The division of the Roman Empire into four Praetorian prefectures originated in the age of the Tetrarchy yet outlived that period. In 318 AD, the Roman emperor Constantine the Great organized four prefectures: the Prefecture of Gaul, the Prefecture of Italy and Africa, the Prefecture of Illyricum, and the Prefecture of the East, each of which was overseen by a praetorian prefect. Each prefecture was subdivided into dioceses, themselves made up of provinces. The Roman Empire is the term conventionally used to describe the Ancient Roman polity in the centuries following its reorganization under the leadership of Octavian (better known as Caesar Augustus). ... The Tetrarchs, a porphyry sculpture sacked from a Byzantine palace in 1204, Treasury of St. ... Constantine. ... The praetorian prefect was commander of the Praetorian Guard until Constantine abolished it in 314. ... In some Christian churches, the diocese is an administrative territorial unit governed by a bishop, sometimes also referred to as a bishopric or episcopal see, though more often the term episcopal see means the office held by the bishop. ... Map of the Roman Empire, with the provinces, after 120 AD. In Ancient Rome, a province (Latin, provincia, pl. ...

Roman Empire c. 350 AD, showing the four prefectures