Repayment schedules differ with the type of bond issued. Municipal bonds typically pay interest semi-annually. Shorter term bonds generally pay interest only until maturity; longer term bonds generally are amortized through annual principal payments. Longer and shorter term bonds are often combined together in a single issue that requires the issuer to make approximately level annual payments of interest and principal. Certain bonds, known as zero coupon or capital appreciation bonds, accrue interest until maturity at which time both interest and principal become due.