Denier (DEN) is a unit of weight used to express the fineness of yarn or fibre, and thus to describe the thickness of hosiery. By comparison, 1 DEN is approximately the diameter of a sixth of a hair. To be more accurate, 1 gram of one denier yarn is equal to the weight in grams of 9000 metres of the yarn, which means that less than 4.5 kg of the material would be enough to circle the Earth along the equator. The finer the fibre, the more transparent the tights, and vice versa. While 10–20 DEN tights are barely visible, 30–40 DEN ones are semi opaque and over 50 DEN ones completely opaque. The thickness and weight of hosiery are important factors defining their style, comfort, look and warmth.