(Alan Rappa)
(Alan Rappa)

Over the years, countless people have looked at 16-digit credit card numbers and said things like “Why do they need so many numbers? It’s not like there are 9.999 quadrillion bank accounts out there.” Well, that’s true. But the numbers on your card aren’t just about how many accounts or cardholders exist. They also indicate information about your card issuer, its network and tells processors whether or not the number is valid.

The first part is the easiest, and one that most people have either learned about or assumed — the initial digit in your credit card number indicates the card network.

You may have noticed on some e-commerce sites that the e-tailer recognizes immediately after you’ve typed in that first number what kind of card you’re using. A “4” is for Visa, “5” for MasterCard, “6” for Discover, and “3” covers American Express, Diner’s Club, and Carte Blanche.

What you may not know is that the next several digits indicate what type of card you’re using. Credit.com points out that a Citi AAdvantage…