A customized rate agreement between investor and issuer that fixes the price or yield of a treasury security. This is used by investors to guarantee a fixed return, or to hedge their risks, and counts as a separate derivative security. For example an investor can buy a treasury lock at 4%, if interest rates rise, making the market rate rise above the lock rate, the seller will pay the difference between the lock and market rates. However if interest rates fall, and the market rate falls below the lock rate, the investor must pay the issuer the difference between the lock and market rates.