A Flat Price is often used by investors as a term relating to interest rates and bond prices. Without any interest accrued to the principal, the price of the security is said to be flat. Flat bonds are also used in the fixed income market as a synonym for a fixed coupon bondThese are the types of bonds that are always purchased for their stability. They are also called money market bonds, as they are generally used to help investors maintain cash flow. They pay a flat, interest rate, no matter how high or low the market value of the underlying bond may fluctuate.