D in socks 200D is the abbreviation of Denier, which is the fineness expression of chemical fiber. It refers to the weight grams of 9000-meter-long silk at a given moisture regain, also known as denier. The bigger the D, the thicker the yarn.

Denier is the fixed-length density unit of fine silk, man-made fiber and nylon textile fiber, which refers to the grams of mass of 9000m long fiber at a fixed moisture regain, and the unit is denier. Denier is a measure of fiber fineness. 9000m long fiber weighs 600g, which is called 600D fiber. 9000m long yarn weighs 600g, which is called 600D yarn. The larger the number, the thicker the yarn or fiber.

It is a non-statutory unit in China, but it is also commonly used in the fields of chemical fiber and silk in foreign trade.

Its calculation formula is: TD = 9000× GK/L; Where: TD- denier of fiber (denier); L- length of fiber (m); GK—— the mass (g) of fiber at a certain moisture regain.