By fusing together low-micron merino wool and polypro fibers, you get an incredibly strong yarn that allows a middleweight to act like a heavyweight in both warm and cold climates. They're strong, wicking, odor and blister resistant, breathable, quick-drying... and they immediately recover in the wash. Made in the USA. Men's shoe sizes- Small(4-5), Medium(5-8), Large(9-12), X-Large(13-15) Women's shoe sizes- Small(4-7), Medium(8-11)