In America, a comforter is a type of bedding made of two lengths of fabric or covering sewn together and filled with insulative materials for warmth, traditionally down or feathers, wool or cotton batting, silk, or polyester and other down alternative fibers. It is a quilted blanket that is used as a top sheet and forms part of a complete bedding set. Comforters are generally fluffier compared to a duvet. They are usually laid over a top bed sheet and used to cover the body during sleep. Comforters are typically sold in bedding sets or bed-in-a-bag sets with other coordinating pieces such as sheets and pillow shams, all made for the same bed size. They are meant to be the final piece and are not meant to be covered with anything or paired with any additional accessories. Comforters are usually used in the winter season when it is very cold, although a variety of different thicknesses means that they can be used in other seasons as well, with lighter examples being used in warmer weather.