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.

 Fitted medical bed sheets are also an important part of a hospital sheet set because fitted medical bed sheets are designed to fit securely to the mattress and stay in place, even when the bed is adjusted or moved.  This is especially important for electric beds, as the fitted medical bed sheets need to accommodate the movement and positioning of the bed without shifting.