If the coupon interest rate remains constant from the time of issue until the bond matures, then the bond is called a bond. The contract that describes the terms of a borrowing arrangement between a firm that sells a bond issue and the investors who purchase the bonds is called the . Issuers can gradually reduce the outstanding balance of a bond issue by using a sinking fund account into which they deposit a specified amount of money each year. To operationalize the sinking fund provision of an indenture, issuers can (1) purchase a portion of the debt in the open market or (2) call the bonds if they contain a call provision. Under what circumstances would a firm be more likely to buy the required number of bonds in the open market as opposed to using one of the other procedures