A fitted sheet is a sheet with elastics at the four corners that help keep it in place on a mattress. A fitted sheet may also be called a bottom sheet. A flat sheet is simply a large rectangle of fabric, usually with finished edges. It goes on top of the bottom or fitted sheet and provides warmth and extra cushioning. Some people prefer to sleep only with a flat sheet, while others find this too chilly. In warm climates, many people forgo using any sheets at all!