As others have said, there is an interior elastic band that is located near the ankle that is so small that After wearing them for less than an hour, I had to take them off and cut through the lining to cut the band.

I bought the Extra Large, as most reviewers said that they run small. I wear 91/2 or 10 boots, and probably could have been OK in the Large. Even though the bootie was larger than I needed, the elastic was still much too tight. I have to wonder if anyone from Sierra Designs have ever tried wearing them.