Not all numbers on credit cards do the same thing. Some identify the card issuer or represent the cardholder's personal account number, while others ensure that the card is recognized by computers as a valid credit card number. Most major U.S. credit cards, such as Visa, Mastercard and Discover, have 16 digits on the front. However, there are exceptions to this rule.

American Express

American Express is the most commonly held credit card that doesn't have a 16-digit number on the front. American Express card numbers have only 15 digits. Cards issued by Diners Club International also do not have 16 numbers. Diners Club cards are issued with 14 numbers on the front. However, some Diners Club cards issued in the U.S. that bear the MasterCard symbol now have 16 digits.