What is an LCCN (Library of Congress Control Number)?

A Library of Congress Control Number (LCCN) is a unique number the US Library of Congress assigns to a catalogue record. Librarians use it to look up the record for a book, which makes it easier for US libraries to order and catalogue your title. It is printed on the copyright page.

For self-publishers in the US, an LCCN is optional. It mainly matters if you want your book in public or academic libraries, where some acquisitions librarians look for one. It does nothing for sales on Amazon or other online stores.

The Library of Congress issues LCCNs free of charge, but only before a book is published — you cannot get one for a book that is already out. Publishers apply online through the Library's pre-publication program (now called PrePub Book Link, which replaced the older Preassigned Control Number program). You typically need an ISBN first, and in return you are expected to send a copy of the finished book to the Library. Eligibility rules, including for books that are only available as eBooks or print on demand, can change, so check the Library of Congress's current requirements before applying.

An LCCN is not copyright registration and it is not CIP data; those are separate. Authors outside the US do not need one — their own national library may run a similar legal deposit or cataloguing program.