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.
The term bed sheet was first used in the 15th century.
Bed sheets come in two main varieties — flat or fitted. A flat sheet is simply a rectangular sheet of cloth, while a fitted sheet has its four corners, and sometimes two or four sides, fitted with elastic, to be used only as a bottom sheet. The fitted sheet may also be secured using a drawstring instead of elastic. The purpose of a fitted bottom sheet is to keep it from slipping off the mattress while the bed is in use. A particular way of folding and tucking while making the bed, known as "hospital corners," is sometimes used when the bottom sheet is flat rather than fitted.
Bed sheets were traditionally white, but now various colors and patterns are used. The quality of bed sheets is often conveyed by the thread count - the number of threads per square inch of material. In general, the higher the thread count, the softer the sheet, but the weave and type of thread may affect the "hand" of the material so that a sheet with a lower thread count may actually be softer than one with a higher count.
New materials such as nonwoven polypropylene fabric allow the bed sheet to be disposable thanks to their low price. Once used in emergency shelters or hospitals, this disposable bed sheet is now used in hotels as well.
Usually a flat bed sheet is overlocked around the edges to form four seams. One of the seams is wider than the other three and helps with orienting the sheet correctly on the mattress. The wider seam goes at the head end of the mattress. Sometimes the sides do not have seams, but are finished with the selvedge only.
One may find that there are certain terms marketed towards the prospective buyer of said product that involves thread-counts or the origin of the materials. It is recommended that optimizing the use of bedding should remain as a subjective variable that is dominated by their own comfort.
See also 
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