A French Window is a pair of casements (openers) fitted side by side into a single frame – they can also be used as entrances by fitting doors and are then referred to as French Doors.
The classic mechanism if for French Windows to open outwards but, as you can see form the image, they can also be manufactured to open inwards; However, opening inwards makes them less secure and more susceptible to being forced open. With an outward opening set, the moving parts will hit the frame if you try to force them inwards and that will make them very difficult to force open.
French windows are originally mainly made from timber, but these days there are excellent uPVC versions in the market. Modern uPVC French Window designs come in many colours and finished surfaces. Find out more at – www.doubleglazingquoter.org.uk/double-glazed-windows/french-windows