Muslin is a tightly woven cotton fabric used for covering of scenery as well as for bounce drops for lighting. Muslin can be either FR (Fire Retardant) or NFR (Non Fire Retardant) and comes in a variety of widths in several colors. Traditionally, a muslin drop, if it is to be seamed, the seams run horizontally. This is so that when it is used as a backdrop, the seam blends into the horizon that our eyes place in our minds. Muslin is also used as a canvas because it takes paint very well. Flats are often covered with muslin, then sized (made to shrink with the use of water and glue or paint) so that it is stretched tight. If you do not tack down muslin before you paint it, it will shrink unevenly and could cause distortion in your image.