|Comparative military ranks|
|Naval officer ranks|
Sub-lieutenant (SLt. or S/Lt.) is a junior military officer rank.
In many navies, a sub-lieutenant is a naval commissioned or subordinate officer, ranking below a lieutenant. In the Royal Navy (RN) the rank of sub-lieutenant is equivalent to the rank of lieutenant in the British Army and of flying officer in the Royal Air Force (RAF).
In some armies, sub-lieutenant is the lowest officer rank. However, in Brazil, it is the highest non-commissioned rank, and in Spain, it is the second highest non-commissioned rank.
The NATO rank code for the British Royal Navy rank is OF-1 (senior).
In 1955, the Royal Navy created the rank of acting sub-lieutenant, although it had existed intermittently in the world wars. Unlike their substantive counterparts, acting sub-lieutenants are subordinate officers, as they hold their ranks by order and not by commission. Upon passing Fleet Board, acting sub-lieutenants were confirmed as sub-lieutenants and issued commissions backdated to the date when they were appointed acting sub-lieutenants. The rank of acting sub-lieutenant was abolished in the Royal Navy around 1993 but remained in the Royal Naval Reserve until 2013; officers in both the RN and RNR now commission into the rank of Midshipman. The rank of Acting Sub-lieutenant remains in the Royal Navy only within the University Royal Naval Unit where Training Officers enter at this rank.
Before its abolition, the rank of acting sub-lieutenant in the Royal Navy corresponded with, but was junior to, the ranks of lieutenant (Army) and flying officer (RAF). This can be seen in print versions of STANAG 2116 edition 5 (1992).
In many Commonwealth navies (e.g. Canada and Australia), the rank of acting sub-lieutenant still exists as a commissioned rank equivalent to second lieutenant, while the rank of sub-lieutenant is equivalent to that of an army lieutenant. As the term "acting" is a designation, both acting and substantive ranks should be addressed as "sub-lieutenant"; the full designation including "acting" should be used in written documents, however. Indeed, when someone is addressed as "Acting sub-lieutenant", it can be seen as a way of patronising an individual in a derogatory manner due to the emphasis of their "acting" rank.
The Royal New Zealand Navy follows the US precedent in titling its lowest commissioned officer ensign.
In the Royal Thai Army, Army Reserve Force Students who complete grade 5 and their B.D. are promoted to the rank of acting second lieutenant (ว่าที่ ร้อยตรี).
In the modern Royal Navy, all officer cadets now commission as midshipmen, regardless of whether they are a graduate or a school leaver. They are subsequently promoted to sub-lieutenant one year after entering Britannia Royal Naval College. Upper Yardsmen commission as Sub-Lieutenants assuming their seniority date is greater than 12 months.
In the Royal Canadian Navy, all undergoing basic officer training join as naval cadets, but upon graduation, those who joined with a bachelor's degree receive an immediate promotion to acting sub-lieutenant, while those who do not retain their rank as naval cadet until such time as they finish more career-related training.
In France, a sub-lieutenant (sous-lieutenant) is the junior commissioned officer in the army or the air force. He wears a band in the colour of his corps (e.g. gold for infantry, silver for armoured cavalry, etc.). During the 18th century a rank of sous-lieutenant de vaisseau existed in the French Navy. It was the equivalent of the master's mate rank of the Royal Navy. It is now replaced by the rank of "first ensign" (enseigne de vaisseau de première classe).
An Argentinian sub-lieutenant wears a single silver sun on each shoulder, Brazilian sub-lieutenants are the most senior non-commissioned rank (called Sub-Officer in the Navy and Air force), wearing a golden lozenge. In Mexico, the sub-lieutenant is the most junior officer in the rank scale, and wears a single gold bar. Thai sub-lieutenants and acting sub-lieutenants wear a single star on each shoulder.
The British Army briefly used the rank of sub-lieutenant from 1871 to 1877, replacing the ranks of ensign in the infantry and cornet in the cavalry. In 1877, it was replaced in turn by the rank of second lieutenant, although this had always been used by the Royal Artillery, Royal Engineers, and rifle and fusilier regiments.
In the Royal Navy, the Royal Australian Navy and the Royal New Zealand Navy, the insignia of both sub-lieutenants and acting sub-lieutenants consists of one medium gold braid stripe with an executive curl. The size of this stripe should not be confused with the narrow stripe, colloquially referred to as "spaghetti strap," used on the Royal New Zealand Navy rank of ensign and the Royal Canadian Navy's naval cadets. The Royal Air Force also followed this example of braiding when developing their rank system (see flying officer).
The insignia of sub-lieutenants in most commonwealth countries are identical to the United States Navy and United States Coast Guard grade of ensign (although US ranks do not use the executive curl), even though its equivalent grade in the USN is actually lieutenant junior grade.
In the Royal Canadian Navy, acting sub-lieutenants display one medium stripe. A sub-lieutenant adds a narrow stripe below the medium stripe to maintain the executive curl on the top. The equivalent air force lieutenant rank has the narrow stripe above the medium stripe since these elements do not need to maintain spacing on top due to the lack of an executive curl; the Royal Canadian Navy followed this pattern before the addition of the curl in 2010.