Down is the soft undercoat of the bird, rather than feathers. Down in down comforters is warm, lightweight, natural, and packable, retaining the loft of the down comforter or duvet for an extended time. The fill power of a comforter is the amount of space an ounce of down occupies, with higher ratings providing more insulation and loft. Down usually comes from fowl such as geese, as in the goose down comforter. A down alternative comforter is filled with synthetic fiber such as rayon or polyester which is resistant to allergens. This type of comforter is heavier than a traditional down comforter and is resistant to moisture, although a down comforter is quite breathable. Down alternative comforters keep their shape well, similarly to down comforters. A comforter with gel fiber can be comparable to down in warmth. Synthetic fibers in bedding such as Comforel have been designed to mimic the lofting qualities of down. PrimaLoft is a synthetic microfiber which has comparable insulation, compression, and density to down but with superior wetting and drying characteristics and loft retention when wet. It allows the comforter to retain heat in the presence of moisture, making it a durable bedding material. A comforter with synthetic fill can be machine washed, whereas a down comforter needs to be dry cleaned. Alternatively, use a duvet cover with a down comforter to avoid having to clean comforter often. The face of a comforter can be a variety of materials. While cotton is common, nylon is 35% lighter than cotton, and microfiber makes for a soft feel. Cotton sateen has softness and sheen, giving a smooth feel to the comforter. Egyptian cotton has an extra long staple length and is used in luxury bedding and comforter sets. Silk is also used to provide ultimate softness to the comforter. A silk comforter is lightweight yet thick, breathable, and lacks bulkiness. A comforter cover of a higher thread count is softer and insulates and protects the filling better. A stitching technique called baffling, or baffle box construction, keeps the down or synthetic fiber of the comforter in place.