In multifold areas of the world a second planate bed sheet is laid on Bed Sheets top of the sheet covering the mattress. This is manifest as a "top sheet" and when a first sheet is used, the covering the mattress is known as a "bottom sheet". Blankets, comforters, and other bed covers are then placed on foremost of the second bed sheet.
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.