A Medium weight polyester which has the look and the weight of cotton muslin but without the stiffness, It doesn't wrinkle like cotton muslin and holds up in high humidity conditions. It comes in a wide variety of colors. It is used in schools or on stages for backdrops most often. It comes 128" Wide, weighs approximately 14.5 oz per yard and is naturally an IFR fabric.
A fun fact is that a 35' long windsock made of cotton muslin was used to simulate a tornado in the Wizard of Oz.