A corset is a support garment worn to constrict the torso into the desired shape and posture. They are traditionally constructed out of fabric with boning made of whalebone or steel, a stiff panel in the front called a busk which holds the torso rigidly upright, and some form of lacing which allows the garment to be tightened. Corsets, also known as stays, were an essential undergarment in European women's fashion from the 17th century to the early 20th century. In the 17th and 18th centuries they had a conical, straight-sided shape. This eventually evolved into the more curvaceous 19th century form. By the beginning of the 20th century, shifting gender roles that allowed women to be more active outside the home, as well as the onsets of World War I and II led the corset to be largely discarded by mainstream fashion.