In the US a duvet isn't always used. During summertime, a flat sheet on top of the fitted sheet may be just enough to keep you snug. If you do use a duvet, the way to dress is it generally layered over the top of the flat sheet, and let it hang over the sides for an effortless look. An oversized duvet will also help to conceal any unattractive side of your box spring or bed frame or any underbed storage.