These are rollers to mount horizontally on both sides of a gate to help stabilize it from tipping over. They are sold individually but a pair would be required for true stabilization. They are made of rubberized UV protected hard plastic with seal bearing and a zinc plated bracket. Designed to stand up to the rigors of even the most extreme climates, these rollers will last for years and are maintenance free due to the internal sealed ball bearings. You can affix them to posts next to your gate via welding or bolting.