A crane is an important device used for hoisting heavy loads, a familiar sight in warehouses, at docks, on building sites and elsewhere. It has a winch or winding mechanism operating a rope (usually steel) attached to a pulley block on which there is a hook. In many cranes the rope passes over a pulley at the far end of a boom or jib from the winch; in others it hangs from a horizontal beam or girder. A derrick is a crane where the distance of the end of the jib from the crane's vertical support (e.g., pillar) may be varied: simplest is the ship's derrick where the jib is pivoted near the base of a braced or guyed mast. The tower derrick is similar, though here the pivoted end of the jib may be moved up and down the pillar as needed. Mobile cranes move on rails or caterpillar tracks: in particular, the gantry crane is shaped like a bridge, the two verticals having wheels and the pulley block being suspended from the horizontal. In traveling bridge cranes the horizontal runs on wheels on tracks mounted high on facing walls of, say, a workshop. Cranes may also be mounted on trucks, railroad freight cars, or ships.