In the latter half of the 2nd century, the Roman Empire was governed by the emperor and the vice emperor, and during the time of emperor Diocletianus, the Empire was divided into the west and the east and governed by two emperors and two vice emperors.
2世紀後半、ローマ帝国では正帝と副帝による分治が起こり、ディオクレティアヌス帝の時代に帝国が東西に分けられ、2人の正帝と2人の副帝によってテトラルキアされた。