<P> Librarians all over the world use this unique identifier in the process of cataloging most books which have been published in the United States . It helps them reach the correct cataloging data (known as a cataloging record), which the Library of Congress and third parties make available on the Web and through other media . </P> <P> In February 2008, the Library of Congress created the LCCN Permalink service, providing a stable URL for all Library of Congress Control Numbers . </P> <P> 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 (1898, 1899, and 1900) 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 . </P> <P> Serial numbers are six digits long and should include leading zeros . The leading zeros padding the number are a more recent addition to the format, so many older works will show less full codes . The hyphen that is often seen separating the year and serial number is optional . More recently, the Library of Congress has instructed publishers not to include a hyphen . </P>

What is the library of congress control number