Adjustable-Rate Mortgages, known as "ARMs", come in even more varieties. Generally, ARMs determine what you must pay based on an outside index, perhaps the 6-month Certificate of Deposit (CD) rate, the one-year Treasury Security rate, the Federal Home Loan Bank's 11th District Cost of Funds Index (COFI), or others. Most commonly, they may adjust every month, every six months, or once a year. 

Most programs have a "cap" that establishes a maximum your monthly payment can go up at once. There may be a cap on how much your interest rate can go up in one period -- say, no more than two percent per year, even if the underlying index goes up by more than two percent. You may have a "payment cap," that instead of capping the interest rate directly caps the amount your monthly payment can go up in one period. In addition, almost all ARM programs have a "lifetime cap" -- your interest rate can never exceed that cap amount, no matter what.

ARMs often have their lowest, most attractive rates at the beginning of the loan, and can guarantee that rate for anywhere from one month to ten years. You may hear about loans that are called "3/1 ARMs" or "5/1 ARMs" or the like. That means that the introductory rate is set for three or five years, and then adjusts according to an index every year thereafter for the life of the loan. Loans like this are often best for people who anticipate moving -- and therefore selling the house to be mortgaged -- within three or five years, depending on how long the lower rate will be in effect.