Abstract: Members are attachable to the window or door around which a cord can be stretched, under tension, in the form of a polygon. A switch is positioned within the polygon, said switch being attached by further cords to two of the sides of the polygon. The switch is held under a selected tension and the shape of the polygon is such that an intruder cannot enter through the window or door without disturbing the cord polygon and, thereby, the tension on the switch. The switch is designed to respond to a change in the tension applied thereto, to supply a signal to an alarm system.