Look closely at your credit card and you'll find a string of numbers which don't make much sense to the user but believe it or not, are all there for a purpose. All major card companies have the same system when it comes to numbering. Others like department stores, gas cards, and phone cards work on a smaller scale and so have their own methods suiting individual requirements.
The digits on the cards have a designated code to them. The first digit tells you what kind of a card you have. An entertainment or travel card like American Express or Diners Club will begin with a '3'. '4' would stand for Visa cards, including Visa-branded debit cards or cash cards. All MasterCard and MasterCard-branded cards would begin with '5' and '6' is used by Discover.
Companies keep certain identifying numbers specific to themselves. Generally the next set of digits gives the routing number of the bank the card has been issued from, followed by the user's account number and lastly the check digit. This check digit is calculated using a specific formula on the other numbers and this is used as an anti-fraud check. So a string of numbers suddenly gets decoded to a whole lot of information.
Visa uses the digits from two to six to represent the bank number. The seventh to twelfth/fifteenth is the account number and the last is the check digit. Visa cards do not have a specific number count and can have a varying number of digits for various cards.
MasterCard uses the second to the sixth digit as its bank number and the remaining are the card holder's account details with the last being the check digit.
Cards like American Express and Diners Club have a second digit for company identification. The American Express Card uses digits three and four for either business or personal types of accounts along with identifying the currency if the card holder belongs to another country. Digits five to eleven are account numbers and finally digits twelve to fourteen indicate the card number within the account, and the last digit is the check digit.
Maybe this information is vital to the card company, but is it relevant at all to you? Probably not. But for people who like to pick up and hoard up little pieces of trivia in their minds, this is a good one.