A horseshoe is a "U"-shape item made of metal or of modern synthetic materials, nailed or glued to the hooves of horses and some other draught animals. Like a shoe on a human, it is used to protect the animal's feet from wear and tear. Professional horseshoers, called farriers, usually place horseshoes on the solar (outer) surface of the hoof by nailing through the insensitive hoof wall, which is analogous anatomically to the human toenail, though much larger and thicker. Some horseshoes have "calkins", "caulks", or "calks", protrusions at the toe and/or heels of the shoe, to provide additional traction. Horseshoes are available in a wide variety of materials and styles, developed for different types of horses and 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 where a strong, long-wearing shoe is needed, such as polo, eventing, show jumping, and western riding events. Aluminum shoes are lighter, making them common in horse racing, where a lighter shoe is desired; and often facilitate certain types of desired movement, and so are favored in the discipline of dressage.