Down is a natural insulator with a high warmth-to-weight ratio. It is produced from a layer of fine feathers found under the tougher exterior feathers of ducks or geese. Down offers excellent thermal properties, has good lofting characteristics and traps small pockets of air efficiently, which creates a thermal barrier. Down insulation is rated by fill power, measured as the number of cubic inches displaced by a given ounce of down (in³/oz). Higher fill-power down will insulate better than lower fill-power down of the same weight.