If you are a merchant, every now and then a customer may place an order online or over the telephone and forget to tell you what kind of credit card she is using. Without that information, you won't be able to process the order. So, you have to either contact the customer to ask her what kind of credit card she was using or learn how to decipher the account number to determine the issurer.
Look to see if the first digit of the 16-digit credit card number is 4. If so, it is a Visa credit card.
If the first two digits of the card number are 51 or 55, it is a MasterCard credit card.
If the credit card number is 15 digits with the first two digits being 34 or 37, it is an American Express card.
If the credit card number begins with 6011, it is a Discover credit card.
If the card number is 14 digits and the first three numbers are 300 or 305, it is a Diner's Club credit card.
If the card number is 14 digits and the first two digits are 36 or 38, it is a Carte Blanche credit card.