Equivalent ranks worldwide include "ship-of-the-line captain" (e.g. France, Argentina, Spain), "captain of sea and war" (e.g. Portugal), "captain at sea" (e.g. Germany, Netherlands) and "captain of the first rank" (Russia).
Any naval officer who commands a ship is addressed by naval custom as "captain" while aboard in command, regardless of his or her actual rank, even though technically an officer of below the rank of captain is more correctly titled the commanding officer, or C.O. Officers with the rank of captain travelling aboard a vessel they do not command should be addressed by their rank and name (e.g., "Captain Smith"), but they should not be referred to as "the captain" to avoid confusion with the vessel's captain. The naval rank should not be confused with the army, air force, or marine ranks of captain, which all have the NATO code of OF-2.[Note 1]
On large US ships (e.g., aircraft carriers), the executive officer (XO) may be a captain in rank, in which case it would be proper to address him by rank. Often the XO prefers to be called "XO" to avoid confusion with the CO, who is also a captain in rank and the captain of the ship. The same applies to senior commanders on board US aircraft carriers, where the commander and deputy commander of the embarked carrier air wing are both captains in rank, but are addressed by the titles of "CAG" and "DCAG", respectively.
Captains with sea commands generally command ships of cruiser size or larger; the more senior the officer, the larger the ship, but ship commanders do not normally hold a higher rank than captain. In the Royal Navy, a captain might command an aircraft carrier, an amphibious assault ship, or the Ice Patrol Ship, while naval aviator and naval flight officer captains in the U.S. Navy command aircraft carriers, large-deck amphibious assault ships, carrier air wings, maritime patrol air wings, and functional and specialized air wings and air groups.
Maritime battlestaff commanders of one-star rank (commodores or rear admirals lower half) will normally embark on large capital ships such as aircraft carriers, which will function as the flagship for their strike group or battle group, but a captain will retain command of the actual ship, and assume the title of "flag captain". Even when a senior officer who is in the ship's captain's chain of command is present, all orders are given through the captain.
The following articles deal with the rank of captain as it is used in various navies.
Captain insignia of the Royal Australian Navy
Captain insignia of the Egyptian Navy
Aluf-Mishne (captain) insignia of the Israeli Navy
Ploiarchos (captain) insignia of Hellenic Navy
Komandor insignia of the Polish Navy
Marine Nationale française Capitaine de vaisseau
Kapitan of the 1st rank insignia of the Russian Navy
Thng tá insignia of Vietnam People's Navy
Captain Insignia of Iran's Navy
Kapitan of the 1st rank insignia of the Ukrainian Navy (1995-2016)
Kapitan of the 1st rank insignia of the Ukrainian Navy (f. 2016)
When the U.S. Navy's predecessor, the Continental Navy, was established in 1775, the first set of Navy regulations stipulated the commissioned offices of captain and lieutenant. When the United States Navy was created by Congress in 1794, the legislation again provided for the ranks of captain and lieutenant "who shall be appointed and commissioned in like manner as other officers of the United States are." In 1799, master commandant was authorized as a rank between lieutenant and captain. Although master commandant was changed to commander in 1837, this simple rank system survived intact until the Civil War.