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 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.