Adjustable
mortgage rates (ARMs) are the second major type of loan available. With an ARM, your interest rate, and therefore your payment, can go up or down through the life of the mortgage, depending on various economic factors.
The rate is usually tied to a money market index, most commonly the one-year Treasury bill. The lender will usually add between two and four percentage points to the current rate for the Treasury bill to come up with your current adjustable rate. These extra percentage points are called the margin.