The AWS1837 Thermwell Products Window Screen is a simple way to let some air into your home without leaving the window fully open. Leaving an opening like that can allow things into your home that you do not want there. The extension window screen prevents bugs or other creatures from climbing inside. This product has a fingertip adjustment to fit all sash windows. The coated metal screen resists tear and weather and keeps flying bugs out. The durably constructed painted metal rails and varnished wood ends finish off the look of the screen. There are extra-sturdy tenoned corners double nailed to keep screens square. You can install the metal rail screens from inside. The Thermwell screen is 18 x 21-37 letting you adjust the size to fit your window. It will let you have a nice breeze flow throughout your home.