One of the oldest natural materials in the world, down feather is now favored as a premium filling for keeping the body warm throughout the winter. Down filled products are extremely resilient and insulate very well. In duvets, down is the best insulator per ounce over any other fill, natural or manmade. For pillows, down provides incredible re-lofting capabilities, yet is soft and comfortable.
The best way to determine which duvet size will fit your mattress best is to measure your mattress and compare to the above sizing to ensure you'll have the desired coverage and overhang.