I ordered the 37 width, as that is the width of my window.  The actual width of the shade was no more than 35, leaving at least 1 open on each side.  The header measured 36.5.  If it were an inside mount, that would have been okay, but since I am mounting outside, I had to return them and order the 39 width so I can fully cover window.  The quality of the shade is not as thick as Id hoped.