A bed sheet is a rectangular piece of cloth used either singly or in a pair as bedding, which is larger in length and width than a mattress, and which is placed immediately above a mattress or bed, but below blankets and other bedding (such as comforters and bedspreads). A bottom sheet is laid above the mattress, and may be either a flat sheet or a fitted sheet (a sheet which has been sewn with elastic or a drawstring along the hem of the top, sides and bottom of the sheet to prevent it from slipping off of the mattress). A top sheet, in the many countries where they are used, is a flat sheet, which is placed above a bottom sheet and below other bedding.