Did you know that the Byzantine Empire sometimes had two emperors? This was an old tradition dating back to Roman Emperor Diocletian in the late 200s CE, who created a system of four emperors, two senior emperors and two junior emperors. Byzantine co-emperors go back to at least the 400s CE with Leo II crowning his father Zeno co-emperor and promptly dying, making Zeno sole ruler. Not exactly off to a good start. But the co-emperor tradition continued. By the 900s it was common enough that there were distinct terms for the junior co-emperor (basileus) and senior co-emperor (autokratōr or occasionally megas basileus).
One of the more interesting co-emperors had not one co-ruler but four! Romanos I Lekapenos, an Armenian who became a major Byzantine naval commander, seized the royal palace and the reins of government in 919. In March he married his daughter to the reigning emperor, fifteen-year-old Constantine VII. In September Romanos decided that was not enough and had himself crowned co-emperor with his own made-up term for equal emperors “Caesar,” before finally, in December, naming himself the senior co-emperor or autokratōr
Romanos eventually crowned his own sons co-emperors: Christopher in 921, Stephen and Constantine in 924. For the time being, Constantine VII was regarded as first in rank after Romanos himself, Augustus to his Caesar. For his kindness to the man he deposed, Romanos I Lekapenos was given the nickname “the gentle usurper.”