An illustration of an iron derrick, a lifting device composed of one mast or pole which is hinged freely at the bottom. It is controlled by lines (usually four of them) powered by some means such as man-hauling or motors, so that the pole can move in all four directions. A line runs up it and over its top with a hook on the end, like with a crane. It is commonly used in docks and onboard ships. Some large derricks are mounted on dedicated vessels, and are often known as "floating derricks".