A mortgage-backed instrument which is comprised of two parts: interest and principal. In some cases, the security is made up of a certain percentage of both interest and principal. The ideal situation for investors is for the security to either be an interest-only strip or a principal-only strip. In these scenarios, the securities are very sensitive to the change in interest rate, so an investor will choose to purchase one or the other based on the direction he/she believes the interest rates are headed.