Mizentop, maintop, foretop
Service in these masts has nothing whatever to do with age or merit. A “top” is a platform fixed over the head of a lower mast, resting on the trestle-trees, to spread the rigging of the topmast. The mizenmast is the aftermost mast of a ship; the foremast is in the forward part of a ship; the mainmast is between these two.
“He was put into the mizentop, and served three years in the West
Indies; then he was transferred to the maintop, and served five years
in the Mediterranean; and then he was made captain of the foretop, and
served six years in the East Indies; and at last he was rated captain's
coxswain in the Druid frigate.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894