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The Secret of Credit Card Numbers 
by:
Gordon Goh 
Have you ever really looked at your credit card and tried to figure out what that huge string of numbers really means? Do these card issuers have so many customers that your account number has to be 16 digits long?
You may be surprised to know that all those numbers you see actually do stand for something, and it's not just who you are. Let's take a look.
Most of the major credit card companies operate on the same system when choosing a credit card number. Other cards like gas cards, department store cards and phone cards go their own way. Let's concentrate on the ones that all play by the same rules.
The very first digit in the series will be a 3,4,5, 0r 6. This number designates the type of card as follows:
3 = a Travel & Entertainment Card like American Express or Diners Club.
4 = Visa and Visabranded debit cards, cash cards, etc.
5 = MasterCard and MasterCardbranded debit cards, cash cards, etc.
6 = Discover
American Express and Diners Club use the second digit to identify the company. That means that Diners Club cards will start with either "36" or "38", and American Express cards will use either "34" or "37".
The remaining numbers in the series are used for different purposes depending upon the card type and issuer.
In most cases, the next group after the opening series of numbers represents the routing number of the cardissuing bank, the group after that is the user's account number, and the final digit is a check digit. The check digit is a number that is calculated by applying a special formula to all of the other numbers. The check digit is the result of that formula and is used as an antifraud check.
To keep things from getting too confusing, look at your card as you follow along for the next steps.
American Express
The American Express Card uses digits three and four for type (business or personal) and the currency of the cardholder's country of origin. The next digits from the fifth through the eleventh are account numbers. Digits twelve through fourteen indicate the card number within the account and the last digit is the check digit.
Visa
With Visa, digits two through six represent the bank number. Beginning with the seventh digit and running through the twelfth or the fifteenth represents the account number and the last number is the check digit. Since all Visa cards do not have the same amount of numbers in the sequence, the number of digits in a group may vary.
MasterCard
For MasterCard, the second digit, through to anywhere between the third and the sixth digit is the bank number. All remaining digits, except the check digit at the end of the series, identifies that cardholder's account.
Now that we've gone over it all, you're probably wondering why you were ever wondering in the first place. Just remember though, knowledge is power. Some things are just fun to know.

