Baby Safety Locks & Guards: A must for multi-level homes. The safest window guard on the market! Others are permanently mounted, which could spell disaster in a fire. This one withstands 70kg. of pressure, yet removes easily with emergency release buttons. The steel bars are closer together than others (9.8cm apart; not 4-12.7cm .) The only brand recommended by the city of Boston. Works with single and double-hung windows, as well as sliding and hinged casement windows (does not work with crank-out windows.) Side posts screw mount into your window frame - it's very easy. Please measure your window widths (frame to frame) before ordering.