The down feathers in a pillow helps trap small air pockets. This provides a thermal barrier. Down may also be used in jackets, sleeping bags and bedding.
Depending on the size and shape of the pillow down can be packed down into small areas allowing for many down feathers to be included in one pillow.
The down feathers are surrounded with a cover or shell, which is normally made of either cloth or silk. A variety of designs may be made on the pillow casing.
When down feathers become wet they lose their thermal properties and become poor insulators, even more so than synthetic-covered pillows. Many people may be allergic to down.
Down pillows tend to cost up to double the price of synthetic-filled pillows.