Format
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. More recently, the Library of Congress has instructed publishers not to include a hyphen.
Read more about this topic: Library Of Congress Control Number