Roman Prefectures
The empire was administratively divided into praetorian prefectures (praefectura praetorio, plural praefecturae praetorio) during the tetrarchy.
History
The title praetorian prefect (praefectus praetorio) originated as the commanders of the Praetorian Guard. Over time, the role evolved into that of an advisor, and finally into that of a grand vizier.
Under the tetrarchy, each of the four quarters of the empire were administered by a praetorian prefect.
Prefectures
Galliarum
Composed of the dioceses of Gaul, Britain, Spain, and Vienna.
Italiae
Composed of northern and southern Italy, Africa, and Pannonia.
In 347, Italiae was split to create Illyrici; Dacia, Macedonia, and Pannonia were removed. Pannonia returned to Italiae by 395.
Illyrici
Composed of Dacia and Macedonia.
Split out from Italiae in 347. Pannonia was originally part of Illyrici in that reorganization, but was moved back to Italiae by 395.
Orientis
Composed of Thracia, Asia, Pontus, the East, and Egypt.
Egypt was elevated to a diocese, but retained in the Diocese of the East, in 380.