Hoop skirts are a type of women's undergarment, related to the crinoline, which enjoyed a brief period of high fashion in the mid 19th century. Both were later replaced by the bustle, as women's fashion started to give way to more practical garments. The hoop skirt was worn on formal occasions by fashionable upper class women, and would not have appeared on workers and lower class women. Navigating in a hoop skirt is challenging, as any historical re-enactor will inform you, and wearing one required some training.
A hoop skirt is made from a wide bell of sturdy material with casings sewn into it at multiple points along the length of the skirt. The casings hold stiffening materials such as whalebone, metal, rope, and twisted cloth. This causes the hoop skirt to project stiffly out from the wearer's body. When dresses are worn over hoop skirts, they take on the bell shape of the hoop skirts. This gives the wearer's lower body the look of a large triangle, and it was at one point very fashionable, although impractical.
For 19th century women, hoop skirts actually represented a huge improvement in their undergarments. Previously, women had to wear multiple layers of heavy petticoats to achieve the full bell shape which hoop skirts created. With hoop skirts, a woman only needed two petticoats, the hoop skirt itself, and the dress. The first petticoat was slim, rather than full, and worn under the hoop skirt for modesty. The second petticoat was worn over the hoop skirt to smooth its lines, so that the dress did not appear bunched or lumpy. All told, a woman would still be wearing a substantial amount of fabric, thanks to the immense flowing skirts which were so popular among the upper classes.
In the United States, hoop skirts are most frequently associated with Civil War fashion, and they often appear at history balls and events commemorating this period in American history. The full skirts actually represented serious hazards to the wearer. It would been difficult for women in hoop skirts to sit without exposing themselves, and the skirts also needed to be lifted to avoid puddles and filth on the roads. More importantly, a hoop skirt could get caught in a doorway or piece of machinery, or could catch fire. Nurses were specifically prohibited hoop skirts, as the skirts would have made it impossible for them to attend to patients. Many women welcomed the slimmer style of the bustle with pleasure.