Actually, the word casement refers to the part of a window that opens, so a French casement window is just a typical French window – and that is a pair of windows within a single frame that open from the centre.
Some French windows have a mullion or centre post.
Classic designs will feature small panes of glass set into panels within the casement, these separators are sometime called Georgian bars or mullions.
They can work well as either a window or a door and can commonly seen to be made from timber or uPVC.
If you are looking to buy some French windows, then make sure they have a good locking system, as older examples were favourites for burglars as the centre locking area was seen as a weak point.