A two-step mortgage is a combination of both a fixed rate mortgage and an adjustable rate mortgage. It is usually expressed in confusing numbers such as 5/15, 5/25, or 7/23. The first number indicates how long the first fixed rate mortgage lasts. The second number is the duration of the second fixed rate mortgage, which is usually tied to an interest rate index. Unlike many other adjustable rate mortgages, this type only changes the interest rate once.