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A horn is a sound-making device installed on motor vehicles, trains, boats, and other types of vehicles. The sound it makes usually resembles a “honk” (older vehicles) or a “beep” (modern vehicles). The driver uses the horn to warn others of the vehicle's presence or approach, or to call attention to some hazard. Motor vehicles, ships and trains are required by law in some countries to have horns. Trams, trollies, streetcars, and even bicycles are also legally required to have an audible warning device in many areas.