Pixel Underwear Girls

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.
As dancing became a favourite pastime of young flappers, the garter belt was invented to keep stockings from falling. The increased sexual awareness of the flapper also made underwear sexier than ever before. It was the flappers who ushered in the era of lingerie.