All frames were replaced with new ones equipped with insulating glass.
At the front there were the original sash-windows. These were replaced by Tilt & Turn windows.
These originated in Germany and through a simple mechanism have the ability to both tilt inwards from the top like a hopper window, or open inwards from side hinges like a casement window.
In appearance the modern frames are indistinguishable from what existed before, however they are much safer and far easier to use.