Please note: That the security lock can't be used on VELUX Top-Hung windows, GPL, GPU, GHL and GHU. To check what type of VELUX roof window you have, you need to have a look at the type sign of the window by opening the sash and have a look at the ventilation flap. Features • Key operated window lock for GGL/GGU centre-pivot windows. • Simple lock that reinforces the 'click shut' mechanism at the top of your VELUX window. Extra security comes with this simple lock that reinforces the 'click shut' mechanism at the top of your VELUX Roof Window. You can fit this lock in the frame yourself. When the safety device is locked, it is not possible to open the sash, but the ventilation function of the window will still work.