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Skis are runners that are designed to glide over snow. Skiers employ them in pairs, attached to ski boots with ski bindings, with either a free, lockable, or partially secured heel. For climbing slopes, ski skins can be affixed at the base of each ski to prevent them from sliding backwards. Originally used as a means of travel over snow, skis have become specialized for recreational and competitive alpine and cross-country skiing. Skis may also be attached to vehicles, such as aircraft or snowmobiles.