Elastane, also known by the brand name spandex in some regions, is a synthetic fiber known for its ability to stretch significantly and then return to its original length. In denim, it is blended with cotton yarn in small amounts, typically around 1 to 3 percent of the fabric by weight, which is enough to add noticeable give without changing the fabric's overall character.
Because only a small share of the fabric is elastane, most stretch jeans still look and largely feel like cotton denim, while gaining flexibility at the knees, hips and waist. The higher the elastane content, generally, the more the fabric stretches and the more it may relax or sag slightly during wear before recovering with a wash.
Checking the fabric composition on a garment label is the most direct way to know how much elastane a pair of jeans contains, since this is not always obvious from touch alone. See our stretch versus rigid guide and the entry on stretch denim for more on how this affects fit and comfort.
