# ISBN

**ISBN** (**International Standard Book Number**) is a 10-digit number and barcode (if published before 2007), or a 13-digit subset of EAN-13 barcode (if published on or after 2007), used to reference a book. Although the overall ISBN has a fixed width of 10 or 13 digits, its constituent elements are variable width.

The original 10-digit standard took the form of a "registration group" (language or country); a "registrant" (publisher); a "publication" (specific book); and a single-digit modulo 11 checksum.

Since 2007, this was replaced with a 13-digit standard that takes the form of the EAN prefix for ISBN (affectionately known as the country code for "Bookland"), namely 978 or 979; the registration group; the registrant; the publication; and a single-digit EAN-13 checksum.

The registration group, publisher, and book are all variable-width. While they always total 9 digits, these can be divided up one of many ways. For example, English-language books have a single-digit registration group (always 0 or 1), followed by anything from a 7-digit publisher and 1-digit book for the many smatterers, to a 2-digit publisher and 6-digit book for the few stalwarts.

As a result, the number tends to be hyphenated at various different points to emphasise these semantic divisions, such as x-xxx-xxxxx-x (as with *Analog Days*), and x-xxxxxxx-x-x (as with *The A-Z of Analogue Synthesisers*).

In short, an ISBN consists of an optional prefix of 978 or 979, followed by 9 digits that can be split up in various predictable ways, suffixed with a single-digit checksum of one of two types, depending on whether the prefix is present or not.

**Standards**: ISBN | SI unit | Useful Unicode characters