PRAETORIAN PREFECTURE

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 Praetorian prefecture of Gaul,

★ the Praetorian prefecture of Italy and Africa,

★ the Praetorian prefecture of Illyricum,

★ the Praetorian prefecture of the East,
each of which was overseen by a praetorian prefect. Each prefecture was subdivided into dioceses headed by a Vicarius, the dioceses themselves made up of provinces.
After the end of the Western Roman Empire, the Praetorian prefecture of Italy was retained by the new Ostrogothic Kingdom, which was still theoretically subordinate to the Empire, and Theodoric the Great even re-established the Praetorian prefecture of Gaul when he took the Provence.
After the reconquest of Northern Africa during the Vandalic War of 533-534, the new provinces were grouped by Justinian into a new Praetorian prefecture of Africa, which would later be transformed into the Exarchate of Africa. The Praetorian prefecture of Italy was also re-established after the end of the Gothic War, before too evolving into an exarchate.