Each praetorian prefect oversaw one of the four quarters created by Diocletian, which became regional praetorian prefectures for the young sons of Constantine ca 330 A.D. From 395 there two imperial courts, at Rome (later Ravenna) and Constantinople, but the four prefectures remained as the highest level of administrative division, in charge of several dioceses (groups of Roman provinces), each of which was headed by a Vicarius.