What was the title of the rulers of the Byzantine and Frankish empires?

They bore the title of emperor. Since the time of the split of the Roman Empire into two parts, the Byzantines received the right to call their rulers – emperors. The Frankish kingdom has long been ruled by kings. Only in the 8th century, after the extensive conquests of Charlemagne, Francia turned into an empire. Thus, the increased authority of the Frankish ruler, Charles, demanded a new title, which became “emperor”. So in 800 the Pope crowned Charlemagne. After that, Francia began to be called an empire.