D=Thickness

The smaller the "D" number, the thinner and more transparent the stockings will be. More durable socks generally have a higher "D" number. Generally, 40-60D are spring and autumn socks, and 60D and above are winter socks. The DEN number of summer stockings is generally below 20D, which is cool, light, and transparent. Nowadays, ultra-thin stockings even reach 5D.