A duvet (UK: , US: ; from French duvet [dyvɛ] 'down'), usually called a comforter or (down-filled) quilt in U.S. English, and a doona in Australian English, is a type of bedding consisting of a soft flat bag filled with either down, feathers, wool, cotton, silk, or a synthetic alternative, and is typically protected with a removable cover, analogous to a pillow and pillow case. The term duvet is mainly British, especially in reference to the bedding; when rarely used in US English, it often refers to the cover. Sleepers often use a duvet without a top bed sheet, as the duvet cover can readily be removed and laundered as often as the bottom sheet. Duvets originated in rural Europe and were filled with the down feathers of ducks or geese. The best quality feathers are taken from the eider duck, which is known for its effectiveness as a thermal insulator.