A bedsheet is a rectangular piece of cloth that is placed on top of a mattress. You may get them in several materials, ranging from cotton to linen to silk and more. Bedsheets are available in two styles: fitted and flat. Each serves a particular purpose on the mattress. A fitted sheet has elastic woven into the seam throughout the whole sheet. It is meant to fit down over the corners of the mattress and tuck underneath it for a snug fit. A fitted sheet helps prevent the sheet from slipping off the mattress while the bed is in use. A flat sheet is then placed immediately on top of the mattress. On most beds, it is tucked under the foot of the bed. In most cases, fitted sheets serve as the top sheets, while flat sheets serve as the bottom sheets.