What credit card numbers mean

Specifications for credit card numbering have been drawn up by the International Standards Organization (ISO/IEC 7812-1:1993) and the American National Standards Institute (ANSI X4.13).

The first digit of your credit card number is the Major Industry Identifier (MII), which represents the category of entity which issued your credit card. Different MII digits represent the following issuer categories:

MII Digit Value	Issuer Category
0	ISO/TC 68 and other industry assignments
1	Airlines
2	Airlines and other industry assignments
3	Travel and entertainment (such as American Express and Diners Club)
4	Banking and financial (Visa)
5	Banking and financial (MasterCard)
6	Merchandizing and banking (Discover)
7	Petroleum
8	Telecommunications and other industry assignments
9	National assignment

For example, American Express, Diner's Club, and Carte Blanche are in the travel and entertainment category, VISA, MasterCard, and Discover are in the banking and financial category, and SUN Oil and Exxon are in the petroleum category.

The first 6 digits of your credit card number (including the initial MII digit) form the issuer identifier. This means that the total number of possible issuers is a million.

Some of the better known issuer identifiers are listed in the following table:

Issuer	Identifier	Card Number Length
American Express	34xxxx, 37xxxx	15
VISA	4xxxxx	13, 16
MasterCard	51xxxx-55xxxx	16
Discover	6011xx	16
Diner's Club/Carte Blanche	300xxx-305xxx, 36xxxx, 38xxxx	14

The final digit of your credit card number is a check digit, akin to a checksum. The algorithm used to arrive at the proper check digit is called the Luhn algorithm, after IBM scientist Hans Peter Luhn (1896-1964), who was awarded US Patent 2950048 ("Computer for Verifying Numbers") for the technique in 1960. The most succint description of the Luhn algorithm is: "For a card with an even number of digits, double every odd numbered digit and subtract 9 if the product is greater than 9. Add up all the even digits as well as the doubled-odd digits, and the result must be a multiple of 10 or it's not a valid card. If the card has an odd number of digits, perform the same addition doubling the even numbered digits instead."

For 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.

For 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.

For 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.

Let’s analyze a sample number of 4408 0012 3456 7890 to see whether it is a valid credit card number.

The Major Industry Identifier (MII) is 4 (banking and financial), the issuer identifier is 440800 (a VISA partner), the account number is 123456789, and the check digit is 0. Let's apply the Luhn check to 4408 0012 3456 7890. (4*2)+4+(0*2)+8+(0*2)+0+(1*2)+2+(3*2)+4+(5*2-9)+6+(7*2-9)+8+(9*2-9)+0 = 8+4+0+8+0+0+2+2+6+4+1+6+5+8+9+0 = 63, which is not a multiple of 10. Therefore we conclude that the number 4408 0012 3456 7890 is an invalid credit card number.

Try to use a valid credit card to verify the numbering, and you will see that the first 6 digits match the card issuer and the final digit always makes the result from Luhn algorithm to be a multiple of 10.