A serial debenture is a municipal bond structure where portions of the principal mature and are repaid at regular intervals over the bond's term, rather than all at once at the end. For example, a 20-year serial debenture might have equal portions maturing each year, spreading repayment evenly. This structure results in declining total payments over time as principal reduces. Serial debentures may better match infrastructure depreciation and avoid accumulating large balloon payments. They contrast with term debentures (all principal due at maturity) and sinking fund debentures (municipality makes regular deposits toward final payment). Each structure has different cash flow implications.