While this stool is manufactured to fit the Andersen Tilt Wash double hung window with a 4-9/16 in. (2x4) wall thickness, it can be used anywhere this size meets the project needs. The stool is an interior trim mould serving as a window frame sill cap, giving a finished appearance to the interior of the window unit. If the window unit is, "picture framed" then a stool is not needed.