Invisible from the outside and is operated by simple key action which throws the bolt into the door frame. Two bolts should be fitted to external doors, one top and one bottom. Only one is required for internal doors fitted centrally, with the key operation on the side invisible to the likely intruder. Two bolts, one into the top frame the other into the bottom sill, should be fitted to secure each opening leaf of 'French Windows.' Supplied 1 no door bolt per visi pack card. Keys available separately, 2 per card