An Adjustable-Rate Mortgage (ARM) is a type of loan whose prevailing interest rate is tied to an economic index (like one-year Treasury Bills), which fluctuates with the market. The three most popular types of ARMs are one-year ARMs, which adjust every year, three-year ARMs, which adjust every three years, and five-year ARMs, which adjust every five years. When the loan adjusts, the lender tacks a margin onto the economic index rate to come up with your loan’s new rate. ARMs are considered riskier than fixed-rate mortgages, but their starting interest rates are generally lower than a longer-term rate, and in the past five to ten years, people have done very well with them.