Brittany

Rennes is officially the capital of the modern administrative region of Brittany – one of France's 18 regions.

Significantly, Wikipedia has separate pages for the "cultural region" and the administrative region that share the name of Brittany. The former page informs us that "Since reorganisation in 1956, the modern administrative region of Brittany has controversially excluded the Loire–Atlantique department around Nantes, which now forms part of the Pays de la Loire region."

Wikipedia lists Nantes as one of three cities that could justifiably claim to be the capital of the historical region of Brittany, along with Rennes and Vannes. With a population of 309,346 in 2017, Nantes is France's sixth biggest city; Rennes (216,815) is tenth biggest. Vannes had a population of only 53,352 in 2017; its claim to capital status is based at least partly on the fact that the Estates of Brittany (the provincial assembly, an early form of parliament) met there more often than anywhere else – 19 times, as opposed to 17 times in Nantes (and a lesser, but unspecified, number of times in Rennes).

The Celtic League recognises Brittany as one of the six Celtic nations, along with Ireland, Scotland, Wales, Cornwall and the Isle of Man.