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 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. History The office of the praetorian prefect had a long history dating back to the origins of the Roman Empire: initially, its two holders were the commanders of the Praetorian Guard, but gradually, they became the emperor's chief aides, and amassed considerable administrative and judicial responsibilities. The exact process of transformation to the chief civilian administrator of a specific territorial circumscription is still unclear. A common misconception, based on Zosimus, is that Constantine I established the praetorian prefectures as definite territorial administrations as early as 318, or in 324, after his victory over Licinius. During the Tetrarchy, when the number of holders of the imperial office multiplied (two senior emperors, the Augusti, and two junior colleagues, the Caesares), there is evidence for the existence of only two prefects at each time, presumably assigned to each of the Augusti. At that stage, the prefect's power was still immense. In the words of A.H.M. Jones, he was "a kind of grand vizier, the emperor's second in command, wielding a wide authority in almost every sphere of government, military and judicial, financial and general administration. He was the emperor’s chief of staff, adjutant-general, and quartermaster-general...". Following Diocletian's abdication in 305, civil war erupted among the various co-emperors, during which time each of the contenders appointed his own prefect, a pattern carried on during the period where the Empire was shared between Licinius and Constantine I. In 317 a third prefect was added in Gaul for Constantine's son Crispus. After his execution in 326 this prefect was retained. From 317 there were never less than three, and for years 347–61, 74-79 and 88–91, four, with the addition of prefecture for Illyricum, although in the last two years it comprised only the dioceses of Dacia and Macedonia which