First Lieutenant First Lieutenant is a military rank and , in some forces , an appointment . The rank of Lieutenant has different meanings in different military formations ( see comparative military ranks ) , but 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 . United Kingdom . British Army . In the British Army and Royal Marines , the rank above Second Lieutenant is simply Lieutenant , with no ordinal attached . Before 1871 , when the whole British Army switched to using the current rank of " Lieutenant " , the Royal Artillery , Royal Engineers and Fusilier regiment s used " First Lieutenant " and " Second Lieutenant " . Royal Navy . The First Lieutenant ( 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 , destroyer s and frigate s 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 specialisation 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 ) . United States . 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 . 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 Air Force and Marine Corps . The difference between the two ranks is slight , primarily being experience and higher pay . 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 ( 65 - 150 soldiers ) . 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 . U . S . Navy and U . S . Coast Guard . 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 carrier s , a Lieutenant Commander or even Commander . However , on submarine s , 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 . Other countries . For other countries , the equivalent rank to a US Army First Lieutenant ( O - 2 ) is listed below .