Temporary command and permanent rank in the early 1800s

What was the difference between a captain and a commander in the first half of the nineteenth century?

The example distinguishes permanent rank from a lieutenant's temporary command appointment, which could end when he left the ship.

The example concerns a lieutenant given command of a vessel and described as its master and commander. He might combine responsibility for sailing the ship with acting as its senior officer. The appointment could carry commander insignia and a temporary elevation, followed by reversion to lieutenant when he left the vessel. This explains how an officer described as commanding one ship could later appear as first lieutenant of a larger one. The example concerns appointment and acting status; it does not provide a complete account of substantive commander and captain ranks across the period.