Abstract: A horizontal sliding window construction is disclosed of the type including a fixed sash and a moving vent sash featuring a camming mechanism adapted to force the vent sash directly inward in the position which compresses a peripheral seal against the window frame facing surface to insure a weather tight closure of the window. The camming mechanism is operable in any position of the vent sash to secure the sash in a partially opened position. A closed position lock is created by operation of the mechanism moving the vent sash inward with the vent sash in the fully closed position by moving an aligned cutout on the vent sash frame into registry with a fixed locking projection on the window frame to lock the sash in the closed position.