A sheet may be a rectangular piece of fabric used either singly or during a pair as bedding being placed immediately below or above bed occupants. A top sheet, within the many countries where they're used, may be a flat sheet, immediately under which bed occupants lie. A bottom sheet is laid above the mattress and bed occupants lie thereon . A bottom sheet could also be either a flat sheet, or a contour sheet , which has been sewn to make an outsized pocket round the top, sides and bottom fringe of a mattress, with elastic or a drawstring within the hem to prevent it slipping off rock bottom of the mattress. Blankets, quilts, duvets, comforters and other bed covers are laid over the highest sheet.