Twill is a way of weaving fabric that creates diagonal lines or ridges across the cloth. If you look closely at a pair of jeans, you'll see tiny parallel lines running at an angle: that's twill weaving. The pattern forms because the threads pass over and under each other in an offset pattern, like climbing stairs, rather than going straight over-under-over-under like a checkerboard.

Twill weaving makes fabric stronger and more durable than plain weaving, which is why it's used for work clothes, jeans, chinos, and military uniforms. The technique also helps fabric resist wrinkles and stains. Denim, one of the world's most popular fabrics, is a type of twill.

The diagonal pattern isn't just practical: it gives twill fabrics their distinctive look and feel. Herringbone and houndstooth patterns are fancy variations of twill. When you run your hand across twill fabric, you can often feel those diagonal ridges, which tell you the fabric was woven to last.