If you are paying with MasterCard, Visa, or Discover credit and debit cards, you can find a three-digit card security code on the back signature panel of the card. The code is not embossed like the card number, and is always the final group of numbers printed. For American Express cards you will find a four-digit code printed on the front side of the card above the number. It is printed flat, not embossed like the card number.