Card Security Code For most cards, including Visa and Mastercard, the card security code is the three-digit number printed on the back of the card. For American Express cards, the card security code is the four-digit number printed on the front of the card, above the main number. Card Billing Address The card billing address is the address held by your card issuer and the address to which your card statement is sent. The address you enter on the form must match this, otherwise your purchase may be declined.