For those who want the lightest running sock but still want a super-fine merino wool, the ultra-light fits the bill. Nothing but protection between you and your running shoes. Details: Patented Full Contact Fit hugs the contours of your foot and stays put so there are no loose socks to rub you the wrong way, will not fall down into your shoe and no show height, flat knit design for zero bulk, fully vented top for maximum breathability and no cushioning in the footbed. Size guide: S (3.5-5.5 men's, 5-7 women's), M (6-8 men's, 7.5-9.5 women's), L (8.5-10 men's, 10-11.5 women's), XL (8.5-10 men's, 10-11.5 women's), 2XL (13-15.5 men's, 14.5-16.5 women's). Material: Merino Wool, Nylon, Polyester, Lycra.