This refers to a lock that is installed by carving a rectangular cut-out from the edge of the door. Mortise locks are still made today, but were far more common before the 1950's when cylindrical locks became common. Old “skeleton key” door locks are usually mortise locks, as are residential and commercial locks in which the latch and deadbolt are operated by a single keyhole. (If the keyhole is above or below the knob or lever handle--not built into the handle--you probably have a mortise lock). See also cylindrical lock and rim lock.