Captain (United States)
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In the United States uniformed services, captain is a commissioned officer rank. In keeping with the traditions of the militaries of most nations, the rank varies between the services, being a senior rank in the naval services and a junior rank in the ground and air forces.
For the naval rank, a captain is a senior officer of pay grade O-6 (the sixth officer rank), typically commanding seagoing vessels, major aviation commands and shore installations. This rank is used by the U.S. Navy, U.S. Coast Guard, U.S. Public Health Service Commissioned Corps, the NOAA Commissioned Corps, and the U.S. Maritime Service.
For the ground and air forces rank, a captain is of pay grade O-3 (the third officer rank), usually serving as the commander of a company-sized unit in the ground forces, as a flight leader or other squadron officer in air units, or serving as an executive officer or staff officer for a larger unit such as a battalion or squadron. This rank is used by the U.S. Army, U.S. Air Force, and U.S. Marine Corps.
The rank of captain may also be used in other organizations outside of the military, particularly in fire departments, police, and law enforcement.
Captain (Ground and Air Forces) 
Rank equivalency between services 
Due to the ambiguity created by the common use of "captain" for officers of different grades between the Army, Navy, Air Force, Marine Corps, and Coast Guard, equivalency is conferred between officers by use of identical pay grade rather than title of rank. The higher the grade, the higher the rank of the officer. For example, an Army, Air Force, or Marine captain is equivalent in rank, responsibilities, and grade to a Navy or Coast Guard lieutenant, all of which are of the third officer grade, or O-3. Similarly, a Navy or Coast Guard captain is equivalent in rank, responsibilities, and grade to an Army, Air Force, or Marine colonel, all of which are of the sixth officer grade, or O-6. Thus, Army, Air Force, and Marine colonels together with Navy and Coast Guard captains wear the silver eagle insignia of the grade of O-6, while Army, Air Force, and Marine captains together with Navy and Coast Guard lieutenants wear the double silver bars of the rank insignia of the grade of O-3. Additionally, the O-6 rank of Navy and Coast Guard captain is abbreviated as uppercase "CAPT", while the O-3 rank of captain is abbreviated as "CPT" for the Army and mixed-case "Capt" for the Air Force and Marine Corps.
Early history 
In the United States, the rank of captain first appeared in the Continental Army during the Revolutionary War. A captain was the officer placed in charge of a company of soldiers and was granted a commission from the regimental colonel. A captain was afforded one to several lieutenants, depending on the size of the company, and the captain’s commission could be revoked or expired at the end of a particular military campaign.
The Continental Navy used the rank of captain as the commander of a sailing vessel at war, with the captain having several lieutenants on board, as well as a Sailing Master to assist in their duties. This use of the rank carried over into the U.S. Navy. With the addition of the ranks of commander and lieutenant commander between lieutenant and captain, a Navy captain became equivalent in rank to an Army colonel.
|United States commissioned officer and officer candidate ranks|
|Pay grade / Branch of service||Officer
|Approximate insignia||(no universal insignia)||(no universal insignia)|
|Air Force||Cadet / OT||2d Lt||1st Lt||Capt||Maj||Lt Col||Col||Brig Gen||Maj Gen||Lt Gen||Gen||GAF|||
|Army||CDT / OC||2LT||1LT||CPT||MAJ||LTC||COL||BG||MG||LTG||GEN||GA||GAS|
|Marine Corps||Midn / Cand||2ndLt||1stLt||Capt||Maj||LtCol||Col||BGen||MajGen||LtGen||Gen|||||
|Navy||MIDN / OC||ENS||LTJG||LT||LCDR||CDR||CAPT||RDML||RADM||VADM||ADM||FADM||AN|
|Coast Guard||CDT / OC||ENS||LTJG||LT||LCDR||CDR||CAPT||RDML||RADM||VADM||ADM|||||
|Public Health Service||||ENS||LTJG||LT||LCDR||CDR||CAPT||RADM||RADM||VADM||ADM|||||
|National Oceanic and
 Grade is authorized by the U.S. Code for use but has not been created
 Grade has never been created or authorized
|United States warrant officer ranks|
|Public Health Service|||||||||||
|National Oceanic and
 Grade is authorized for use by U.S. Code but has not been created
 Grade never created or authorized