Another little historically confusing tidbit:
In the age of sail before the intermediate ranks were established, any large ship (usually a third rate or larger) was commanded by a captain, and the other officers were all lieutenants, warrant officers, and midshipmen. So the XO of the ship was actually the most senior lieutenant, and naturally called the First Lieutenant.
In the modern USN, 'first lieutenant' carries no connotation of seniority, but is instead the title given to the division officer in charge of the ship's 'deck division', basically the division responsible for the maintaining the material state of the ship and handling most of the deck seamanship duties.
In the age of sail before the intermediate ranks were established, any large ship (usually a third rate or larger) was commanded by a captain, and the other officers were all lieutenants, warrant officers, and midshipmen. So the XO of the ship was actually the most senior lieutenant, and naturally called the First Lieutenant.
In the modern USN, 'first lieutenant' carries no connotation of seniority, but is instead the title given to the division officer in charge of the ship's 'deck division', basically the division responsible for the maintaining the material state of the ship and handling most of the deck seamanship duties.