According to Ohnsorge, for a long time, it had been the custom of Byzantium to designate the German princes as spiritual "sons" of the Romans. What might have been acceptable in the fifth century had become provoking and insulting to the Franks in the eighth century. Charles came to believe that the Roman emperor, who claimed to head the world hierarchy of states, was, in reality, no greater than Charles himself, a king as other kings, since beginning in 629 he had entitled himself "Basileus" (translated literally as "king"). Ohnsorge finds it significant that the chief wax seal of Charles, which bore only the inscription: "Christe, protege Carolum regem Francorum protect Charles, king of the Franks, was used from 772 to 813, even during the imperial period and was not replaced by a special imperial seal; indicating that Charles felt himself to be just the king of the Franks. Finally, Ohnsorge points out that in the spring of 813 at Aachen Charles crowned his only surviving son, Louis, as the emperor without recourse to Rome with only the acclamation of his Franks. The form in which this acclamation was offered was Frankish-Christian rather than Roman. This implies both independence from Rome and a Frankish (non-Roman) understanding of empire.