Denim is a type of woven twill fabric, usually made from cotton. It consists of two yarns that are woven together, with the yarn that runs across, known as the weft, threaded over and under the yarn that runs downwards, which is called the warp. This twill weave produces a diagonal ribbing that distinguishes it from cotton duck. Denim is usually yarn-dyed with indigo, meaning the warp yarns are dyed before they’re woven into denim, while the weft yarns are left undyed or bleached. This allows makers to only dye one set of yarn, the warp, which is why denim is blue on the front and white on the back.

All denim is made from cotton fibers using a twill weave, which creates the diagonal ribbing that is iconic to denim material. While denim is ultimately the same in appearance, there are some types of denim that have slight variations. For example, raw denim has a 3x1 twill construction, making it strong and durable, while bull denim is often used for upholstery. Denim fabric is also available in many colors, with blue denim made using indigo dying, while other shades such as black, pink, grey, and green are created using sulfur dying processes.

In summary, denim is made of cotton using a twill weave, which creates the diagonal ribbing that is characteristic of the material. The warp yarns are dyed with indigo, while the weft yarns are left undyed or bleached, resulting in denim being blue on the front and white on the back.