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The commanding officer (CO) or sometimes, if the incumbent is a general officer, commanding general (CG), is the officer in command of a military unit. The commanding officer has ultimate authority over the unit, and is usually given wide latitude to run the unit as they see fit, within the bounds of military law. In this respect, commanding officers have significant responsibilities (for example, the use of force, finances, equipment, the Geneva Conventions), duties (to higher authority, mission effectiveness, duty of care to personnel), and powers (for example, discipline and punishment of personnel within certain limits of military law).
In some countries, commanding officers may be of any commissioned rank. Usually, there are more officers than command positions available, and time spent in command is generally a key aspect of promotion, so the role of commanding officer is highly valued. The commanding officer is often assisted by an executive officer (XO) or second-in-command (2i/c), who handles personnel and day-to-day matters, and a senior enlisted advisor. Larger units may also have staff officers responsible for various responsibilities.
In the British Army, Royal Marines, and many other Commonwealth military and paramilitary organisations, the commanding officer of a unit is appointed. Thus the office of CO is an appointment. The appointment of commanding officer is exclusive to commanders of major units (regiments, battalions and similar sized units). It is customary for a commanding officer to hold the rank of lieutenant colonel, and they are usually referred to within the unit simply as "the colonel" or the CO. "The colonel" may also refer to the holder of an honorary appointment of a senior officer who oversees the non-operational affairs of a regiment. However, the rank of the appointment holder and the holder's appointment are separate. That is, not all lieutenant colonels are COs, and although most COs are lieutenant colonels, that is not a requirement of the appointment.
Sub-units and minor units (companies, squadrons and batteries) and formations (brigades, divisions, corps and armies) do not have a commanding officer. The officer in command of a minor unit holds the appointment of "officer commanding" (OC). Higher formations have a commander (usual for a brigade) or a general officer commanding (GOC). Area commands have a commander-in-chief (e.g. C-in-C Land Army, C-in-C British Army of the Rhine). The OC of a sub-unit or minor unit is today customarily a major (although formerly usually a captain in infantry companies and often also in cavalry squadrons), although again the rank of the appointment holder and the holder's appointment are separate and independent of each other.
In some cases, independent units smaller than a sub-unit (e.g. a military police platoon that reports directly to a formation such as a brigade) will also have an OC appointed. In these cases, the officer commanding can be a captain or even a lieutenant.
Appointments such as CO and OC may have specific powers associated with them. For example, they may have statutory powers to promote soldiers or to deal with certain disciplinary offences and award certain punishments. The CO of a unit may have the power to sentence an offender to 28 days' detention, whereas the OC of a sub-unit may have the power to sentence an offender to 3 days' restriction of privileges.
Commanders of units smaller than sub-units (e.g. platoons, troops and sections) are not specific appointments and officers or NCOs who fill those positions are simply referred to as the commander or leader (e.g. platoon commander, troop leader, section commander/leader, etc.).
In the Royal Air Force, the title of commanding officer is reserved for station commanders or commanders of independent units, including flying squadrons. As with the British Army, the post of a commander of a lesser unit such as an administrative wing, squadron or flight is referred to as the officer commanding (OC).
In the Royal Navy and many others, commanding officer is the official title of the commander of any ship, unit or installation. However, they are referred to as "the captain" no matter what their actual rank, or informally as "skipper" or even "boss".
In the United States, the status of commanding officer is duly applied to all commissioned officers who hold lawful command over a military unit, ship, or installation.
The commanding officer of a company, usually a captain, is referred to as the company commander (or the battery/troop commander for artillery/cavalry) units. The commanding officer of a battalion (or squadron of cavalry/armored cavalry) is usually a lieutenant colonel. The commanding officer of a brigade, a colonel, is the brigade commander. At the division level and higher, however, the commanding officer is referred to as the commanding general, as these officers hold general officer rank.
Although holding a leadership position in the same sense as commanders,
The individual in charge of a platoon, the smallest unit of soldiers led by a commissioned officer, is referred to as the "platoon leader," not the "platoon commander." This officer, typically a second lieutenant, does have command of the soldiers under him but does not have many of the command responsibilities inherent to higher echelons. For example, a platoon leader cannot issue non-judicial punishment.
