Flute Board is an extremely versatile, extruded twin-wall plastic sheet. Being durable and versatile, it is a great substitute for poster board or card stock for interior applications. Its surface is not affected by most oils, solvents or water, so it may also be used outdoors. Corflute is aligned with flutes that generally run horizontally across the width of the sign. Note: In the sizes below the first measurement is the flute direction.