The rank of lieutenant has different meanings in different military formations, but in the majority of cases it is common for it to be sub-divided into a senior (first lieutenant) and junior (second lieutenant) rank. The NATO equivalent rank for land force officers is OF-1 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 regiments used "first lieutenant" and "second lieutenant".
The first lieutenant (often abbreviated "1st Lt") in a Royal Navy ship is a post or appointment, rather than a rank.
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, destroyers, frigates, and submarines, 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 his 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). The first lieutenant may hold the rank of sub-lieutenant, lieutenant or lieutenant-commander.
In the U.S. Army, U.S. Marine Corps, U.S. Air Force, and U.S. Space Force, a first lieutenant is a junior commissioned officer. It is just above the rank of second lieutenant and just below the rank of captain. It is equivalent to the rank of lieutenant (junior grade) in the other uniformed services.
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.
In the U.S. Navy and U.S. Coast Guard, "first lieutenant" is the name of a billet and position title, rather than rank. Officers aboard early sailing ships were the captain and a number of lieutenants. The senior among those lieutenants was known as the first lieutenant, and would have assumed command if the captain were absent or incapacitated. As modern ships have become more complex, requiring specialized knowledge of engineering, communications, and weapons, the "first lieutenant" is the officer in command of the deck department responsible for line handling during mooring and underway replenishment. On smaller ships, the officer of the "first lieutenant" billet holds the rank of lieutenant, junior grade or ensign. On larger vessels, the position of "first lieutenant" is held by a lieutenant or, in the case of extremely large warships such as cruisers or aircraft carriers, the position of "first lieutenant" may be held by a lieutenant commander or even commander. However, on submarines and in aircraft squadrons, where the deck department may only have a few junior sailors, the "first lieutenant" billet may be filled by a first-class petty officer or chief petty officer. What is known in the U.S. Navy as the "first lieutenant division" is usually composed of junior sailors (E-3 and below) who are completing their ninety days of temporary assigned duty, or TAD, that all enlisted personnel are required to perform when initially assigned to a command. The primary mission of the division is servicing, cleaning, organizing and inventorying items within a command.
The term "first lieutenant" had a dual meaning in the U.S. Revenue Cutter Service. The position title of first lieutenant was held by a junior officer who was in charge of deck operations and gunnery. The rank of first lieutenant was the equivalent to lieutenant in the current rank structure of the U.S. Coast Guard and U.S. Navy. The next senior officer ranking above first lieutenant was captain and the next two lower officer ranks were second and third lieutenant, respectively. The rank of first lieutenant carried over to the formation of the U.S. Coast Guard in 1915 and was used until 1918, when the rank structure of the U.S. Navy was adopted.
In Indonesia, "First lieutenant" is known as Letnan Satu (Lettu). The Lieutenant rank has two levels, which are: Second lieutenant (Letda) and First lieutenant (Lettu).
In the Israel Defense Forces, the rank above second lieutenant is simply lieutenant. The rank of (? ("? (katsín miktsoí akademai or "kama"), a professional academic officer (that is, a medical, dental or veterinary officer, a justice officer or a religious officer), is equivalent to a professional officer of the second class in the reserve and equivalent to first lieutenant.
For other countries, the equivalent rank to a US Army first lieutenant (O-2) is listed below.