The International Standard Book Number (ISBN) consists of nine digits followed by one check digit (which may be X).

Here is how some (but not all) internet sites define how the check digit is determined -
	 :	To calculate the check digit, you must multiply the last digit of the true number by 2, the last but 1 by 3 etc. and add these results. The number needed to fill this sum to the next multiple of 11 is the check digit. If it is 10, the check digit is replaced by the letter 'X' (historically correct of course).

(See , for example, http://www.cwi.nl/~dik/english/codes/isbn.html.)

Seems a bit over-complicated, doesn't it? Why not multiply the ''n''th digit by ''n'', and let the check digit be the remainder when the sum is divided by 11 (or X if the remainder is 10)? Try it - it always gives the same result as the previous method.

(If you have examples of your own to add, feel free to change the page name to Unnecessary''''Obfuscation, or something like that.)

-- VickiKerr

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Since 1 January 2007, the old ISBN standard ("ISBN 10") has been deprecated in favour of a 13-digit standard (ISBN 13, now officially "the" ISBN). (Partly, it seems, because the publishing industry has been experiencing something similar to the IpVersionFour address exhaustion problem.) http://www.isbn.org/standards/home/isbn/transition.asp#01 See IsbnThirteenBug.

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CategoryInternationalization