Down is classified by weight and fill power. Fill power is the number of cubic inches that an ounce of down takes up. Fill power is a measurement of efficiency; the higher the fill power number, the better the down and the greater the insulating value. The fill power measurement represents the number of cubic inches filled by an ounce of down. We carry down comforters with a variety of fill powers, from 600 to 900, and thread counts up to 500. Fill power is directly related to the size of the down cluster. Large down clusters occupy more space than smaller down clusters. The higher the fill power, the larger the clusters, and thus, the more air that is trapped between the clusters. The trapped air is what gives down its insulating properties. The size of the bird and the climate in which it lives, determines the size of its down cluster. The colder the climate, the larger the down cluster required to keep the bird warm. For example, it is colder in Canada than it is in Hungary. Therefore, the birds need larger down clusters in Canada to stay warm. With a weight and a loft for everyone, one will be just right for you! To protect your down comforter, we recommend a duvet cover. For machine washable comforters, down wash is a detergent formulated especially for cleaning down.