Shackles are U-shaped pieces of metal secured with a pin or bolt across the opening, or hinged metal loops that are secured with a quick-release locking mechanism. There are three basic types of shackles. All meet or exceed U.S. Federal Specification RRC-271D, "Chains and Attachments: Welded and Weldless", and are made of alloy steel or carbon steel. They are available in painted, galvanized or self-colored finishes and use quenched or tempered pins. Class 1 shackles pass a rounded pin through holes at the base of the shackle. A spring-actuated cotter pin or hair pin holds the butt end of the shackle in place. Class 2 shackles use screw-pins that pass through an opening on one only side of the shackle. The pins use a key-like design and exhibit extreme deformation before failure. Class 3 shackles have safety anchors or chains that are held in place with a nut and bolt on opposite ends of the pin. Shackles that meet Amendment 1 to Federal Specification RRC-271D are also available.