An ISBN, which is an acronym for International Standard Book Number, is a unique identifier for books and eBooks based on format—hardcover, paperback, or eBook. The ISBN, generated by R.R. Bowker, identifies the publisher, the title number of the book associated with that publisher, and a "check" digit, which ensures the uniqueness of the full number. Until 2007, ISBNs were 10 digits in length. Now, they are 13 digits in length and begin with the three-digit number 978 (or 979).