Post-ship
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Post-ship was a designation used in the Royal Navy during the Age of Sail to describe a ship of the Sixth Rate (see rating system of the Royal Navy) which was smaller than a frigate (in practice, carrying fewer than 28 guns) but by virtue of being a rated ship (with at least 20 guns) had to be commanded by a post-captain instead of a lieutenant or commander.
Unlike other uses of the term "ship" during this era, "post-ship" in itself implies nothing on the rig of the vessel; however, all Sixth Rates were in practice ship-rigged, i.e. were square-rigged on three masts.