Adjustable Rate Mortgage (ARM)
An ARM is a mortgage with an interest rate that may vary over the term of the loan  usually in response to changes in the prime rate or Treasury Bill rate. The purpose of the interest rate adjustment is primarily to bring the interest rate on the mortgage in line with market rates.
Mortgage holders are protected by a ceiling, or maximum interest rate, which can be reset annually. ARMs typically begin with more attractive rates than fixed rate mortgages  compensating the borrower for the risk of future interest rate fluctuations.
Choosing an ARM is a good idea when:

Interest rates are going down

You intend to keep your home less than 5 years
ARMs have the following distinguishing features:

Index

Margin

Adjustment Frequency

Initial Interest Rate

Interest Rate Caps

Convertibility
Index
An adjustable rate mortgage’s interest rate increases and decreases based on publicly published indexes. ARMS are based on different indexes including:
 United States Treasury Bills (Tbills)
 The 11th District Cost of Funds Index (COFI)
 London Interbank Offering Rate Index (LIBOR)
 Certificate of Deposit Indexes (CODI)
 12Month Treasury Average (MTA or MAT)
 Cost of Savings Index (COSI)
 Bank Prime Loan (Prime Rate)
Margin
Margin is a fixed percentage amount that is pointed added to the index – accounting for the profit the lender makes on the loan. Margins are fixed for the term of the loan.
interest rate = index + margin
Adjustment Frequency
Adjustment frequency reflects how often the interest rate changes – also known as the reset date. Most ARMs adjust yearly, but some ARMs adjust as often as once a month or as infrequently as every five years.Initial Interest RateThe initial interest rate is the interest rate paid until the first reset date. The initial interest rate determines your initial monthly payment, which the lender may use to qualify you for a loan. Often the initial interest rate is less than the sum of the current index plus margin so your interest rate and monthly payment will probably go up on the first reset date.
Interest Rate Caps
Interest rate caps put limits on interest rates and monthly payments.
 Common caps:
 Initial Adjustment Cap
 An initial adjustment cap limits how much the interest rate can change at the first adjustment period.
 Example:
 If your ARM has a 1% initial adjustment cap, your interest rate may only increase or decrease by a maximum of 1% at the first adjustment period.
 Periodic Adjustment Cap
 A periodic adjustment cap limits how much your interest rate can change from one adjustment period to the next. Usually a sixmonth adjustable rate mortgage will have a one percent periodic adjustment cap while a oneyear adjustable rate mortgage will have a two percent periodic adjustment cap.
 Example:
 If your loan has a 2% periodic adjustment cap, your interest rate may only increase or decrease by a maximum of 2% per adjustment period.
 Lifetime Cap
 A lifetime cap sets the maximum and minimum interest rate that you may be charged for the life of the loan. Most ARMs have caps of 5% or 6% above the initial interest rate.
 Example:
 If your loan has a 6% lifetime cap, your interest rate may only increase or decrease by a maximum of 6% for the life of the loan.
 Initial Adjustment Cap
Initial adjustment caps, periodic adjustment caps, and lifetime caps make up an adjustable rate mortgage’s cap structure, and are usually represented as three numbers:
 Example: 1/2/6 — Initial adjustment cap is 1 %/ periodic cap is 2% / lifetime cap is 6%.