Sash window restoration is an art in practice.
Sash windows can be removed for sucsesful restoration, the weighted pulley system that hangs inside the frame is more difficult to get to, but it can be fixed and restored as well.
Casement windows can give the same appearance as a sash window from afar, but it is not the same type of window at all.
The sash window slides up and down thanks to a counterweight that is affixed inside the window box.
The casement window is opened via a crank system, allowing the window to open at an angle rather than up and down, Similar to a doors function.
Sash window restoration has become an art form as there are fewer skilled crafts people that can truly handle the job. Fortunately, the sash window is still iconic enough that more and more people are looking toward restoration over replacement.