Line officer

In the United States armed forces, the term line officer or officer of the line refers to a Navy, Marine, Air Force or Coast Guard officer who exercises general command authority and is eligible for operational command positions, as opposed to officers who normally exercise authority within a specialty. The equivalent Army term is "competitive category".

Officers who are not line officers are those whose primary duties are in non-combat specialties including chaplains, attorneys, supply and medical services. The distinction between line and non-line officers often blur; line officers may be assigned non-combat roles, and non-line officers are often assigned to tasks normally performed by line officers. Also, non-line officers at the squadron or Group level (and higher) are also issued "G-Series" orders which gives them the same relative power of 'line officers' of equivalent rank. A line officer may even hold authority over a non-line officer of higher rank by the nature of their job, but is otherwise expected to observe normal customs and courtesies outside that role.

History
The expression "officer of the line" is possibly rooted in the 18th- and 19th-century Royal Navy practice of employing sail-powered warships in line formations to maximize the effectiveness of side-mounted cannons. The ships were called ships of the line and their officers were termed line officers. The term also derives from "walking the line" and in many military circles is believed to have come from a "line in the sand" which two groups of officers once used in a political argument to gain power.

United States forces
In the United States Navy, line officers are divided into unrestricted line officers and restricted line officers. Unrestricted Line (URL) officers hold combat warfare specialties as Naval Aviators and Naval Flight Officers, Surface Warfare Officers, Submarine Warfare Officers and/or Special Warfare (SEAL) officers and are eligible for operational combatant command at sea, as well as command of major installations and commands ashore. Restricted Line officers command only within their particular specialty. Line officers wear an inverted gold star above their rank stripes on their dress blue uniforms and, in the case of Captains and below, on their shoulder boards in whites. Line officer admirals will wear solid gold shoulder boards with a silver metallic thread anchor and one, two, three or four silver metallic thread stars below the anchor. When wearing khakis or utility/working uniforms, they wear their rank insignia on both collar points. The Navy refers to non-line officers as Staff Corps officers. Both Line and Staff Corps officers may be assigned as "staff officers" serving on the staff of a senior officer. Staff Corps officers wear their corps insignia, rather than the line officer's star, placed over their sleeve/shoulder board stripes on blues and whites and on their left collar point on khakis and utility/working uniforms.

In the United States Marine Corps, all officers including warrant officers and limited duty officers (LDOs) are line officers, trained to command combat units. Unlike the Navy, the Marine Corps does not have Staff Corps, consequently all Marine Engineer, Supply, and Judge Advocates are line officers. The Marine Corps, as a service under the Department of the Navy also lacks its own medical corps officers, dental corps officers, nurse corps officers and chaplains. These positions are filled by US Navy staff corps officers in those specialties assigned to the Fleet Marine Force.

In the United States Air Force, officers assigned to the Medical, Judge Advocate, Nurse, Medical Services (healthcare administration), Biosciences, and Chaplain Corps are professional officers. All other officers belong to the Line of the Air Force (LAF).

All officers of the United States Coast Guard are considered line officers and wear the Coast Guard shield in lieu of the inverted star.

Other forces
The expression "line officer" is no longer current in the Royal Navy and Commonwealth affiliates, officers trained in the "Executive Department" of a warship are the only ones trained for command. In the Royal Canadian Navy, officers in the Maritime Surface/Sub-Surface (MARS) occupation hold a similar function, but are not distinguished by any identifiable badge.