Twill is a weave technique that gives the fabric a characteristic pattern of diagonal lines. Twill weave is not limited to a certain type of material and can be applied to cotton, silk, linen, wool, or any combination of these materials.
All twill fabrics consist of warp threads and weft threads. The warp threads run along the length of the fabric and the weft across the width. The way in which these threads are crossed determines the strength and look of the woven fabric. Thread quality and width also influence the fabric’s flexibility and sustainability.
Denim is often specified as 3×1 twill, which refers to the number of weft threads per warp thread. Denim fabric is traditionally woven using 3×1 twill, as opposed to a more lightweight denim (under 10.5 oz.) with 2×1 twill. With a 3×1 fabric, the weft thread is woven three times over the warp thread, one time under, then again three times over the warp thread, and one time under, and so on.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary