HINGS come in threes: this article is the third in a trilogy inspired by TV. I'm going to start getting out more after this.

It was a MasterCard commercial. The opening shot showed a supermarket checkout, with groceries rolling along the little plastic mat towards the cashier. The voiceover said: "Frozen dinner, frozen dinner, frozen dinner, frozen dinner. $12." Then it added: "Breaking in your new flat-screen plasma TV. Priceless."

So far, so good. Then the MasterCard logo came up and the voiceover said something clever. Then the logo turned into a bag of groceries and the voice said something like: "MasterCard. It's for groceries."

No, it isn't.

I thought I was hallucinating the first time I saw the ad. There's a rule in money wisdom that says if you're buying your groceries on credit, you are headed for trouble. I would accept that the financial milieu in which we live has changed since that statement was absolute, but financial discipline is clear on this. If you must borrow, use money you already have to buy food, and borrow the money to buy the thing you can't afford and so must borrow to buy.

I'm not saying that you shouldn't have a credit card (although in many cases, I would say that, if you asked me directly). Used wisely, a credit card that is paid off every month is an efficient way of paying bills. The problem with credit cards is that, sooner or later, you don't pay one off. You're on vacation and miss the bill, or you just decide for one reason or another to miss the bill. It gets missed, is what I'm saying.

Then you carry a balance made of up of last month's groceries, and then you have the burden of interest to pay, and then you can't afford to buy a new hat for cash or credit. So you increase your credit balance and ? well, the end of this story is you lose your children and go insane.

We journalists are bound by what is called the rule of full disclosure. As we saw last week, I will break rules, but not this one. The fact is that I buy my groceries with a credit card. This is not a case of "I may, you mustn't", although I can save you the cost of postage by saying that I understand that it certainly looks that way.

I keep a small balance in my favour on my credit card. That way, when I use the card, I am simply spending cash in the same way that I would if I wrote a cheque. I forget what it costs if you a write a lot of cheques. I stuff everything through the MasterCard, because I have an air mileage card, and like to earn "free" tickets to places near and far. It was recently revealed to me that one can pay one's utilities on MasterCard, so I now duly put them on the card. I wish my landlord took MasterCard.

The credit is one air mile for one dollar spent. A trip to New York, if you can show a small amount of flexibility and live with an advance booking, costs 30,000 miles. You do the math. OK, OK, I'll do it. An air ticket to New York, at its cheapest, costs about $400 by the time you add in taxes. That means that 75 air miles equals a dollar saved. That gives me a 1.3 percent discount on everything I spend. Thank you.

The economic downside of this equation is that, although you save the airfare, you have to take more trips. That can destroy your savings programme.

You might think that the bank hates me, but I expect they don't. (Let's not find out.) The bank earns a fee from the supplier each time I charge something, and I pay $95 a year for the card, which is about $60 more than a basic non-mileage card.

Revealing all this manic detail to you is intended to point out that you have a choice: you can run your financial affairs with forethought, or you can adopt a more relaxed approach. The manic part is entirely optional, but I submit that you already have that going for you. It's the part that stops you forgetting to pay your bill and then going insane. So we're just talking degree, is all.

Used sensibly, a credit card is your best friend. You can visit establishments and lands far away, wave the plastic and Bob's your uncle. If you prepay it, or religiously pay it off when it falls due, you win. If you need to borrow money to buy something major, a bank loan is a better and cheaper deal. It also lets you understand the commitment you have made to whatever you borrowed the money to buy, and from such associations comes knowledge, which many of us believe to be a useful tool.

It need not be as manic as it sounds. Every now and then, I send MasterCard some money, when whatever is already in there starts running out, or when I receive a cheque I don't immediately need. When I go on a "free" trip, I usually replenish the card when I get back.

More disclosure: now that I think about it, on Wednesdays, I buy my groceries for cash. Five percent off beats 1.3 percent off. Life's like poker: you gotta play the odds.