Post ship
Post ship was a designation used in the Royal Navy during the second half of the 18th century and the Napoleonic Wars to describe a sixth-rate ship (see rating system of the Royal Navy) that 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 have as its captain a post-captain rather than a lieutenant or commander. Thus ships with 20 to 26 guns were post ships, though this situation changed after 1817.