A comforter is a type of blanket filled with natural or synthetic fiber and encased in a shell. Comforters are relatively light weight and can offer comfort in hot or cold climates. A duvet is a type of comforter which is thicker and used without additional sheets. Duvets are soft flat bags filled with a variety of materials such as down, wool, or synthetic fibers. A removable duvet cover is used with a duvet for protection, style, and added warmth. The duvet cover can then be laundered when necessary. Duvets can vary in thermal insulation, which is measured by togs. A duvet for summer can be 4.5 tog and a winter duvet can be 9.5 tog. A down comforter, like a duvet, can be used with a duvet cover. Doing so allows the comforter or duvet to be washed annually or less. Comforter price is determined by the quality of the fill rather than its weight. The fill power of a comforter is the amount of space an ounce of down occupies, with higher ratings providing more insulation and loft. The thread count of a comforter is the number of threads per square inch of fabric, and higher thread counts seal the comforter while adding softness and durability. Baffle box construction is the lining of the inside of comforters with vertical strips in order to keep the filling from moving or bunching up. A down alternative comforter is filled with hypoallergenic, synthetic fiber which differentiates it from a traditional down comforter, which is filled with down or feathers. Comforter sets come with pillows and a mattress topper or pillow shams. Comforters can come in microfiber, cotton, sateen cotton, Egyptian cotton, nanofiber, silk, nylon, and more and can be filled with gel fiber, polyester, down, goose down, feathers, silk, microfiber, or PrimaLoft. 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.