Banks issue credit cards with 16 digit numbers. If you've
never thought about it before you may not realize it, but there are
specific rules for what those numbers can be. For example, the
first few digits of the number tell you what kind of card it is -
all Visa cards start with 4, MasterCard numbers start with 51
through 55, American Express starts with 34 or 37, etc. Automated
systems can use this number to tell which company...