Credit card issuers now use 14 different methods of charging interest. If you pay your bill in full, this usually won't affect you. But if you take cash out of a cash machine with your credit card, or pay anything less than the full amount on your statement, you'll normally be charged interest.

In fact, if you had two cards with the same interest rate and used them in exactly the same way, one could end up costing over twice as much as the other just because it calculated your interest differently.

This is because the amount you are charged depends not just on the card's rate but on when it starts and stops charging interest.