Abstract: Cranked windows can be retrofitted with a locking system which prevents their forced entry by burglars while still permitting the windows to be opened by a predetermined amount to allow ventilation. Such a retrofitted locking system comprises a case-hardened bolt to disposed through aligned bores drilled through overlapping flanges on a movable fame of the cranked window and the corresponding stationary frame. The bores are drilled through the overlapping flanges at a position in the frames opposite a hinge about which the movable frame rotates. The bores are drilled with a diameter slightly larger than the diameter of the bolt disposed through them to permit the bolt to be inclined, thereby allowing a predetermined degree of opening of the frames relative to each other. An integral head portion of the bolt is disposed outside the movable frame, while the opposing threaded portion of the bolts is provided with a pair of cinched nuts to act as a stop.