Angel locks or 'Restrictors' are designed to restrict the amount a sash window’s lower sash can open. Fitted and used properly, they can prevent your child from opening the sash far enough to fall through. They do not require a key to operate and are unique in the way they reset automatically when you close the window.
Face-Fix Angels are fitted into the face of the stiles of the top sash, one on either side is required.