Non-commissioned officers may be said to have charge of certain smaller military units. They cannot, however, hold command as they lack the requisite authority granted by the head of state to do so. Those wielding "command" of individual vehicles (and their crews) are called vehicle commanders. This distinction in title also applies to officers who are aircraft commanders ("pilot in command"), as well as officers and enlisted soldiers who are tank and armored vehicle commanders. While these officers and NCOs have tactical and operational command (including full authority, responsibility, and accountability – especially in the case of aircraft commanders) of the soldiers and equipment in their charge, they are not accorded the legal authority of a "commanding officer" under the UCMJ or military regulations.
Warrant officers in the United States Armed Forces are single career-track officers that can, and occasionally do, hold command positions within certain specialty units, i.e. Special Forces and Army Aviation. However, warrant officers usually do not command if a commissioned officer is present; normally they serve as executive officer (2IC).
The commanding officer of a company, usually a captain, is referred to as the company commander or the battery commander (for field artillery and low altitude air defense units). The commanding officer of a battalion or a squadron (Marine aviation), is usually a lieutenant colonel. The commanding officer of a regiment, aviation group, or Marine Expeditionary Unit (MEU), a colonel, is the regimental/group/MEU commander. At the Marine Expeditionary Brigade (MEB), Marine Logistics Group (MLG), Marine Division (MARDIV), Marine Aircraft Wing (MAW), Marine Expeditionary Force (MEF), and Fleet Marine Force (FMF) levels; however, the commanding officer is referred to as the commanding general, as these officers hold general officer rank.
The officer in charge of a platoon, the smallest tactical unit of Marines usually led by a commissioned officer, typically a first or second lieutenant, is referred to as the platoon commander. This distinction in title also applies to officers who are aircraft commanders, as well as officers, staff non-commissioned officers (staff sergeant – master sergeant), and non-commissioned officers (corporal and sergeant) who are tank and armored vehicle commanders. While these officers, SNCOs, and NCOs have tactical and operational command (including full authority, responsibility, and accountability—especially in the case of aircraft commanders) of the Marines and equipment in their charge, they are not accorded the legal authority of a "commanding officer" under the UCMJ or military regulations.
In the United States Navy and United States Coast Guard, commanding officer is the official title of the commander of a ship, but they are usually referred to as "the Captain" regardless of their actual rank: "Any naval officer who commands a ship, submarine or other vessel is addressed by naval custom as 'captain' while aboard in command, regardless of their actual rank."  They may be informally referred to as "Skipper", though allowing or forbidding the use of this form of address is the commanding officer's prerogative.
A prospective commanding officer (PCO) is a U.S. Navy officer who has been selected for his/her own command. The term is used in correspondence or in reference to the officer before they assume command of the unit (ship, squadron, unit, etc.).
If the sailor in command of a unit is an enlisted member, rather than a commissioned or warrant officer, he or she is referred to as the "officer in charge" rather than "commanding officer." In the Coast Guard it is common for smaller cutters to be commanded by a chief petty officer.
In the United States Air Force, the commanding officer of a unit is similarly referred to as the unit commander, such as squadron commander, group commander, wing commander, and so forth. Squadron commanders (the base unit of the United States Air Force) are usually majors or lieutenant colonels. Group commanders (made up of two or more squadrons) are usually colonels, while wing commanders may be colonels (typical wings) or generals (larger wings).
A battalion is a military unit, typically consisting of 300 to 1,200 soldiers commanded by a lieutenant colonel, and subdivided into a number of companies. In some countries, battalions are exclusively infantry, while in others battalions are unit-level organizations.
A company is a military unit, typically consisting of 80–250 soldiers and usually commanded by a major or a captain. Most companies are formed of three to seven platoons, although the exact number may vary by country, unit type, and structure.
Military ranks are a system of hierarchical relationships, within armed forces, police, intelligence agencies or other institutions organized along military lines. The military rank system defines dominance, authority, and responsibility in a military hierarchy. It incorporates the principles of exercising power and authority into the military chain of command—the succession of commanders superior to subordinates through which command is exercised. The military chain of command constructs an important component for organized collective action.
A brigade is a major tactical military formation that typically comprises three to six battalions plus supporting elements. It is roughly equivalent to an enlarged or reinforced regiment. Two or more brigades may constitute a division.
Commander is a common naval officer rank. Commander is also used as a rank or title in other formal organizations, including several police forces. In several countries this naval rank is termed frigate captain.
