The Library of Congress Control Number is a number given to an item in the Library of Congress. It is used to identify items in the Library's collection. Each number is issued as part of a series. It is not based on a classification system. The numbering system began in 1898, and it was called the Library of Congress Card Number.
The Library made cards for its library catalogue, and they also made cards which could be sold to other libraries. They introduced the number to identify the sets of cards.
Librarians around the world still use the number, especially when identifying books published in the United States.
The number has two parts; the first is a two digit number based on the year, and the second is the next number in the series, written as a six digit number. Since 2001, the year has been written as a four digit number.