Library of Congress Control Number
The Library of Congress Control Number (LCCN) is a serially-based system of numbering and cataloguing records in the United States Library of Congress. The number includes the year of registration and a serial number in its most basic form. The year has two digits from 1898 to 2000 and four digits beginning in 2001. The size of the serial number distinguishes the three ambiguous years (1998-2000).
Librarians worldwide use this unique identifier in the process of cataloging most books that have been published in the United States. It helps them access the correct cataloging data (known as a cataloging record), which the Library of Congress and third parties make available on the Web and other media.
The Library of Congress reserves the right to refuse cataloguing titles. However, they do not provide catalog numbers for books under 50 pages unless they are children's books. They also do not catalog transitory or consumable materials, such as coloring books, comic books, and cut-out books. In addition, the Library of Congress will not assign card catalog numbers for books that fall under any of these categories.