The praetorian prefecture (, in Greek variously named ) 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 (r. 306-337), 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 East 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.