The first time I had to replace a credit card (stolen) I noticed that many of the digits were the same. My wife has a card from the same bank and she also shares most digits with me.
Obviously the first few digits are bank specific.
This page has a brief outline of the meaning behind the numbers.
The first digit is the type of card:
- 3 = Amex or Diners Club
- 4 = VISA
- 5 = Mastercard
The rest of the digits depend on the type of card.
The format of VISA credit card numbers
- T is for the type of card. In this case always 4 as this is a VISA card
- B is a six digit code for the bank
- A is a 9 digit account number
- C is a check digiti calculated from all of the other digits