Commodore was one of the officer ranks of the Royal Navy.
They were commissioned officers, and the rank existed between Post Captain and Rear Admiral. Commodore was generally a tempory post, and not a actual promotion. After serving the posting as Commodore, the officer would generally be shifted back to a Post Captain. Many officers were promoted from Post Captain without ever at any point being a Commodore.
The appointment of Commodore dates to the mid-17th century: it was first used in the time of William III. There was a need for officers to command squadrons, but it was not deemed desirable to create new admirals (as Post-Captains were promoted to Rear-Admiral in order of seniority). Captains assigned squadron command were given the title of Commodore, but it was not an actual rank. The officer so designated kept his place on the list of Captains. In 1748 it was established that Captains serving as Commodores were equal to Brigadier-Generals in the Army.
The Royal Navy Commodore was eventually split into two classes. Those of the first class had a Captain under them to command their ship and were allocated one-eighth of all prize money earned by ships under their command. Those of the second class commanded their own ship as well as the squadron. In 1783, Commodores of the first class were allowed to wear the uniform of a Rear-Admiral, a distinction which continued with some variation until the two classes of Commodore were consolidated in 1958. In 1996, Commodore was made a substantive rank in the Royal Navy.
- Commodore on Wikipedia