Young Brunette Girl Posing On The Bed With A White Fitted Sheet

In many areas of the world, a second flat bed sheet is laid on top of the sheet covering the mattress. This is known as a "top sheet" and when a top sheet is used, the sheet covering the mattress is known as a "bottom sheet". One sleeps between the two bed sheets. Blankets, comforters, and other bed covers are then placed on top of the second bed sheet.
Varieties
Bed sheets come in two main varieties--flat or fitted. A flat sheet is simply a rectangular sheet of cloth, while a fitted sheet has its four corners, and sometimes two or four sides, fitted with elastic, to be used only as a bottom sheet. The fitted sheet may also be secured using a drawstring instead of elastic. The purpose of a fitted bottom sheet is to keep it from slipping off the mattress while the bed is in use. A particular way of folding and tucking while making the bed, known as "hospital corners," is sometimes used when the bottom sheet is flat rather than fitted.