Denim is a woven cotton fabric built on a twill weave, which gives it the diagonal rib you can see and feel on the fabric surface. In classic denim, the warp yarns running lengthwise are dyed with indigo, while the weft yarns running crosswise stay undyed. That is why denim looks blue on the outside of a garment and pale on the inside.
When you are choosing jeans, denim is usually described by three things: its weight in ounces per square yard, its weave direction, and whether it has been washed before sale. Lighter denim, around 10 to 12 oz, feels soft and flexible from the start, while heavier denim above 14 oz is stiffer at first and takes longer to soften with wear. Knowing this helps you judge how a pair will feel once it has been worn a few times, not just on the first try.
Denim also varies in how it is finished. Raw denim has not been washed after dyeing and will fade in a way that follows your own wear pattern, while pre-washed denim starts softer and closer to its final look. For a full walk through the different fabric options and what suits different jeans, see our denim guide.
