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447579 (3) [Avatar] Offline
Old (9 character) ISBNs use a checksum character, not a digit. It is calculated in base-11, with X used for 10. It is common (but I don't know if it was part of the standard) to have a hypen before the checksum character, making the string 11 characters.

ISBNs are, since 2007, 13 digits (including the checksum). The checksum is also calculated in a different way and can only be a digit. The official format is to separate the number in five variable size parts with hyphens between them, but it is quite common to have the first three numbers separated from the rest with a space character and it is also common without any extra characters...
In the official format, the hypens provide information, the same information can be recovered from the number by having a database with information about the different available series.
447579 (3) [Avatar] Offline
Sorry, I should have read the rest of the chapter before commenting... smilie
Dan Bergh Johnsson (11) [Avatar] Offline
You are absolutely right. We are taking some short-cuts for pedagogical reasons.