Two kinds of hair, built for different jobs
Terminal hair is the coarse, pigmented hair most people readily notice: it forms scalp hair, eyebrows, eyelashes, and—after puberty, in many people—beards and other body hair. Its size and color make it useful as a physical barrier. Eyebrows and eyelashes help keep sweat, dust, and debris away from the eyes; scalp hair can shield skin from sunlight and reduce heat loss.
Vellus hair is shorter, finer, and usually lightly colored. It covers much of the body, including areas where it is hard to see. Because it lies close to the skin, it does not provide the same coverage as a thick terminal hair. But its follicles can still register movement: a slight brush can bend a hair and stimulate nearby nerve endings, adding to the skin’s ability to sense contact.
How one type can become the other
Hair type depends partly on the follicle—the small structure in the skin that produces each hair—and on signals the follicle receives. During puberty, hormones called androgens can cause certain follicles to produce thicker, darker terminal hairs. The change happens only in responsive follicles, which helps explain why hair becomes coarse in some body regions but remains fine in others. Hormones, genetics, age, and individual variation all influence the pattern.
The distinction is therefore more than a difference in appearance, but it is not an absolute division of labor: both kinds grow from follicles, and both can contribute to touch sensing. What changes most is scale. A tiny vellus hair and a sturdy terminal hair are made through the same basic process, but their follicles produce very different kinds of coverage.
