The International Standard Book Number (ISBN) is a unique commercial book identifier barcode. Each ISBN code identifies uniquely a book. ISBN have either 10 or 13 digits. All ISBN assigned after 1 Jan 2007 have 13 digits. Typically, the ISBN is printed on the back cover of the book.

For example, the ISBN code for J.K.Rowling's "Harry Potter and the Deathly Hallows", Adult Edition, Paperback, UK edition is 978-0747595823, and this code identifies uniquely this book and edition.