A picture frame mat is a board that is cut to have a window in it so that it borders your picture. You use them on things like pastels, watercolors, posters, and photographs. They are enhancers; basically, they add color to your artwork or keep the art from touching the glass. They come in all different colors and variations. Normally they are cut with a bevel so you see the core of the mat board. Some mats have colored core or black core, but the most common bevel is a white core. The other option is to have the mat cut with a reverse bevel. This is great when a bright white line might be distracting.