A bed sheet is a rectangular piece of cloth used either singly or in a pair as bedding, being placed immediately below or above bed occupants. A top sheet, in the many countries where they are used, is a flat sheet, immediately under which bed occupants lie. A bottom sheet is laid above the mattress and bed occupants lie on it. A bottom sheet may be either a flat sheet, or a fitted sheet, which has been sewn to form a large pocket around the top, sides and bottom edge of a mattress, with elastic or a drawstring in the hem to stop it slipping off the bottom of the mattress. Blankets, quilts, duvets, comforters and other bed covers are laid over the top sheet.