It’s a fabric covering cotton batting of a futon. Also called “futon-gawa.” The cotton, synthetic fabric such as polyester and silk are usually used to make a futon. About a wabuton, the cotton is often used. The same 100% pure cotton can be turned into the various kinds of fabrics such as satin (satin-weave), twill (twill-weave), and broad (plain-weave) by using the different weaving methods. The cotton name varies depending on its staple (fiber) length. There are the short-staple cotton (up to 0.82-inch long), the medium-staple cotton (between 0.82 and 1.1-inch long), the long-staple cotton (between 1.1 and 1.38-inch long), and the extra-long staple cotton (not less than 1.38-inch long). The extra-long staple cotton or silk is often used to make a high-grade hane-buton (feather quilt).