Young Black Haired Girl In The Bed With Yellow Bed-clothes

A bed sheet is a piece of cloth used to cover a mattress. It is this sheet that one typically lies on.
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.