When ultimate warmth and comfort are what you need, pull on the PhD(R) Heavy OTC. Smartwool has updated this classic style with Indestructawool(TM) technology for ultimate durability and the 4 Degree(TM) elite fit system to keep the sock in place. Heavy cushioning provides warmth for even the coldest days.FeaturesPhD: Built for performance in the highest degree4 Degree Elite Fit System: For a dialed, performance-oriented fit with flex zone at ankle jointIndestructawool: Indestructawool technology features a patent-pending construction method for exceptional durability and comfortMesh Venting: Mesh Ventilation zones provide optimum moisture management and maximum breathilityHeavy Cushion: Heavy impact absorption and warmth for extended excursions