The first digit is always 3, 4, 5 or 6. Credit card companies use a standardized system of assigning numbers. 3 means it is an American Express or Diners Club credit card. Credit cards beginning with 4 means that it is a Visa card. If your credit card numbers starts with 5 it represents a MasterCard. And in the case of a credit card number beginning with 6, it is a Discover credit card.

The other numbers are used to carry various other information. Visa a major credit card issuer uses the digits 2 through 6 to identify the bank. 7 to 12 or 15 are used to identify the account number and the last number is called the check digit. The pattern in Visa credit cards can vary greatly because all of Visa credit cards do not have the same amount of digits on credit cards.

Similar is the case with MasterCard. Digits 2 to 6 represent the bank and digits 7 onwards are used to identify the account with which the credit card is associated. The last digit is the check digit.

The numbering pattern with American Express credit cards is slightly different. The second digit in these credit cards is used to identify the company. For example if the credit card is issued in American Express’s name it will have the first two digits 34, or 37 and if it carries Diners club logo it will have its first two digits as 36 or 38. The 3rd and 4rth digits are used to identify whether the credit card is a business or personal type. The digits 5 to 11 represent the account number and digits 12 to 14 shows the card number attached to that account. The last digit is the check digit.

In case of Discover credit card the pattern is similar like those of Visa and MasterCard. Read more from: http://www.credit-card-gallery.com/article/381,Credit_card_numbers_What_do_they_really_mean

If it’s an American Express Card it has only 15 numbers.