A line officer (or otherwise termed "officer of the line") is 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-direct combat specialties (as opposed to those assigned to non-combat duties for a given tour or rotation) such as chaplains, lawyers, supply officers and medical officers (both nurses and doctors). The navy refers to them as Staff Officers. In the United States military, qualified line officers, regardless of rank, would have authority over higher ranking non-line officers in times of combat. A chaplain is typically a member of the clergy serving a group of people who are not organized as a mission or church; lay chaplains are also found in some settings such as universities. ... Staff officers in the U. S. Navy provide specialized support to Line Officers and to all other persons in the organization. ...

In the United States Navy, a component of the Department of the Navy, line officers are easily recognized by an inverted star placed over their sleeve/shoulder stripes, or by their rank insignia on both collars. They are divided into Unrestricted Line Officers and non-staff Restricted Line Officers. USN redirects here. ... Seal The United States Department of the Navy was established by an Act of Congress on April 30, 1798, to provide administrative and technical support, and civilian leadership to the United States Navy. ... Unrestricted Line Officers (URL Officers) are Officers of the Line in the U.S. Navy who are qualified to command ships and aviation squadrons. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

The uniform of line officers differs from staff officers somewhat. 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. Staff officers have their specialty insignia placed over their sleeve/shoulder stripes and on their left collar.

All officers of the United States Coast Guard are considered line officers and all wear a replica of the Coast Guard shield in lieu of the inverted star. The United States Coast Guard (USCG) is a branch of the United States armed forces involved in maritime law enforcement, mariner assistance, search and rescue, and national defense, among other duties of coast guards elsewhere. ...

In the United States Marine Corps, also part of the Department of the Navy, all officers except warrant officers and limited duty officers (LDOs) are considered line officers, trained to take command of combat units. The United States Marine Corps (USMC) is a branch of the U.S. military responsible for providing power projection from the sea,[1] utilizing the mobility of the U.S. Navy to rapidly deliver combined-arms task forces to global crises. ... A Limited Duty Officer (LDO) is a Line Officer in the U.S. Navy who was selected for commissioning based on his skill and expertise, and are not required to have a Bachelors Degree. ...

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 commanders were termed line officers.