The rank of lieutenant has different meanings in different military formations. In the majority of cases it is common for it to be sub-divided into a senior (first lieutenant) and junior (second lieutenant) rank. In navies it may relate to a particular post rather than a rank.
U.S. Army, U.S. Marine Corps and U.S. Air Force
In the United States Army, Air Force, and Marine Corps, 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.
In the U.S. Navy and Coast Guard, first lieutenant is a position title, instead of a rank. It is held by the officer in command of the deck department. On smaller ships, the first lieutenant holds the rank of lieutenant, junior grade or ensign. On larger vessels, the position is held by a lieutenant or, in the case of extremely large warships such as aircraft carriers, a lieutenant commander or even commander. However, on submarines, where the deck department may only have a few junior sailors, the first lieutenant may be a senior enlisted member, such as a first-class petty officer or chief petty officer.
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