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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.
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 immediately under which bed occupants lie. 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. A duvet cover consists of two rectangles sewn together on all but one side to create a sleeve for the duvet, which can be taken off and washed. When no other blankets are lain atop the covered duvet, it can provide several advantages over a top sheet. First, respecting sanitation, with a top sheet the bed occupant could accidentally in the night remove the barrier between himself and the duvet (which is less washable); with a duvet cover this is impossible. Second, for households with children, a bed with a covered duvet is easier to make: the child need only tug the corners of the duvet back towards the corners of the bed. Tucking in a top sheet before arranging the duvet is an extra step. Third, switching a duvet cover is a cheaper way to change color schemes than buying a new comforter.
However, a top sheet has its own advantages. First, a tucked-in top sheet can provide a comfortable feeling of snugness that duvets cannot, as duvets are not usually large enough to tuck under the mattress. Second, top sheets allow a wider range of blanket choices besides duvets, such as quilts.
In some Asian countries, such as China, top sheets are not used. Flat sheets are often used in place of fitted sheets or together with them as bottom sheets.[clarification needed] Most families prefer to use duvet covers to cover the quilts.
The term bed sheet was first used in the 15th century. Bed sheets were traditionally white and made of linen, cotton or silk, but now various colors and patterns are used.
Bed sheets come in two main varieties: flat and 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.
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. When placing a flat sheet on a bed, the manufacturer has designed the printed side to be softer, and thus it should be placed on the bed printed side "down". When folding back the covers, this also allows the printed side to show, for aesthetic purposes. When one makes a bed, the patterned or monogrammed side of the top sheet is placed facing down and then the top edge is folded towards the foot of the bed, exposing the design.
In the US and Canada, sheets are often sold in a four-piece set consisting of a fitted sheet, a flat sheet and two pillowcases. In China, a four-piece set consists of a duvet cover, two pillowcases and either a fitted or flat sheet.
Common materials used to create bed sheets include cotton, linen, satin, silk, rayon, bamboo fibre, polypropylene spunbond, and blends of cotton with polyester. Some new materials, such as nonwoven polypropylene fabric, allow the sheet to be disposable due to their low price. Disposable sheets are typically used in emergency shelters or hospitals, and sometimes in hotels.
The quality of woven 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. Yarn quality also plays a part in the look and feel of sheets, as finer yarns tend to create a finer sheet fabric. The ply also plays a role in how heavy the sheet feels. Ply represents how many fibers are twisted together as the sheet is being created. A 2 ply 300 thread count sheet will feel heavier than a single ply 600 thread count sheet. "The heavier ply generally means a more durable sheet as well."
The most common weaving styles are percale, a plain weave in which the warp and weft cross each other one at a time, and sateen, which has multiple threads (usually three or four) over, and one under.