PolyPro is a combination of Spandex (10%) and Polypropylene (90%) creating the ultimate layering material. The brushed ´fleece like´ inside surface provides good thermal qualities and at the same stays remarkably dry to the touch since the Polypropylene material repels water. Wear these socks in conjunction with your existing footware to give yourself a real heat boost! We love wearing these socks underneath our Hot Socks which combine to create a fantastic heat barrier without adding bulk to the foot. Rooster PolyPro Leggings and PolyPro Top also available for the ultimate thermal base layer.