Abstract: A deep groove or slot is provided at the joint between each stile and each rail to permit the accomodation, within the thickness of the element, of an appropriately shaped stiffener of suitable material such as stainless steel, aluminum alloy, galvanized sheet, brass, etc. In addition, both the stiles and the rails of each of the two elements of the window are provided on one of their surfaces with two deep grooves designed to accept suitable seals, while their outer surface is provided with steps, which match corresponding steps on the opposing element. The stiffener has an L-, C-, or Z-shaped cross section and forms essentially a right angle. After the stiffeners have been inserted in the stiles and the associated rails they thus disappear from view.The rails and the stiles can be of laminated construction, and after the individual layers have been bonded, they form either the grooves which house the stiffeners or the grooves which house the seals.