Abstract: In a double-hung window system of the type having upper and lower ventilating sliding sashes and a lock having a lapping assembly having upper and lower vertical lapping members the closer ends of which form a stop assembly together with a stop means cooperative therewith for stopping upward movement of the lower lapping member with respect to the upper lapping member. The upper lapping member is attached to the upper sash and the lower lapping member is attached to the lower sash whereby, with the window system open for ventilation, the amount of opening space available to a burglar who seeks to enter is limited so long as the lapping remains in place. The new feature thereof being means anchoring the upper end of the lapping assembly to the window frame so that a burglar cannot reach from the outside to the upper end of the frame anchored lapping assembly for forcing it inward out of its locking position.