Park it on a 0% card?

Two runs of the same days with the same dollars. In one the balance stays on the line of credit, accruing daily at its rate the way a HELOC is billed. In the other it moves to the card: the fee goes on the balance on day one, the promo rate accrues daily (usually nothing), and each month the card's minimum comes off. The same payments come off the line of credit in the first run, so on the day the promo ends the only difference between the two balances is what each lender charged: the fee against the interest. That difference is the number at the top.

The fee as a rate: divide it by the balance and scale to a year. On the whole amount for the whole promo that is the simple figure a lender would quote. But the minimums shrink the balance every month, so the fee bought less than it looks: the second rate is the fee over the balance you actually carried, day by day, and it is the one to hold against the line of credit. The break-even is the line-of-credit rate at which the two runs tie.

A card's minimum is in its agreement — MBNA's is the greater of $10 and 1% of the balance, plus any interest; Quebec's floor is 5% — and what is left when the promo ends goes back on the line of credit, or stays on the card at its regular rate, which is the trap. Purchases on the card are charged at that rate from day one, and a missed payment usually ends the promo.