Twill
A weave with diagonal lines, used for chinos, denim and workwear.
In a twill weave, the weft passes over one or more warp yarns and under two or more, creating a diagonal pattern. The floats make the fabric stronger and denser than a plain weave of the same weight.
Common forms are 2/1 and 3/1 twill. Denim is a twill, which is why it has a diagonal face and a lighter back.