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Slips are familiar to most women, as thigh to full-length undergarments meant to be worn underneath sheer dresses. In the 1930s, especially as the flapper movement had relaxed rigid rules about wearing restricting corsets, several designers began offering the slip dress. In that time, the slip dress was usually a fitted bust and a narrowly fitting dress with a long skirt, which looked very much like the traditional undergarment. Source