The problem of two emperors or two-emperor problem is the historiographical term for the historical contradiction between the idea of the universal empire, that there was only ever one true emperor at any one given time, and the truth that there were often multiple individuals who claimed the position simultaneously. The term is primarily used in regards to medieval European history and often refers to in particular the long-lasting dispute between the Byzantine emperors in Constantinople and the Holy Roman emperors in modern-day Germany and Austria as to which monarch represented the legitimate Roman emperor.
Gold solidus depicting Empress Irene (left) on the obverse, and her son Constantine VI (right) on the reverse.
Refusing to recognize the Eastern Empire, Pope Leo III crowned Charlemagne as emperor
Denarius of Frankish king Charlemagne, who was crowned as Roman Emperor Karolus Imperator Augustus in the year 800 by Pope Leo III due to, and in opposition to, the Roman Empire in the East being ruled by Irene, a woman. His coronation was strongly opposed by the Eastern Empire.
Coin of Byzantine emperor Basil I which titles him as Basilios Augustus
The word emperor can mean the male ruler of an empire. Empress, the female equivalent, may indicate an emperor's wife, mother/grandmother, or a woman who rules in her own right and name. Emperors are generally recognized to be of the highest monarchic honour and rank, surpassing kings. In Europe, the title of Emperor has been used since the Middle Ages, considered in those times equal or almost equal in dignity to that of Pope due to the latter's position as visible head of the Church and spiritual leader of the Catholic part of Western Europe. The emperor of Japan is the only currently reigning monarch whose title is translated into English as "Emperor".
Gaius Octavianus Caesar "Augustus", or simply Augustus, was the first emperor of the Roman Empire, reigning from 27 BC until his death in AD 14.
A statue of dictator Julius Caesar.
Under Justinian I, reigning in the 6th century, parts of Italy were for a few decades (re)conquered from the Ostrogoths: thus, this famous mosaic, featuring the Byzantine emperor in the center, can be admired at Ravenna.
Holy Roman Emperor Charles V in the 1550s, after Titian