The window kit has two pieces that slide together. Each is 20.5 inches, BUT if your window is bigger than 20.5 inches and you have to use the slider extension, you can’t put it in a window that’s narrower than 30 inches or a bit wider, because the second piece can’t cover the vent opening. So you have to cut a piece off of the slider, which is metal in my case and not something I have tools for, or jerry rig a different way of setting up the venting system. I also do not believe that two pieces that are 20.5 inches will cover an opening wider than 40 inches, so the manufacturer ‘s statement of 59 inches is mathematically impossible.