We provide four uPVC screws with each restrictor. Installation is very straight forward for these restrictors as you can drill straight into the frame without the need for any pre-drilling. It works on a spring loaded hook mechanism, which can be released by hand to allow the window to open completely if needed. Our video below gives you some advice on how to install it: Our releasable uPVC window restrictor is a great little product to restrict the opening distance of your windows. Releasable PVC restrictors come in left and right hand variants but both the hook and receiver can be fitted to either frame or window sash. Used for safety and convenience, these window restrictors are safety compliant and can be retro fitted.