A bond, usually tax-exempt, which is made up of many bonds with different maturities. They are issued on the same date. This structure helps the issuer to spread out the debt service and make approximately equal annual principal payments. For example, bonds may mature each year for twenty years after the date of issue of the series. Each maturity can pay a different coupon rate - usually the longer the maturity the higher the coupon. In addition, most tax-exempt issues are callable.