The Library of Congress Classification (LCC), loosely based on Thomas Jefferson's book arrangement, was first developed in the late nineteenth and early twentieth centuries to organize and arrange the book collections of the Library of Congress. Over the course of the twentieth century, the system was adopted for use by other libraries as well, especially large academic libraries in the United States. It is currently one of the most widely used library classification systems in the world. The Library's Cataloging Policy and Support Office maintains and develops the system, posting weekly lists of updates on its Web page at loc.gov.
The system categorizes knowledge into twenty-one basic classes, each identified by a single letter of the alphabet. Most of these alphabetical classes are further divided into more specific subclasses, identified by two-letter, or occasionally three-letter, combinations. Each subclass includes a loosely hierarchical arrangement of the topics pertinent to the subclass, going from the general to the more specific.
Individual topics are often broken down by specific places, time periods, genres, or bibliographic types (such as periodicals, anthologies, etc.). Each topic (often referred to as a caption) is assigned a single number or a span of numbers. Whole numbers used in LCC may range from one to four digits in length, and may be further extended by the use of decimal numbers. Some subtopics appear in alphabetical, rather than hierarchical, lists and are represented by decimal numbers that combine a letter of the alphabet with a numeral.