Most Americans carry around some sort of credit or debit card, embossed with a 15- or 16-digit card number on the front. But what exactly do those numbers mean? Are they sequential, random or do they follow some other type of pattern?

It turns out that there is meaning behind those numbers. Looking at the first number on your card: If it starts with 3, it is an American Express, Diner’s Club, or Carte Blanche; 4 is reserved for Visa, 5 for MasterCard, and 6 for Discover. The next five digits will indicate the card issuer such as the bank or credit union, as well as the type of credit card. So for example, all Citi AAdvantage Executive MasterCards will begin with 546616 with the 5 representing MasterCard, and the 46616 representing Citi as well as the fact that it is one of their American Airlines AAdvantage Executive cards.

The remainder of the 16 digits (or 15, in the case of American Express), represent both the cardholder’s account number as well as one or more “check digits.” A check digit is applied to an unusual formula that helps determine if your credit card number is actually valid. Using this formula, it takes only a fraction of a second for a computer to confirm that a credit card number is valid, but about a minute for someone with high school level math skills to do the same.

In addition, credit cards now have additional digits that are often required to process a transaction, especially when the card is not present, such as when placing a telephone or Internet order. These are the three-digit numbers on the back of MasterCard, Visa and Discover cards, or the four-digit numbers on the front of an American Express card. These codes were introduced in the late 1990s in response to the need to validate the growing number of Internet transactions.

## How to Tell If a Credit Card Number Is Valid

You, too, can crack the credit card code to see if the number you’re looking at is a valid one. For 16-digit numbers, start with the first number on the left, and double each alternating number on the card. (For 15-digit numbers, start with the second number from the left.) Add any double-digit numbers so that they reduce to a single-digit number. (For example, if one of the numbers is 8, it doubles to 16, then you add 1 + 6 to get 7.) Next, add those together with the alternating numbers that you did not double. If the total you get is divisible by 10, the credit card number is likely valid. So clearly, the check digits are included to just to turn an otherwise random account number into a valid one.

## Credit Card Numbers Versus Account Numbers

When your card is lost, stolen, damaged or expired, it will need to be replaced. Some card issuers, such as American Express, will replace expired cards with one that has the same number, while others will have a card printed with a new number.

To make matters more confusing, your credit card account number may be different from the number on your credit card. So if your new card has a new number, it can still be linked to your existing account number, which doesn’t change. For example, American Express issues separate credit card numbers for each authorized user within an account, but there is just one account number for the primary account holder.

## Why Does This Matter?

While this system is very important to credit card issuers and merchants, it actually matters very little to the average credit card user. You will always want to protect your credit card number from being stolen, and it might be helpful to know that the first six digits are the same for everyone with the same kind of credit card you have. You can also think of these first six digits as the equivalent of the badges on your car indicating the make and model, and the remaining digits kind of like a license plate that is unique to your vehicle.

As credit cards continue to evolve, the EMV smart chips embedded on the cards will eventually replace magnetic strips as a means of transmitting credit card account numbers and verifying transactions, but the credit card number itself should still be with us for a long time.

### More on Credit Cards:

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• robert hanna

Does having the last three numbers on the back of a credit card allow scammers/hackers to use the card?

• http://www.credit.com/ Credit.com Credit Experts

It might now, in some card-not-present transactions, but that should soon be changing. Credit.com writer Jason Steele said this: The next steps are toward chip and PIN cards that require cardholders to enter a personal identification number, followed by even smarter cards that will have a dynamic CVV (the three-digit code on the back of the card). That number will change each time you use it, preventing a criminal from making a transaction over the Internet, even when they already have your credit card number.

• http://www.jasonsteele.com/ Jason Steele

Some digits represent the bank, others represent the product. So two different cards from the same bank will have different numbers for the first 6.

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