French windows are a great product, they have two adjoining sashes that usually open outwards (although they can be manufactured to open inwards if you prefer). They combine all the benefits of a casement window with the features of French doors.
When both sashes are open fully, you have a clear, unobstructed opening. This gives a great ambient feel to the room, whilst also providing an easy escape route should there ever be an emergency.
French windows operate in a similar way to French doors. One sash is the Master, which can be opened and closed independently. The second is the Slave sash, that can only be opened when the Master is open.
A shootbolt locking system fitted to both sashes ensures the security of the window isn't compromised to achieve the functionality.