A bed sheet is a rectangular piece of cloth used as bedding, being placed immediately below or above bed occupants. Bed sheets can generally be divided into two categories: "top" and "bottom" sheets.
A bottom sheet is laid above the mattress and bed occupants lie on it. It may be either a flat sheet (extra fabric is usually tucked under the mattress ), or a fitted sheet, which is sewn in a pocket - like shape to go down over the corners of the mattress, and has an elastic band around the edges of the sheet to prevent the sheet from slipping. However, this can make it more difficult to fold when not in use.
Bottom sheets are standard in developed countries. They are more easily washable than a mattress, and when used properly can protect the longevity of the mattress and provide better sanitation for bed occupants.
A top sheet is a flat sheet which bed occupants lie underneath. Blankets, quilts, duvets/comforters and other bed covers are laid over the top sheet, but because of the top sheet they do not directly touch the bed occupant.
Top sheets are standard in the U.S., but much of Europe prefers to use duvet covers.
