ISBN

ISBN (International Standard Book Number) is a 10 digit number which is protected by a special numerical check.
Each digit is multiplied by a factor ranging from 10 to 1. The calculation results are then summed together. Finally, to be a valid ISBN, the sum must be divisible by 11.
The last digit can be a X, which has a value of 10.

Example (ISBN: 90-274-3123-X):

9 0 - 2 7 4 - 3 1 2 3 - X
* * * * * * * * * *
10 9 8 7 6 5 4 3 2 1
= = = = = = = = = =
90 + 0 + 16 + 49 + 24 + 15 + 4 + 6 + 6 + 10 = 220 = N*11

Example (ISBN: 90-210-0242-6):

9 0 - 2 1 0 - 0 2 4 2 - 6
* * * * * * * * * *
10 9 8 7 6 5 4 3 2 1
= = = = = = = = = =
90 + 0 + 16 + 7 + 0 + 0 + 8 + 12 + 4 + 6 = 143 = N*11

Inside the ISBN four digit groups are used:

GroupMeaning
1Language (e.g. 2 = French; 4 = Spanish)
2Publisher code
3Publication number
4Check

Publisher codes can be 2 to 7 digits long:

Group lengthCode range
200 - 19
3200 - 499
45000 - 6999
570000 - 79999
6800000 - 899999
79000000 - 9999999 (unique publications)

2000-02-15