Duvet and cover. A duvet is essentially a soft flat bag filled with down, feathers or a synthetic alternative. It is recommended that duvets are inserted into a removable cover, much like a pillow and pillow case. Duvets and duvet covers are designed to be used alone (with no sheets) because the insert can be removed and the outer "bag" can be laundered, but many people use them with sheets anyway. Many people like duvets because they reduce the complexity of making a bed, as it is a single covering instead of the combination of bed sheets, blankets, quilts and other bed covers.