Although most people nowadays think of aviation when they hear the word "pilot", the term was originally used to refer to local seamen whose task was to guide ships through coastal waters and into a port. In fact, the English word "pilot" evolved from the Dutch term for a plumb lead, a length of rope with a weight on the end of it used for measuring the depth of water. Today, nearly every port employs pilots licensed by the government of the country, and the use of their services is compulsory, except for local ships whose master holds a current pilotage exemption for a specific port. Pilots are responsible for the safe conduct and navigation of any ship from the time they assume control from the master after boarding at sea until the vessel is safely moored at the allocated wharf, or vice versa in the case of ships that are leaving a harbour.