Vellus-like hairs are < 0.03 mm in diameter and rarely grow > 1-2 mm.   Terminal hairs are coarse > 0.06 mm in diameter and can grow up to 3 feet.   A true vellus hair does not have an attached arrector pili muscle.  Only miniaturized vellus-like hairs of androgenetic alopecia have arrector pili muscle.