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.