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No matter what credit card you carry in your wallet (or the variety of credit cards you keep on hand for various purchases), it shares some physical characteristics with the cards used by consumers across the U.S. The common features of the many, many different credit cards in circulation are their size and the appearance of a number emblazoned across the front.

Credit cards have a similar size to one another because of an organization known as the ISO. The ISO, or International Organization for Standardization, is the standard-setting body of international representatives that produces global commercial and industrial standards. The ISO establishes standards for the size of credit cards and other ID cards, as well as determining what cards are made of and ensuring their standards are ideal for a wide array of uses.

The ISO is the reason your credit cards are made of PVC and are generally the same size across the board, which allows for easier processing and recognition. It ensures that your credit cards remain standardized.

Credit cards also share a common numbering scheme. A credit card number contains a single-digit major industry identifier (MII), a six-digit issuer identifier number (IIN), an account number and a single-digit checksum.

The Major Industry Identifier number corresponds to the issuer category. For example, a “1” denotes airlines, “2” airlines and other industry assignments, “3” travel and entertainment, “4” banking and financial, etc. American Express would fall into the travel and entertainment category, while Visa, MasterCard and Discover are in the banking and financial category.

The first six digits of the credit card number (including the initial MII digit) make up the issuer identifier, which denotes the credit card network to which the number belongs. Some of the better-known issuer identifiers include:

American Express — 34xxxx and 37xxxx

Visa — 4xxxxx

MasterCard — 51xxxx-55xxxx

Discover — 6011xx

The maximum length of a credit card number is 19 digits, with the maximum length of the account number field being 12 digits (initial six-digit issuer identifier minus the final digit check number). Each issuer has 1 trillion possible account numbers.

The final digit of your credit card number is a check digit, similar to a checksum. The algorithm used to arrive at the correct digit is called the Luhn algorithm, after IBM scientist Peter Luhn. The check digit is used to confirm the initial digits of the card number. The benefits of using a checksum is that it prevents casual attempts to invent credit card numbers, as only one in ten will be valid. Additionally, it serves to prevent mistakes when the credit card number is manually recorded.

Credit cards also feature issue and expiration dates, as well as extra codes such as issue numbers and security codes. However, not all credit cards have the same sets of extra codes used to identify that the card is authentic.

See related: Anatomy of a credit card