The numbers found on credit cards have a certain amount of internal structure, and share a common numbering scheme.
The card number's prefix, called the Bank Identification Number, is the sequence of digits at the beginning of the number that determine the bank to which a credit card number belongs. This is the first six digits for Mastercard and Visa cards. The next nine digits are the individual account number, and the final digit is a validity check code.
In addition to the main credit card number, credit cards also carry issue and expiration dates (given to the nearest month), as well as extra codes such as issue numbers and security codes. Not all credit cards have the same sets of extra codes nor do they use the same number of digits.