ISBN (International Standard Book Number)

A product number for a book. Each edition and format (hardback, paperback, e-book) has its own ISBN.

ISBNs had 10 digits until the end of 2006. Since January 1, 2007, they’ve had 13 digits, usually starting with 978 or 979.

978-0-123456-47-2 (illustrative)

A book gets a new ISBN for each format, such as hardback, paperback and EPUB, and for each substantially revised edition. Small corrections or a new price don’t get a new one. That’s why the same title can have many ISBNs.

Most citation styles, including APA, don’t put ISBNs in your reference list. They’re still handy for looking a book up, and PaperCitation uses them to find book records.