|Comparative military ranks|
A petty officer (PO) is a non-commissioned officer in many navies and is given the NATO rank denotation OR-5 or OR-6. In many nations, they are typically equal to a sergeant in comparison to other military branches. Often they may be superior to a seaman, and subordinate to more senior non-commissioned officers, such as a chief petty officers.
Petty officers are usually sailors that have served at least several years in their respective navies. Petty officers represent the junior and mid-grade non-commissioned officer ranks of many naval services, and are generally responsible for the day-to-day supervision of ranks junior to them. They may also serve as technical specialists within their rating (military occupation).
The modern petty officer dates back to the Age of Sail in the Royal Navy. Petty officers rank between naval officers (both commissioned and warrant) and most enlisted sailors. These were men with some claim to officer rank, sufficient to distinguish them from ordinary ratings, without raising them so high as the sea officers. Several were warrant officers, in the literal sense of being appointed by warrant, and like the warrant sea officers, their superiors, they were usually among the specialists of the ship's company. The Oxford English Dictionary suggests that the title derives from the Anglo-Norman and Middle French "petit", meaning "of small size, small, little".
Two of the Royal Navy petty officer's rates, midshipman and master's mate, were a superior petty officer with a more general authority, but they remained no more than ratings. However, it was quite possible for a warrant officer (such as the armourer), in his role as a superior officer, to be court-martialed for striking a midshipman. This is because the midshipman was regarded as future sea officer, with the all-important social distinction of having the right to walk the quarterdeck. Midshipmen wore distinctive uniforms, master's mates dressed respectably, and both behaved like officers. The master's mate rating evolved into the rank of sub-lieutenant, and midshipman evolved into naval cadet.