Library of Congress Control Number

The Library of Congress Control Number or LCCN is a serially based system of numbering books in the Library of Conress in the United States. This numbering system has been in use since 1898. It has nothing to do with the contents of any book, and should not be confused with Library of Congress Classification.

In its most elementary form the number includes a year and a serial number. The year has two digits for 1898 to 2000, and four digits beginning in 2001. The three ambiguous years are distinguished by the size of the serial number. There are also some peculiarities in numbers beginning with a "7" because of an unsuccessful experiment applied between 1969 and 1972.

Serial numbers are six digits long and should include leading zeros. The hyphen that is often seen separating the year and serial number is optional.

See http://www.loc.gov/marc/lccn_structure.html for more details.