The down in down comforters is the fine, fluffy "undercoat" taken from the breast of a goose or duck. The older the goose or duck is, the better-developed and thus better quality the down is.
Most down comforters are a mix of down and fine feathers. The down itself is such an effective insulator because it traps a lot of air in its three-dimensional clusters--that trapped air is what actually does the insulating.
The feathers are much less effective insulators, but they're cheaper, thus the mixing--especially in less expensive down products.
Down is typically either hand plucked from a dead goose or removed via a plucking machine, after the animal has been dipped in scalding water to loosen the feathers. A notable exception is eider down, which is gathered from the nests of the rare eider duck in Europe, after the down is naturally shed.
The rarity of this down, and the amount of work that goes into gathering it, makes eider down comforters very rare and expensive.
Once you have some down, you can make the most rudimentary of down [comforters](https://society6com/comforters?utm_source=SFGHG&utm_medium=referral&utm_campaign=2232) by simply stitching two sheets together and stuffing the space between them with down, then sewing them shut. But because down is so light it tends to shift around and bunch up in one corner or another of such a simple comforter, leaving cold spots where the down layer is scantily distributed.
Sewn-through stitching prevents this: When the comforter is sewn closed, full of evenly distributed down, the seamstress pins seams that pass the length and width of the comforter, usually in a diamond or square pattern, then stitches all the way through the top and bottom layers of the comforter. This traps down within individual squares or diamonds between seams and keeps it from shifting around, allowing even insulation.
Baffles and Baffle Boxes
Air can penetrate through the tiny holes made by sewn-through stitching, and there's not room for down to fully fluff within the tight confines of a sewn-through comforter. The very highest quality in down comforters will be constructed with baffles, strips of fabric that are perpendicular to the top and bottom sides of the down comforter.
These baffles help keep the top and bottom sides of the comforter apart, leaving the down room to fluff up and trap more air, thus achieving optimal insulation. Baffles also keep the down from shifting around, just like sewn-through stitching.
Look for fully closed baffle boxes--literally boxes within the comforter's structure, defined by a set of intersecting baffles--which means the boxes are completely closed in so that down won't shift at all, as opposed to partially open baffles or baffle boxes which might leave room for some shifting of down.