"D" refers to the fineness unit of sock fiber. The smaller the D value, the thinner and more transparent the socks are, while the thicker the socks are, the higher the D value is. Every 9000 meters, the weight of optical fiber is as many grams as that of D, but there is basically no difference between 15D and 10D with naked eyes. They are all ultra-thin stockings. Only by weighing them with a very accurate scale can we distinguish the subtle differences in weight.