Moravia, the first Western Slavic polity, arose through the unification of the Slavic tribes settled north of the Danube. However, its formation is scarcely described by contemporaneous sources. The archaeologist Barford writes that the first report of the emerging Moravian state was recorded in 811. In the autumn of this year, according to the "Royal Frankish Annals", Avar rulers and the "duces" or "leaders of the Slavs who live along the Danube" visited the court of Emperor Louis the Pious (r. 814–840) in Aachen. The earliest certain reference to Moravians or "Maravani" is dated to 822 when the emperor "received embassies and presents from all the East Slavs, that is, Obodrites, Sorbs, Wilzi, Bohemians, Moravians, and Praedenecenti, and from the Avars living in Pannonia" at an assembly held at Frankfurt.