In libraries, a classification system is used to systematically group together items that have similar characteristics. The classification assigned to library materials is known as the "call number". The first part of a call number is derived through a classification system. Materials are classified for basically two reasons: 1) to enable them to be found quickly and easily; and 2) to bring together on the shelf materials that deal with similar subjects. Libraries group or classify materials by subject as well as by format (for instance, video or audio), size (regular or oversize), circulation policy (reference, reserve, or circulating), or type (periodicals, government documents).
Knowing about call numbers is essential for finding materials because they are shelved in numerical and alphabetical order by call number. It will help you understand how information is organized in a library and improve the ability to do research. A call number is a unique number that combines an item's classification or class number (composed of both letters and numerals) as well as an author or title number (also composed of letters and numerals). The latter may be followed by a date indicating the item's year of publication. The call number thus indicates each item's major subject content and physical shelf location.
Subject classification enables materials dealing with similar subject matter to be grouped together. There are many classification systems, but libraries in the U.S. most often use two: most special**, college, and university libraries use the Library of Congress (LC) Classification System, and public and school libraries usually employ the Dewey Decimal System (DDC). The second part of a call number, usually letters, represents the author’s last name. In biographies (921 in DDC) the second part represents the first letters of the the person the book is about so that all books about a person are shelved together.
**Special libraries may include research, business, tribal or museum libraries with a narrow focus for its specialized patrons.
Library of Congress Classification System (LC) –used mostly in special, college and university libraries.
The LC Classification System, developed in 1897, divides knowledge into 21 broad subject classes and identifies each by single letters of the alphabet, A through Z. (Five letters--I, O, W, X, and Y--are not used.) Combinations of letters and numbers (alpha-numeric system) indicate subtopics within classes and subclasses. For a basic list of the LC subject headings, go to the Library of Congress website.
Dewey Decimal Classification System (DDC) –used mostly in school and public libraries.
The Dewey Decimal Classification (DDC) was developed by Melvil Dewey and first published in 1876. DDC divides knowledge into ten subject areas and uses three digit numbers plus decimals to classify materials. It is universally used in school libraries.
You can find the DDC of a book by looking up the MARC (machine-readable cataloging) record on many of the electronic databases accessible through SLED. Another source is the CIP (cataloging in publication) information printed on the verso (back side) of the title page of the book itself. A summary of the DDC is reprinted on the next page.
1. How to organize and operate a small library / by Genore H. Bernhard. Fort Atkinson, Wis. : Highsmith Co., c1975.