Apologies if this Q is a bit bone.......I've noticed recently when watching docs about the RN that the 1st Officer "No. 1" is now referred to in what appears to be the USN style i.e. as the "Executive Officer or XO"......why??
Because there's been a move away from the 1st Lt's (as he was properly known) job towards an Executive Role. The former concentrates mainly on ropes, wires, bogs and logs; the latter is the CO's 2-i-c and Chief of Staff.
Hang on a ******* second you complete tube, I quite clearly recognised the fact that the OP was referring to the 1Lt but calling him by the wrong name, whereas you clearly have no ******* idea about the subject whatsoever (see post #3) yet still gob off. Never mind comprehending the question, you don't even have a clue about the subject. You are not even a dipstick, as at least that provides useful information.
Other way round I think. In days of yore, the Royal Navy had loads of big ships that were commanded by Captains. There would be a load of commanders who were head of different branches or departments aboard ship, marine engineering, supply etc. The commander in charge of the executive branch was known as the executive officer and was second in command of the ship.
On big ships, I believe the 1st Lieutenant was the most senior Lieutenant Commander in the executive branch, effectively third in command of the ship.
Now, most frigates, destroyers and submarines are commanded by a commander and his heads of department are lieutenant commanders so there is really no distinction between XO and 1st Lt who will likely be the same person.
A 'big ship' would be commanded by a post-Captain with a Commander appointed as 'Executive Officer and Second-in-Command', an arrangement which reached down to the County class DLGs.
A destroyer or frigate squadron would also be led by a Captain who would also be appointed directly in command of one of the ships ('HMS Nonsuch in Command and as Captain(F) nth Frigate Squadron'). His ship would have a Lt Cdr appointed as 'Executive Officer and Second-in-Command', as would a private destroyer or frigate commanded by a Commander; he was always formally referred to as the 1st Lt, informally as 'No.1' to his face, 'Jimmy' behind his back; the leader would also have Commanders as heads of the engineering branches, double-hatted 'for Squadron duties', but the 1st Lt would still be the second in command as engineering specialists could not succeed to Sea Command; only fish-heads know how to drive, and the Command is personally responsible for safe navigation. The succession to Squadron command would follow the seniority of the other COs in the Squadron.
The same arrangement trickled down to the smallest commissioned ship where the Lt in command was still 'the Captain' and his 1st Lt was still 'No.1'.
The term 'First Lieutenant' goes back to before the rank of Commander was invented; in the old sailing navy we managed to thrash the French and all sorts of lesser fry with only two ranks below the Flag - Captain and Lieutenant. Small ships such as HM Armed Transport 'Bounty' had a CO appointed as 'Lieutenant-in-Command'.
That the 1st Lt should now be referred to as the 'XO' is an Americanism that crept in during the eighties I should think. Jackspeak like the rest of English does not stand still.
The RN rank of Lieutenant is correctly pronounced "L'tenant", goodness knows where pongoes got their F from.
And just to complicate matters, the senior Executive (in 1956 downgraded to 'Seaman') Lieutenant Commander in a big ship was called the First Lieutenant and operated under the Commander as Mate of the Upper Deck. Such officers were perhaps not really expecting to be promoted and I can think of a couple who compounded this by seriously blotting their copy book, one of them by becoming the anti-hero of a mutiny, which little excitement was eventually sorted out by a midshipman where the Master at Arms and others had failed, the MAA being pelted with tomatoes which, as they were still in their tins, was rather painful.