A mortgage on which half the monthly payment is paid twice a month. It should be called a 'semi-monthly mortgage' but market practice often trumps logic. In contrast to a biweekly, a bimonthly mortgage involves no extra payments. The 24 half payments a year add to the same total as 12 full payments. Advancing the payment by half a month saves a little interest, but the effect is negligible. A 7% 30-year loan pays off in 29 years, 11 months.