Yak down, also known as yak hair, is a type of natural fiber that is obtained from the undercoat of the Yak (Bos mutus), a large bovine species native to the Mongolian plateau and the Himalayan region of Asia. Yaks are well-adapted to the high altitude and cold temperatures of their native habitat, and as a result, they have a thick, double layer of fur that helps to insulate their bodies and keep them warm. The outer layer of fur is made up of long, coarse hairs, while the inner layer is made up of shorter, softer hairs known as down.

Yak down is prized for its warmth, softness, and durability and it is often used to make clothing, blankets, and other items that are worn or used in cold environments. The fibers have a diameter that ranges between 15-20 microns, which makes them finer than the fibers of other types of wool. Its unique characteristics make it an ideal insulation material since it is able to trap air and retain heat while being lightweight and easy to care for. 

Additionally, yak down fibers has naturally water-resistant and moisture-wicking properties, which make them well-suited for use in cold, wet environments. It is also known for its hypoallergenic properties, as it is less likely to cause allergic reactions than other types of fibers. The fibers come in a range of colors, including shades of chocolate brown, light brown, black, and grey, and the coloring is a natural characteristic, which means that the fibers have not been treated with any chemicals or dyes.

The process of obtaining yak down involves combing the undercoat of the yak during the moulting season, usually from early spring to early summer, when the animals shed their winter coat. The fibers are then cleaned, sorted, and spun into yarn. The yarn can be used to make various products, such as clothing, blankets, and accessories.

In addition to its practical uses, yak down is also valued for its cultural significance in the regions where yaks are found. Yaks have long been an important source of food, transportation, and other resources for the people who live in these areas, and the fiber produced from their down has played a significant role in the traditional crafts and industries of these communities. Yak down is also an important source of income for many communities that live in the remote high-altitude regions where yaks are found, and thus supporting yak down production can contribute to sustainable development in these areas.