Card ID Explanation Card ID - Visa, MasterCard, Discover The Card ID is a 3-digit security code that is printed on the back of your Visa, MasterCard or Discover Card. The number appears on the signature strip after the last four digits of your account number. The signature strip may contain your entire account number or just the last four digits of your account number. Either way, the Card ID will appear after the last four digits of your account number. The Card ID is used to verify that you have possession of the credit card you are attempting to use. If there is not a Card ID number on the back of your card then please leave the field blank. Card ID - American Express The Card ID is a 4 digit, non-embossed number printed above your account number on the face of your card.