For the uninitiated, these are "liners" or "liner socks," which are designed to be worn under your main hiking/athletic socks. These are meant to help you prevent blisters of your heel, sole, and toe areas. You can of course use them as standalone socks for casual shoes. That's where these liners or "socks" really shine: versatility.

They are thin, which means they dry fast despite the small percentage of cotton (37% Coolmax Polyester, 34% Cotton, 25% Polyester, 4% Spandex). The fast drying aspect is important for me as a backpacker.

They are feet hugging, which is important for mitigating blisters (don't throw out the duct tape though!). I am a 9-1/2 shoe size and the Large fits snugly but not too tightly. According to their chart: Large (9-12.5), X-Large (13-16), and Medium (4-8.5). So, it looks like their sizing is right on the money.