The Lightning CX Gray Socks has a closed structure and is thicker than the prolen sock. The addition of cotton makes the material somewhat rougher and much warmer. This sock is ideal for temperatures between 5 and 15 degrees. This model sits in between our merino wool socks and the prolen socks. The Lightning CX Gray Socks are made of high quality polypropylene (92%), cotton (5%) and elastane (3%).