The Smartwool PhD Outdoor Light Micro Sock is a light cushioned sock for running. Trails, urban, on the track, whatever tickles your fancy. The 4 Degree Elite Fit System dials in the Fit while mesh ventilation zones help the foot breathe. Made in the USA with imported yarn, featuring merino wool, nylon and elastane for a temperature regulation, durability and stretchy comfort. Features of the Smartwool PhD Outdoor Light Micro Sock Indestructawool Technology Features a wool-based, patent-pending yarn and construction method for exceptional durability and comfort 4 Degree elite Fit system Uses two elastics for greater stretch and recovery to keep the sock in place Mesh zones designed for High-output activities or warmer temperatures provide ventilation Virtually Seamless toe Made in USA