An executive officer is a person who is principally responsible for leading all or part of an organization, although the exact nature of the role varies depending on the organization. In many militaries and police forces, an executive officer, or "XO", is the second-in-command, reporting to the commanding officer. The XO is typically responsible for the management of day-to-day activities, freeing the commander to concentrate on strategy and planning the unit's next move.
Adjutant is a military appointment given to an officer who assists the commanding officer with unit administration, mostly the management of human resources in an army unit. The term adjudant is used in French-speaking armed forces as a non-commissioned officer rank similar to a staff sergeant or warrant officer but is not equivalent to the role or appointment of an adjutant.
Wing commander is a senior commissioned rank in the British Royal Air Force and air forces of many countries which have historical British influence, including many Commonwealth countries but not including Canada and South Africa. It is sometimes used as the English translation of an equivalent rank in countries which have a non-English air force-specific rank structure. It ranks immediately above squadron leader and immediately below group captain.
Officer Cadet is a rank held by military cadets during their training to become commissioned officers. In the United Kingdom, the rank is also used by members of University Royal Naval Units, University Officer Training Corps and University Air Squadron; however, these are not trainee officers with many not choosing a career in the armed forces.
Second-in-command is a title denoting that the holder of the title is the second-highest authority within a certain organisation.
Commandant is a title often given to the officer in charge of a military training establishment or academy. This usage is common in English-speaking nations. In some countries it may be a military or police rank. It is also often used to refer to the commander of a military prison or prison camp.
A senior officer is an officer of a more senior grade in military or other uniformed services. In military organisations, the term may refer to any officer above junior officer rank, but usually specifically refers to the middle-ranking group of commissioned officers above junior officer ranks but below flag, general or air rank. In most countries, this includes the ranks of lieutenant commander/major/squadron leader, commander/lieutenant colonel/wing commander and naval captain/colonel/group captain, or their equivalents. In some countries, it also includes brigadiers and commodores.
United States military seniority is the method by which the United States Armed Forces determines precedence among commissioned officers, in particular those who hold the same rank. Seniority is used to determine assignments, tactical commands, promotions and general courtesy. To a lesser extent, historical seniority is used to recognize status of honor given to early United States military leaders such as inaugural holders of certain ranks or those officers who served as leadership during major wars and armed conflicts.
In the United States Army, U.S. Marine Corps, U.S. Air Force and U.S. Space Force, major is a field-grade military officer rank above the rank of captain and below the rank of lieutenant colonel. It is equivalent to the naval rank of lieutenant commander in the other uniformed services. Although lieutenant commanders are considered junior officers by their services, the rank of major is that of a senior officer in the United States Army, the United States Marine Corps, and the United States Air Force.
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. Many fire departments and police departments in the United States also use the rank of captain as an officer in a specific unit.
The United States Marine Corps is organized within the Department of the Navy, which is led by the Secretary of the Navy (SECNAV). The most senior Marine commissioned officer is the Commandant of the Marine Corps, responsible for organizing, recruiting, training, and equipping the Marine Corps so that it is ready for operation under the command of the unified combatant commanders. The Marine Corps is organized into four principal subdivisions: Headquarters Marine Corps, the Operating Forces, the Supporting Establishment, and the Marine Forces Reserve.
A company commander is the commanding officer of a company, a military unit which typically consists of 100 to 250 soldiers, often organized into three or four smaller units called platoons. The exact organization of a company varies by country, service, and unit type. Aviation companies can have as few as 40 personnel, while some specialized companies such as maintenance or training units are considerably larger and may number as many as 500 personnel. In some forces, the second-in-command of a company is called the executive officer (XO).
In the United States, commander is a military rank that is also sometimes used as a military billet title—the designation of someone who manages living quarters or a base—depending on the branch of service. It is also used as a rank or title in non-military organizations; particularly in law enforcement.
In the United States Army (USA), U.S. Marine Corps (USMC), U.S. Air Force (USAF), and U.S. Space Force (USSF), captain is a company-grade officer rank, with the pay grade of O-3. It ranks above first lieutenant and below major. It is equivalent to the rank of lieutenant in the Navy/Coast Guard officer rank system and is different from the higher Navy/Coast Guard rank of captain. The insignia for the rank consists of two silver bars, with slight stylized differences between the Army/Air Force version and the Marine Corps version.