A type of stepped-payment loan in which the borrower's payments are initially lower than those on a comparable level-rate mortgage. The payments gradually increase over a predetermined period (usually 3, 5, or 7 years), and then are fixed at a level-pay schedule, which will be higher than the level-pay amortization of a level-pay mortgage originated at the same time. The difference between what the borrower actually pays and the amount required to fully amortize the mortgage is added to the unpaid principal balance.