Young Blonde Girl Strips Her White Underwear

By the end of the decade, trouser-like "bloomers", which were popularized by Amelia Jenks Bloomer (1818–1894) but invented by Elizabeth Smith Miller, gained popularity with the so-called Gibson Girls who enjoyed pursuits such as cycling and tennis. This new female athleticism helped push the corset out of style. The other major factor in the corset's demise was the fact that metal was globally in short supply during the First World War. Steel-laced corsets were dropped in favour of the brassiere.
Meanwhile, World War I soldiers were issued button-front shorts as underwear. The buttons attached to a separate piece of cloth, or "yoke", sewn to the front of the garment, and tightness of fit was adjusted by means of ties on the sides. This design proved so popular that it began to supplant the union suit in popularity by the end of the war. Rayon garments also became widely available in the post-war period.
In the 1920s, manufacturers shifted emphasis from durability to comfort. Union suit advertisements raved about patented new designs that reduced the number of buttons and increased accessibility. Most of these experimental designs had to do with new ways to hold closed the crotch flap common on most union suits and drawers. A new woven cotton fabric called nainsook gained popularity in the 1920s for its durability. Retailers also began selling preshrunk undergarments.
Women's bloomers became much shorter and stockings covered the legs instead. The shorter bloomers became looser and less supportive as the boyish flapper look came into fashion. By the end of the decade, they came to be known as "step-ins", very much like modern panties but with wider legs. They were worn for the increased flexibility they afforded.