The International Standard Book Number, or ISBN, is a unique identifier for books, intended to be useful commercially. There is another quite similar system, the International Standard Serial Number (ISSN), for periodical publications such as magazines. The ISBN system was created in the United Kingdom in 1966 by the booksellers and stationers W H Smith and originally called Standard Book Numbering or SBN. It was adopted as international standard ISO 2108 in 1970.
Each edition and variation (except reprints) of a book receives its own ISBN. The number consists of four parts:
The different parts can have different lengths and are usually separated by hyphens. These hyphens are not strictly necessary however, since prefix codes are used which ensure that no two codes start the same way.
The country field is 0 or 1 for English speaking countries, 2 for French speaking countries, 3 for German speaking countries, etc. (The original SBN lacked the country field, but prefixing 0 to a 9-digit SBN creates a valid ISBN.) The country field can be up to 5 digits long; 99936 for instance is used for Bhutan. See this complete list (http://www.isbn-international.org/en/identifiers/allidentifiers.html).
The publisher number is assigned by the national ISBN agency, and the item number is chosen by the publisher.
Publishers receive blocks of ISBNs, with larger blocks going to publishers that are expected to need them; a small publisher might receive ISBNs consisting of a digit for the language, seven digits for the publisher, and a single digit for the individual items. Once that block is used up, the publisher can receive another block of numbers, with a different publisher number. As a consequence, different publisher numbers occasionally correspond to the same publisher.
The check digit is the sum of the digit number times the digit, modulo 11, with "10" represented by the character "X". For example, to find the check digit for the ISBN whose first nine digits are 0-306-40615:
1×0 + 2×3 + 3×0 + 4×6 + 5×4 + 6×0 + 7×6 + 8×1 + 9×5 = 0 + 6 + 0 + 24 + 20 + 0 + 42 + 8 + 45 = 145 = 13×11 + 2
So the check digit is 2, and the complete sequence is ISBN 0-306-40615-2. Since 11 is a prime number, this scheme ensures that a single error (in the form of an altered digit) can always be detected.
Because of a pending shortage in certain ISBN categories the international standards organization will soon be moving to a thirteen digit ISBN. This move will also bring the ISBN system into line with the UPC barcode system. There is a FAQ document (http://www.nlc-bnc.ca/iso/tc46sc9/isbn.htm) about this change.
Currently, the barcodes found on the backs of books are EAN13 "Bookland". There is a detailed description of the format here (http://www.barcodeisland.com/ean13.phtml). "978", the asset code for books, is prepended to the ISBN in the barcode data, and the check digit is recalculated according to the EAN13 formula (modulo 10, 1x and 3x weighting on alternate digits).