What makes Darn Tough socks so tough? All of our socks are knit on small needle, fine gauge knitting machines. This approach produces durable, high density stitching without bulk. Less bulk means a better fit. The better the sock fits, the longer it will last. The wool we use in our wool socks is fine-gauge, low micron 100% merino wool (the good stuff). We reinforce the outside and inside of the sock with nylon and lycra for added comfort, durability, and fit. Throw in 3 generations of knitting know-how, and you get one darn tough sock.