The quality of the down: it is determined by the size of the tufts. The size is measured in filling power: larger tuft = more volume = better quality = greater lightness What does filling power mean? Filling power means the weight to volume ratio of the down, and is measured in cubic inches. The larger the individual tufts are, fewer are needed to get a certain volume and hence a duvet with a certain caloric coefficient.
The quality of the down