The bridge of a ship is the room or platform from which the ship can be commanded. When a ship is underway the bridge is manned by an OOW (officer of the watch) aided usually by an AB (able seaman) acting as lookout. During critical maneuvers the captain will be on the bridge supported, perhaps, by an OOW as an extra set of hands, an AB on the wheel and sometimes a pilot if required.
Wheelhouses are the small enclosed parts of a bridge which historically held the ship's steering wheel. Today, ship bridges do not have a separate wheelhouse. In some cases, the term wheelhouse is used interchangeably to refer to the bridge.
Traditionally, sailing ships were commanded from the quarter deck, aft of the mainmast. With the arrival of paddle steamers, engineers required a platform from which they could inspect the paddle wheels and where the captain's view would not be obstructed by the paddle houses. A raised walkway, literally a bridge, connecting the paddle houses was therefore provided. When the screw propeller superseded the paddle wheel, the bridge was retained.
Traditionally, commands would be passed from the senior officer on the bridge to stations dispersed throughout the ship, where physical control of the ship was exercised, as technology did not exist for the remote control of steering or machinery. Helm orders would be passed to an enclosed wheel house, where the coxswain or helmsman operated the ship's wheel. Engine commands would be relayed to the engineer in the engine room by an engine order telegraph, which displayed the captain's orders on a dial. The engineer would ensure that the correct combination of steam pressure and engine revolutions were applied. The bridge was often open to the elements, therefore a weatherproof pilot house could be provided, from which a pilot, who was traditionally the ship's navigating officer, could issue commands from shelter.
Iron, and later steel, ships also required a compass platform. This was usually a tower, where a magnetic compass could be sited far away as possible from the ferrous interference of the hulk of the ship. Depending upon the design and layout of a ship, all of these terms can be variously interchangeable. Many ships still have a flying bridge, a platform atop the pilot house, open to weather, containing a binnacle and voice tubes to allow the conning officer to direct the ship from a higher position during fair weather conditions.
Larger ships, particularly warships, often had a number of different bridges. A navigation bridge would be used for the actual conning of the ship. A separate admiral's bridge could be provided in flagships, where the admiral could exercise strategic control over his fleet without interfering with the Captain's tactical command of the vessel. In older warships, a heavily-armored conning tower was often provided, where the vital command staff could be located under protection to ensure that the ship could be commanded and fought under fire.
Modern advances in remote control equipment have seen progressive transfer of the actual control of the ship to the bridge. The wheel and throttles can be operated directly from the bridge, controlling often-unmanned machinery spaces. Aboard modern warships it is common that actual commanding is carried out on the bridge while weapon systems are controlled from a room usually deep inside the vessel. This also enables a redundant setup in which the vessel is still able to fight in case one of the command instances has been hit or destroyed.
See also 
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