Down and feathers are collected from geese and ducks, usually birds killed for meat. Goose down is preferred for bedding because it carries little or no smell.

Raw down is washed and rinsed over and over to strip out dust and contaminants. Cleaner down lofts higher and is less likely to set off allergies.

Wet down goes into dryers that steam it quickly and thoroughly. The clusters come out open and lofty, and some makers add a sanitizing chamber heated to 200 °F.

A separator blows the fill upward through a series of chambers. Light down clusters float farther, heavier feathers drop out sooner, and each chamber collects one grade.

Technicians put one ounce of down in a graduated cylinder, drop a small weight on top, and read how many cubic inches it holds up. Higher numbers mean bigger clusters and more warmth for the weight.

Cotton yarn is woven tight enough that down cannot work its way through. Guides call for at least 180 threads per square inch, and 250 or more is better.

Thin strips of fabric are sewn between the top and bottom sheets, standing up like walls to form three-dimensional pockets. Cheaper sewn-through designs stitch the two layers straight together, which leaves cold seams.

A filling pipe blows an equal weight of down into each pocket through a small opening, which is then sewn closed. Sealed pockets keep the fill from shifting, and the finished duvet is checked again.