With most ARMs, the interest rate and monthly payment change every year, every three years, or every five years. However, some ARMSs have more frequent interest and payment changes. The period between one rate change and the next is called the adjustment period. So, a loan with an adjustment period of one year is called a one-year ARM, and the interest rate can change once every year.

The Index
Most lenders tie ARM interest rate changes to changes in an "index rate." These indexes usually go up and down with the general movement of interest rates. If the index rate moves up, so does your mortgage rate in most circumstances, and you will probably have to make higher monthly payments. On the other hand, if the index rate goes down your monthly payment may go down.

Lenders base ARM rates on a variety of indexes. Among the most common are the rates on one-, three-, or five-year Treasury securities. Another common index is the national or regional average cost of funds to savings and loan associations. A few lenders use their own cost of funds, over which-unlike other indexes-they have some control. You should ask what index will be used and how often it changes. Also ask how it has has behaved in the past and where it is published.

The Margin
To determine the interest rate on an ARM, lenders add to the index rate a few percentage points called the "margin". The amount of the margin can differ from one lender to another, but it is usually constant over the life of the loan.

Let's say, for example, that you are comparing ARMs offered by two different lenders. Both ARMs are for 30 years and an amount of $65,000.

Both lenders use the one-year Treasury index. But the first lender uses a 2% margin, and the second lender uses a 3% margin. Here is how that difference in margin would affect your initial monthly payment.

In comparing ARMs, look at both the index and margin for each plan. Some indexes have higher average values, but they are usually used with lower margins. Be sure to discus the margin with your lender.