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Online credit card typo not someone else's problem

As I was shopping online the other day, the site spotted that I was off a digit when I gave them my debit card number.  As we've all done once or twice, the card simply bounced back and prompted me to put the card in correctly. It was then that I realized I've never had a freebie online because I was off a number, nor have I ever seen something funny on my statement because of someone else's goof. I'm sure that's how it's supposed to be, but how do they do that?

— Curious Shopper, via email

If only because we could peruse Amazon for "research," we like your question, Mr. or Mrs. Shopper.  Although we've been known to dial a wrong number or two here at Since You Asked headquarters, typos putting in a MasterCard don't equate to free stuff.

That's because although your bank card number's lengthy, it's far from random.  

You may have noticed that all Visa cards start with a "4," MasterCards with at "5," American Express with a "3" and Discover with a "6." While the first number indicates the card network, information about the issuing bank and/or lending institution is found within the first six digits of the card. Those first six digits are used as the "Major Industry Identifier" number and identifies whether that card is issued from Bank of America or Deutsche Bank, although many institutions have more than one MII number.

After those first six digits is a variable-length account number, plus a digit that ensures the card number fits within an algorithm. That algorithm was named after IBM engineer Hans Peter Luhn who developed the system in the 1950s and is used in other lengthy numbers, such as the International Mobile Identity number issued to each mobile phone.  The Luhn Algorithm isn't for fraud prevention; rather it's a safeguard against human error used in the case of bank cards to prevent a very expensive "oops" moment.

Before you start wondering why bad guys don't just start typing 16 digits that start with the number "4," you also can thank statistics for being another safeguard against a very expensive "oops" moment. There were 849 million Visa cards issued worldwide in 2014, according to CreditCards.com, and a theoretical 10 trillion combinations.

Send questions to “Since You Asked,” Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to youasked@mailtribune.com. To see a collection of columns, go to mailtribune.com/youasked. We’re sorry, but the volume of questions received prevents us from answering all of them.