There are several terms for referring to the second in command in military ranks. Some are formal indicators of rank, and others are titles commonly given as a way of distinguishing one particular officer from others but which carry no official authority. The names also tend to vary by country, type of service, and era in history.
The executive officer or XO is used in modern naval and military ranks to refer to the second in command on a ship, and sometimes in Army battalions as well. The term comes from the fact that the XO is often given the duties of seeing that orders of the commanding officer or CO are carried out, or executed. Executive officers are responsible for seeing that all routine duties are carried out on a ship, for approving leave, reporting readiness and assuming command if the CO becomes incapacitated.
First mate is similar to the executive officer title except that it's commonly used on merchant vessels, or informally in military naval structures. It also refers to the officer in charge of the ship immediately under the authority of the captain and is responsible for all crew on deck and the safe operation of the ship. The first mate is also the watch officer, who's responsible for bridge operations and considered the primary pilot of the ship after the captain.
Chief mate is the official international designation on Merchant Marine ships for the second in command. A chief mate has to have significant experience as an officer of the watch before being promoted to the post and must comply with relevant laws and regulations of the flag state under which the ship sails. Among the duties a chief mate shares in common with other second in command officers in other organizations, he or she must be fully aware of the rules of the Resolutions International Marine Organization Convention governing the movement of commercials ships in international waters.
First officer is a more general and somewhat informal designation for second in command, and can apply to discrete military units in Navy, Army or Air Force service. It's also commonly used in the airline industry to indicate a co-pilot who's second in command to the main pilot of a commercial airliner. The title is also widely used in recreational sailing on yachts and entails many of the same duties as a second in command on cargo ships or other commercial vessels.
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