First lieutenant is a commissioned officer military rank in many armed forces; in some forces, it is an .
The rank of lieutenant has different meanings in different military formations, but in most forces it is sub-divided into a senior (first lieutenant) and junior (second lieutenant) rank.
In navies, while certain rank insignia may carry the name lieutenant, the term may also be used to relate to a particular post or duty, rather than a rank.
Before 1871, when the whole British Army switched to using the current rank of "lieutenant", the Royal Artillery, Royal Engineers and fusilier used "first lieutenant" and "second lieutenant".
Historically the lieutenants in a ship were ranked in accordance with seniority, with the most senior being termed the first lieutenant and acting as the second-in-command, unless the ship was complemented with a commander. Although lieutenants are no longer ranked by seniority, the post of "first lieutenant" remains. In minor war vessels, , , and , the first lieutenant is second in command, executive officer (XO) and head of the executive branch; in larger ships where a commander of the warfare specialization is appointed as the executive officer, a first lieutenant is appointed as their deputy. The post of first lieutenant in a shore establishment carries a similar responsibility to the first lieutenant of a capital ship. Colloquial terms in the Royal Navy for the first lieutenant include "number one", "the jimmy" (or "jimmy the one") and "James the First" (a back-formation referring to James I of England).Partridge, p 612, p 621, p 884 The first lieutenant may hold the rank of sub-lieutenant, lieutenant or lieutenant-commander.
Promotion to first lieutenant is governed by Department of Defense policies derived from the Defense Officer Personnel Management Act of 1980. DOPMA guidelines suggest all "fully qualified" officers should be promoted to first lieutenant. A second lieutenant (grade O-1) is usually promoted to first lieutenant (grade O-2) after 18 months in the Army or 24 months in the Marine Corps and Air Force. The difference between the two ranks is slight, primarily being experience and a higher pay grade. It is not uncommon to see officers moved to positions requiring more experience after promotion to first lieutenant. For example, in the Army and Marine Corps these positions can include leading a specialty platoon, or assignment as the executive officer for a company-sized unit (70–250 soldiers or marines). In the Air Force, a first lieutenant may be a flight commander or section's officer in charge with varied supervisory responsibilities, including supervision of as many as 100+ personnel, although in a flying unit, a first lieutenant is a rated officer (pilot, navigator, or air battle manager) who has just finished training for his career field and has few supervisory responsibilities.
File:09.RABDF-1LT.svg|First lieutenant
(Antigua and Barbuda Regiment)
File:Argentina-Army-OF-1a.svg|Teniente primero
(Argentine Army)
File:Australian Army OF-1b.svg|First lieutenant
(Australian Army)
File:Botswana-Army-OF-1b.svg|First lieutenant
(Botswana Ground Force)
File:Brazil-Army-OF-1b.svg|Primeiro tenente
(Brazilian Army)
File:Cape_Verde-army-OF-1c.svg|Primeiro tenente
(Cape Verdean National Guard)
File:Cuba-Army-OF-1c.svg|Primer teniente
(Cuban Revolutionary Army)
File:Denmark-Army-OF-1b-M11.svg|Premierløjtnant
(Royal Danish Army)
File:Capona de Primer Teniente Ejercito Nacional Dominicano.svg|Primer teniente
(Dominican Army)
Yliluutnantti M05.svg|Yliluutnantti
(Finnish Army)
File:08.Gambian Army-1LT.svg|First lieutenant
(Gambian National Army)
File:07. Guinea-Bissau 1LT.svg|Primeiro tenente
(Army of Guinea-Bissau)
File:14-TNI Army-1LT.svg|Letnan satu
(Indonesian Army)
File:blank.svg|First lieutenant
(Liberian Ground Forces)
File:Luxembourg-Army-OF-1b.svg|Lieutenant en premier
(Luxembourg Army)
File:Netherlands-Army-OF-1b.svg|1e Luitenant
(Royal Netherlands Army)
File:New Zealand-Army-OF-1b.svg|First lieutenant
(New Zealand Army)
File:09-Nicaragua Army-1LT.svg|Teniente primero
(Nicaraguan Army)
File:Paraguay-Army-OF-1b.svg|Teniente primero
(Paraguayan Army)
File:PA 1st Lt.png|First lieutenant
(Philippine Army)
File:US Army O2 (Army greens).svg|First lieutenant
(United States Army)
File:blank.svg|Teniente primero
(National Army of Uruguay)
File:PTTEEJB-GNB.png|Primer teniente
(Venezuelan Army)
US Marine O2 shoulderboard vertical.svg|First lieutenant
(United States Marine Corps)
Generic-Navy-O2.svg|Primeiro-tenente
(Cape Verdean Coast Guard)
08.Botswana Air Force-1LT.svg|First lieutenant
(Botswana Defence Force Air Wing)
File:US Air Force O2 shoulderboard.svg|First lieutenant
(United States Air Force)
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