(MONEY Magazine) – If you lowered your payments by opting for an adjustable-rate loan, do you know what could happen to your "amount due" this year or beyond? Look up these key terms in your mortgage paperwork to find out.

1 INDEX. Your rate is tied to an index with a baffling name like the cost-of-funds index (COFI) or the constant-maturity Treasury (CMT). You can find current and historical data on 16 indexes at mortgage-x.com.

2 MARGIN. The interest rate you pay (now and in the future) equals the index rate plus your lender's margin, usually a few percentage points. Calculate the total now to see if you should consider a refi.

3 CAPS. To preview how bad it could get, look for three numbers: The "first adjustment cap" is the most your rate can rise the first time; the "periodic cap" restricts each rate jump after that; and the "lifetime cap" limits how much your rate can ever go up. Generally, things have to get a whole lot worse for these caps to offer much protection.

4 EARLY TERMINATION FEE. This ding (typically 1% to 3% of your balance) might kick in if you refinance out of an ARM in the first couple of years--and wipe out the benefits of a new loan.