The term line officer (or "officer of the line") is used in the United States Navy, Coast Guard and Marine Corps to describe a military officer who is trained to command a warship, ground combat unit or combat aviation unit. Officers who are not line officers are those whose primary duties are in non-combat specialties including chaplains, lawyers, supply officers and medical officers. Nevertheless, line officers may occasionally be assigned non-combat roles. In operational circumstances line officers may hold positional authority over non-line officers of higher rank.
The expression "officer of the line" is rooted in the 18th- and 19th-century British naval 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.
United States forces
In the United States Navy, line officers are divided into unrestricted line officers
and non-staff restricted line officers
. Line officers wear an inverted star above their rank stripes on their dress blue uniforms and on their shoulder boards in whites. When wearing khakis, winter working blues or coveralls they wear their rank on both collars. The navy refers to non-line officers as staff officers
, though this term should not be confused with the non-navy definition. Staff officers have their specialty insignia placed over their sleeve/shoulder stripes and on their left collar.
In the United States Marine Corps, all officers except warrant officers and limited duty officers (LDOs) are considered line officers, trained to take command of combat units.
All officers of the United States Coast Guard are considered line officers and wear the Coast Guard shield in lieu of the inverted star.
The expression "line officer" is no longer current in the Royal Navy
affiliates, though officers holding positions in the executive chain of command
are usually distinguished with the "executive curl" - a loop over the upper rank bar, which was replaced in U.S. Navy practice with a star. In the Canadian Navy
, officers in the Maritime Surface/Sub-Surface
(MARS) trade hold a similar function, but are not distinguished by any identifiable badge.