A credit card number is the long set of digits displayed across the front or back of your plastic credit card. It is typically 16 digits in length, often appearing in sets of four, and it is used to identify both the credit card issuer and the account holder.

Credit card numbers are not randomly assigned; they’re coded to identify the issuer, the network, the account and also to validate the complete number, all in an effort to prevent theft and fraud. The credit card number must fit a complex pattern in order to work. Your credit card number consists of:

## The First Number

The first six numbers are used to identify the industry and/or credit card issuer. And the first digit is known as the Major Industry Identifier (MII). Here are some popular MIIs and what they represent:

1: Airlines

2: Airlines and Financial

3: Travel and Entertainment (including American Express)

4: Banking and Financial (Visa)

5: Banking and Financial (MasterCard)

6: Merchandising and Banking (Discover)

7: Petroleum

8: Health Care and Communications

9: Government

## The Next 5 Numbers

These are referred to as the card’s Issuer Identification Number (IIN), or Bank Identification Number (BIN), and further indicate which credit card company it originates from and clarifies to which card network it belongs.

Following the numbers that identify the issuer are the numbers that identify the account holder. Each issuer has one trillion possible numerical configurations with which to create account numbers and different credit cards use slightly different numbering systems.

## The Next 9-12 Numbers

The seventh and all remaining digits in a card number, except for the last, identify that card’s individual account. Your account number may have as few as nine or as many as 12 digits.

## The Last Number

The last digit of a credit card number is known as a check sum. It is a key that shows whether a credit card is indeed valid. The “check number” or “key” is created by a formula known as the Luhn Algorithm.

The Luhn algorithm was developed by inventor Peter Luhn, an IBM engineer who played a role in the early development of the internet. This complex algorithm is able to immediately detect errors when people inaccurately transcribe credit card numbers. It can tell, for instance, when someone accidentally hits the 9 key instead of the 6 key, as well as many other common errors.

As you can see, credit card companies are not just throwing digits out randomly. Every Discover credit card comes with a unique set of numbers with a utilitarian purpose. Like a snowflake, every credit card number is unique.

Published March 22, 2017.

Updated August 30, 2021.

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