I wanted to add 4 more locks in addition to the ones installed by the aluminum company. Unfortunately, although I managed to install one on a sliding balcony door, when I tried to install one on a sliding window, the screw hit the glass from the inside and it broke. The guides for the screws are limited in distance from the height of the glass and they should be placed slightly further towards the side of each vertical end, which means that in order to be securely installed, the sliding part needs to be pulled out about 1cm (before closing). However, it is a good construction and quite presentable. The safer option, though, is the choice of Cal Doublex Classic or CAL PYROU DOUBLEX LOCKS.