Regensburg is a city in eastern Bavaria, at the confluence of the Danube, Naab and Regen rivers. It is capital of the Upper Palatinate subregion of the state in the south of Germany. With more than 150,000 inhabitants, Regensburg is the fourth-largest city in the State of Bavaria after Munich, Nuremberg and Augsburg. From its foundation as an imperial Roman river fort, the city has been the political, economic and cultural centre of the surrounding region; it is still known in the Romance languages by a cognate of its Latin name of "Ratisbona" (the version "Ratisbon" was long current in English). Later, under the rule of the Holy Roman Empire, it housed the Perpetual Diet of Regensburg.