It is analogous to a binary parity bit used to check for errors in computer-generated data.
Check digit algorithms are generally designed to capture human transcription errors.
(An alternative is simply to avoid using the serial numbers which result in an "X" check digit.)
These three methods use a single check digit and will therefore fail to capture around 10%[citation needed] of more complex errors.
The final character of a ten-digit International Standard Book Number is a check digit computed so that multiplying each digit by its position in the number (counting from the right) and taking the sum of these products modulo 11 is 0.
ISBN 13 (in use January 2007) is equal to the EAN-13 code found underneath a book's barcode.
An extended digit is constrained to betanumeric characters, which are alphanumerics minus vowels and the letter 'l' (ell).
This restriction helps when generating opaque strings that are unlikely to form words by accident and will not contain both O and 0, or l and 1.
Having a prime radix of R=29, the betanumeric repertoire permits the algorithm to guarantee detection of single-character and transposition errors[9] for strings less than R=29 characters in length (beyond which it provides a slightly weaker check).