The early U.S. Navy had three "grades" of officer who were typically placed in charge of warships: captain; master commandant; and lieutenant, commanding (which was not a distinct rank, but a title given to an ordinary lieutenant).
Contemporary paintings show a master commandant's uniform main difference from a captain's uniform was that while a Captain wore an epaulet on each shoulder a master commandant's uniform had a single epaulet on the right shoulder, and a lieutenant commandant wore a single epaulet on the left shoulder.
[4] American naval hero Stephen Decatur notably never held the rank of master commandant.
After leading a daring raid to destroy the captured U.S. frigate Philadelphia in Tripoli Harbor in 1804, Decatur returned to the U.S. as a national hero and was given a direct promotion from lieutenant to captain.
Also in 1804, Master Commandant Richard Somers led a dozen volunteer sailors on USS Intrepid—loaded with explosives—toward the pirate fleet in the harbor of Tripoli, Libya.