To elaborate on your example, if the last digit cast out nines from the previous two digits, valid numbers are 101 (1+0=1), 112, 123, 134, 145, 156, 167, 178, 1

To elaborate on your example, if the last digit cast out nines from the previous two digits, valid numbers are 101 (1+0=1), 112, 123, 134, 145, 156, 167, 178, 189, 191 (1+9=10, 1+0=1), 202. So for example 102 is NOT a valid number — it doesn't exist! And if it is used, and someone somewhere tries to validate it, it will throw an error. Hence the article says: > Shel very quickly removed all ‘checksum software checks’ (which would have made sure it was a legal ISBN) I find it incredibly fascinating (and a little bit depressing) that publishers would twist the system in that way, but maybe it has to be expected if something is sold for a large amount of money. In France ISBN are free; there is a central agency (Electre) that issues them but they don't charge for it (at least not to small publishers). But EAN/UPC codes are issued by a different agency, GS1, that does charge a recurring fee, and it is expensive and "infinite" (you're supposed to keep paying for as long as you're using the codes). But even them don't charge for the range size; the fee is based on the turnover of the company buying the codes.