The string of digits presented on the front of a credit or debit card is enough to confuse even the smartest of shoppers.
Yet, each number plays a crucial role in identifying the card provider, bank, and account information, as well as providing a security check.
The only organization that can assign numbers to specific networks, such as Visa or MasterCard, and financial institutions, like Bank of America or JP Morgan Chase, is the nonprofit American National Standards Institute, the Pittsburgh Post-Gazette reports.
When swiped, the Post-Gazette explains, a terminal uses the magnetic stripe of the card to first route the transaction through the proper card network, and then to the financial institution that is listed on the card so that the transaction can be authorized.
Every card, no matter the network or bank, must be in agreement with the "Luhn System," which determines the validity of a credit card. It's a mathematical algorithm where various combinations of numbers must add up to a number ending in 0. If the total of the combinations adds up to anything other than a multiple of 10, the card is invalid.
Generally, the first number or two of a credit or debit signifies the card provider, followed by digits that determine everything from the currency being used, to the bank processing the transaction, to the individual's account number.
But every network operates a little differently — here's the breakdown.
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American Express uses the first two numbers of the card to identify itself. That two-digit number will be either 34 or 37.
The third and fourth digit signify the type of card and the currency being used, according to Clearpoint Credit Counseling. The next six digits, five through 11, are the number of the account, while the 12th through 14th digits represent the card number within said account.
The final digit is a check digit, which is a random number used to protect against errors and fraud, Jason Oxman, CEO at the Electronic Transactions Association, told the Post-Gazette.
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Visa cards use a very similar formula, although there are a couple of differences.
The first digit assigned to all Visa cards is four, and the second through sixth numbers are connected to the financial institution.
Then, either the seventh through 12th numbers, or the seventh through 15th, are the account number, while that final 13th or 16th digit is the check number.
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MasterCards use the number five as the first in their 16-digit sequence. The second and third, second and fourth, or second and fifth then represent the bank number, Clearpoint Credit Solutions explains.
Following the third, fourth, or fifth digit, every number up through 15 is the account number while that final number, 16, is the check digit.
All Discover cards start with the number six and are 16 digits in length, while petroleum cards start with the number seven and airline cards start with the number one.