Tetrarchy

Diocletian, Roman Emperor reformed the government of the Roman Empire in the face of serious military and economic problems and established a new form of government called the Tetrarchy. This involved dividing his power over the empire into east and west sectors, he retained control of the East and his colleague Maximian controlled the west. Eight years later, feeling more focus was needed on both civic and military problems, he furthered the division of power by naming two "Junior Emperors", or Caesari, under each "Senior Emperor", or Augustus. Thus the Tetrarchy, a rule of four, was established and lasted until c. 324.

Constantine I's father Constantius Chlorus was appointed a Caesar in the Tetrarchy in 293.