into account English language frequency. These were published as the Cutter-Sanborn Tables. The print versions are no longer available, replaced by an online version, the OCLC Dewey Cutter Program. The other uses a brief table assigning numeric values for numbers. In both cases, the final number may be adjusted for entries already in the collection.
Although it is generally not necessary to convert a library's classification from one actively maintained system to another--and both LC and Dewey are continuously updated to reflect current topics and publishing--it is also sometimes useful to have a road map from one to another. There are printed conversion tables, as well as online versions.