Tarrantry is an alternate history shared fictional universe created by various authors at the Naval Fiction Board. It was first developed in the late 1990s and early 2000s and has become one of the more famous older Internet-based AH worldbuilding projects (alongside the likes of Ill Bethisad). Some of the principal creators of the universe include Rick Robinson, Theodore Leverett and Marina O'Leary.
The Tarrantry universe is set in a mildly-ASB timeline that contains a larger island off the coast of Western Europe, the eponymous Tarrantry. It is here that a whole fictional history and new local nationality develops, culminating in the creation and existence of a native Terrentrian monarchy.
The various standalones or series of short stories and novels set in the universe follow Tarrantry's throughout the centuries, with a focus on the exploits of its gradually evolving navy and the role it plays in various naval conflicts of European history. The stories are set in various periods, from the swashbuckling tales of the Age of Sail to the naval clash thrillers of WWI and WWII.
Several long-standing veterans of AlternateHistory.com had taken part in the creation and refinement of Tarrantry fiction or have been affiliated with its writers. Perhaps the most prominent of such members is Grey Wolf and several other members of the Battleship Brothers.
The Saga of Tarrantry - The website of the setting and of the many works and supplemental material (ship artworks, vexilology, etc.) created for it. First created on the Naval Fiction Board, this separate official site is now archived via archive.org.
A page about the Terrentrian naval air service - Supplemental discussion thread at the Naval Fiction Board, includes setting lore on Terrentrian aircraft that's missing from the official site.
Terrentrian Naval Flags - Compilation image, collecting all known Terrentrian naval flags. (Archived on AH.com.)
The Naval Fiction Board - The birthplace of the Tarrantry shared universe. The Naval Fiction Board offers both historical and alternate history naval fiction (both military and civilian).
The Strangerverse - An AH.com shared universe, started in 2008.
Protect and Survive Universe - An AH.com shared universe, started in 2010.