Horseshoes are available in a wide variety of materials and styles, developed for different types of horse and for the work they do. The most common materials are steel and aluminium, but specialized shoes may include use of rubber, plastic, magnesium, titanium, or copper. Steel tends to be preferred in sports in which a strong, long-wearing shoe is needed, such as polo, eventing, show jumping, and western riding events. Aluminium shoes are lighter, making them common in horse racing, where a lighter shoe is desired; and often facilitate certain types of movement, and so are favored in the discipline of dressage. Some horseshoes have “caulkins”, “caulks”, or “calks”: protrusions at the toe or heels of the shoe, or both, to provide additional traction.