What Credit Card Numbers Mean

Although phone companies, gas companies and department stores have their own numbering systems, ANSI Standard X4.13-1983 is the system used by most national credit-card systems.

Illustration by Rosaleah Rautert The front of your credit card has a lot of numbers — here’s an example of what they might mean.

Here are what some of the numbers stand for:

• The first digit in your credit-card number signifies the system:
  • 3 – travel/entertainment cards (such as American Express and Diners Club)
  • 4 – Visa
  • 5 – MasterCard
  • 6 – Discover Card
• The structure of the card number varies by system. For example, American Express card numbers start with 37; Carte Blanche and Diners Club with 38.
  • American Express – Digits three and four are type and currency, digits five through 11 are the account number, digits 12 through 14 are the card number within the account and digit 15 is a check digit.
  • Visa – Digits two through six are the bank number, digits seven through 12 or seven through 15 are the account number and digit 13 or 16 is a check digit.
  • MasterCard – Digits two and three, two through four, two through five or two through six are the bank number (depending on whether digit two is a 1, 2, 3 or other). The digits after the bank number up through digit 15 are the account number, and digit 16 is a check digit.

The Stripe on a Credit Card

­­The stripe on the back of a credit card is a magnetic stripe, often called a magstripe. The magstripe is made up of tiny iron-based magnetic particles in a plastic-like film. Each particle is really a tiny bar magnet about 20-millionths of an inch long.

Illustration by Rosaleah Rautert Your card has a magstripe on the back and a place for your all-important signature.

The magstripe can be “written” because the tiny bar magnets can be magnetized in either a north or south pole direction. The magstripe on the back of the card is very similar to a piece of cassette tape (see How Cassette Tapes Work for details).

A magstripe reader (you may have seen one hooked to someone’s PC at a bazaar or fair) can understand the information on the three-track stripe. If the ATM isn’t accepting your card, your problem is probably either:

• A dirty or scratched magstripe
• An erased magstripe (The most common causes for erased magstripes are exposure to magnets, like the small ones used to hold notes and pictures on the refrigerator, and exposure to a store’s electronic article surveillance (EAS) tag demagnetizer.)

There are three tracks on the magstripe. Each track is about one-tenth of an inch wide. The ISO/IEC standard 7811, which is used by banks, specifies:

• Track one is 210 bits per inch (bpi), and holds 79 6-bit plus parity bit read-only characters.
• Track two is 75 bpi, and holds 40 4-bit plus parity bit characters.
• Track three is 210 bpi, and holds 107 4-bit plus parity bit characters.

Your credit card typically uses only tracks one and two. Track three is a read/write track (which includes an encrypted PIN, country code, currency units and amount authorized), but its usage is not standardized among banks. You can use the debit card instead of using a business check for your transaction