A Brief Tour of Window Mats and Their Openings. In this blog we will try to shed some light on Window Mats and Their Openings, with a particular focus to their external and internal sizing. A window mat ( Customers often call them edges, cardboards, surrounds, masks, etc ) or a window mount, as our English brethren would call one is that white or coloured cardboard that framers often put around images and inside picture frames. Broadly speaking, window mats are placed over and around images for several ...