For those who do not have a door chain, this is a must. It keeps the key in place and the thief cannot turn it with tools. It tightens by turning the black screw on the key and the "ear/curve" catches onto the door handle. Obviously, it cannot be applied if you want to be away from home. Due to the process, sometimes we are too lazy to put it on, but if we had more cases in our area, we would use it.