Warp-faced twill places dyed warp yarns over lighter weft yarns to create denim’s blue outer surface. This structure is a defining feature of traditional denim fabric.
In weaving, warp yarns run lengthwise and weft yarns pass across the width. A twill weave creates diagonal lines by allowing one set of yarns to pass over and under the other in a staggered pattern. In denim, the warp is commonly indigo-dyed, while the weft is often white or natural cotton.
Because the coloured warp dominates the visible face, the fabric looks blue from the outside. The lighter weft can be seen more clearly on the inside or where wear abrades the surface. This contrast contributes to denim’s distinctive fading patterns.
Denim is often described simply as “blue cotton,” but colour alone does not define it. Its twill structure and yarn arrangement are equally important to the classic fabric.