A harbor pilot is a person who takes control of a seagoing ship when it is entering or leaving port. He is expertly familiar with all the channels, shoals, currents, tides, and regulations of his particular port, and is essential to safely steer the ships in and out. It is an ancient and exclusive profession, often passed down from father to son*. In old days, the top two signal flags at right were used by ships entering and exiting a harbor to call for a pilot, while the bottom flag indicated "pilot on board."