Visa/Mastercard and Discover Users
Flip your card over and look at the signature box. You should see either the entire 16-digit credit card number or just the last four digits followed by a special 3-digit code. This 3-digit code is your Card Security Code.
The American Express Card Identification Number (CIN) is a four digit number found on the front of the U.S. issued American Express Card (including Optima). For the American Express card the four digit number is displayed on the front of the card, just above and to the right of the credit card number. For the Optima card it is displayed on the front of the card, to the left and just above the credit card number. American Express and LeatherUp.com are now using this number for added security and customer protection.