The card security code (CSC) is usually
a 3 - or 4 - digit number, which is not part of the credit card number. The CSC
is typically printed on the back of a credit card (usually in the signature field).
On some cards, all or part of the card number appears before the CSC, for example,
1234 567. In this example, 1234 are the last four digits of the credit card number,
and 567 is the CSC. On American Express Cards, the code is on the front.