Isatis tinctoria
Denim was originally dyed with indigo dye extracted from plants, often from the genus "Indigofera". In South Asia, indigo dye was extracted from the dried and fermented leaves of "Indigofera tinctoria"; this is the plant that is now known as "true indigo" or "natural indigo". In Europe, use of "Isatis tinctoria", or woad, can be traced back to the 8th century BC, although it was eventually replaced by "Indigofera tinctoria" as the superior dye product. However, most denim today is dyed with synthetic indigo dye. In all cases, the yarn undergoes a repeated sequence of dipping and oxidation—the more dips, the stronger the color of the indigo.