UGG uses only the highest-grade sheepskin available. Twin-face sheepskin is used in many of our core products. A piece of twin-face sheepskin has been treated on both the fleece side, and the skin side, providing the soft comfort UGG is famous for. Grade-A sheepskin is extremely dense and soft, which provides a more comfortable and durable material. Lesser quality material and synthetics can be coarse, scratchy and non-breathable. Grade-A sheepskin breathes naturally, wicking away moisture, and allowing air to circulate freely. Fleece breathes, wicks away moisture and allows air to circulate keeping feet dry. Sheepskin is naturally thermostatic thus keeping bare feet close to your natural body temperature regardless of the temperature outside.