Before 1817, a rated vessel of at least 20 guns required a post captain, but Royal Navy usage generally reserved frigate for ships of 28 guns or more. The 20- to 26-gun post ship could still be ship-rigged and frigate-built, with quarterdeck and forecastle, yet remained distinct from both frigates and ship sloops. In peacetime these economical vessels substituted for frigates on distant stations; in war they commonly escorted convoys.
Chapter · 2:40:13-2:43:50
Post ships and the lower boundary of a Royal Navy frigate
Post ships reveal that official rating, command rank, rig, and hull form did not produce one simple definition of frigate.