A corset is a support undergarment worn to hold and train 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 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 were commonly known as "stays" and had a more conical shape. This later evolved into the curvaceous 19th century form which is commonly associated with the corset today. By the beginning of the 20th century, shifting gender roles and the onsets of World War I and II led the corset to be largely discarded by mainstream fashion.