The praetorian prefecture was the largest administrative division of the late Roman Empire, above the mid-level dioceses and the low-level provinces. Praetorian prefectures originated in the reign of Constantine I, reaching their more or less final form in the last third of the 4th century and surviving until the 7th century, when the reforms of Heraclius diminished the prefecture's power, and the Muslim conquests forced the Eastern Roman Empire to adopt the new theme system. Elements of the prefecture's administrative apparatus, however, are documented to have survived in the Byzantine Empire until the first half of the 9th century.
The insignia of the praetorian prefect of Illyricum, as depicted in the Notitia Dignitatum: the ivory inkwell and pen case (theca), the codicil of appointment to the office on a blue cloth-covered table, and the state carriage.
In the Late Roman Empire, usually dated 284 AD to 641 AD, the regional governance district known as the Roman or civil diocese was made up of a grouping of provinces each headed by a Vicarius, who were the representatives of praetorian prefects. There were initially twelve dioceses, rising to fourteen by the end of the 4th century.
Emperor Constantine.
