Every once in a while, I like to point out the many little games that are played by the financial services industry as it taps the American pocketbook. As it turns out, there’s a little credit card ‘feature’ out there right now that deserves this spotlight.

Perhaps you’ve seen this in recent commercials: Your credit card company offers to set up a savings account for you. It then will round up every credit card purchase you make to the nearest dollar, and will deposit the difference into the savings account.

On the surface, that sounds like a clever idea. Americans are notorious under-savers, and this appears to be a near-painless way to build up (probably small) amounts of money.

Then you do the math.

Let’s say your credit card has an interest rate of 14% — not at all unusual. Let’s say the savings account pays, oh, 3% — also not unusual (and in fact probably generous).

So: The credit card company takes a $10.10 purchase, charges your credit card $11 for it and deposits 90 cents into a savings account. You’ll pay 14% annual interest on the 90 cents you were charged on your credit card, while it gathers 3% in the savings account.

Guess who makes money off that 11-point gap that you just paid for? Your credit card company. Guess what that company is going to do with your “savings”? Well, it might loan it out to another customer and make a few more points.

And remember, this is pitched as a service to you. You’re being serviced, all right.

I can see how this could work out OK if you pay your credit card bills in full so you don’t pay the float — although people who pay their cards in full are probably smart enough to save without the need for this “assistance.” Otherwise, though, this is no deal at all.

It’s the most sneaky of takeaways, in which you’re nipped in a small way — but the credit card company scales those small nips many thousands of times over until it makes some serious money. You almost have to admire it, but you’re better off just shining a light on it.