Fitted sheets have an elastic to wrap around the mattress and prevent it from moving too much while you sleep. While these sheets protect your mattress from body oils, spills, and other things, using a mattress protector underneath your fitted sheet is a good idea for extra protection. Flat sheets lay on top of fitted sheets to protect your duvet or comforter from body oils and tearing. Not everyone uses these, but we recommend doing so. 

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.