Hi,

Its actually 4 digits giving 10,000 possible account nubmers - low I know! But then remember that is accounts,a s you can have multiple supplementary cards hanging off each account number. Thats why they have multiple numbers for the product code, e.g. Most Centurion cards in the UK are 88 but I am sure that they use another set too as they are more than 10,000 account holders in the UK. I know for Platinum in the UK there are quite a few product codes, like 82, 81 etc...

The last digit is quite rightly a Check digit, and there is an equation that you can do to check to see if the number is a valid AmEx card number.

The ISBN system uses the Modulo 11 check digit system, whihc is why you sometimes see an ISBN end in an X to signify 10 as its a division calaculation on a factor of 11, if the end result is a whole number then its valid. The ten "digits" a-b-c-d-e-f-g-h-i-j in a valid ISBN code must satisfy the equation a + 2b + 3c + 4d + 5e + 6f + 7g + 8h + 9i = j (modulo 11).

Credit/Charge Cards use the Luhn formula for verifiaction, which is Modulo 10. I'm sure a quick search on Google will get you more info if you're interested

Thanks,
Richard


[This message has been edited by RichardMannion (edited 10-16-2003).]