PhD Outdoor Medium features ReliaWool Technology for improved durability in the high wear areas and the 4-Degree Elite Fit System, keep the sock in place and gives you all-sport versatility. 4 Degree Elite Fit System uses two elastics for greater stretch and recovery to keep the sock in place Patented ReliaWool Technology in high impact areas provide longer lasting protection to keep feet comfortable Strategically placed mesh ventilation zones provide ventilation for temperature and moisture management 8.5" overall height 72% Merino Wool, 26% Nylon, 2% Elastane