Captain of the hold
A petty officer (PO) is a non-commissioned officer in many navies and is given the NATO rank denotion OR-6. In many nations, they are typically equal to a corporal or sergeant in comparison to other military branches. Often they may be superior to a seaman, generally the (or one of the) lowest ranks in a navy, and subordinate to a more senior non-commissioned officer, such as a chief petty officer.
The modern petty officer dates back to the Age of Sail. 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 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 both were regarded as future sea officers, 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.
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