Skis are runners, attached to the user's feet, designed to glide over snow. Typically employed in pairs, skis are attached to ski boots with ski bindings, with either a free, lockable, or partially secured heel. For climbing slopes, ski skins can be affixed to 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.