- Brian Herbert Warmington
- and R. S. O. Tomlin
*Diocletian divided Italy and most of the existing provinces into smaller provinces, which he grouped into twelve ‘dioceses’, each administered by a *vicarius. These ‘vicars’ were officially deputies of the praetorian prefects, and facilitated the central bureaucracy's control of provincial governors. The Diocletianic dioceses were Britain, Gaul, Viennensis (see Vienna), Spain, Africa, Pannonia, Moesia, Thrace, Asiana, Pontica, and Oriens. Italy was in practice divided between the vicarius Italiae in the north and the vicarius of Rome in the south. The proconsuls of Asia, Achaia, and Africa were not subject to vicarial authority. The number of dioceses increased when *Constantine I divided Moesia into Dacia and Macedonia, and *Valens detached Egypt from Oriens; the latter were administered by a prefect and a comes respectively. After Constantine, the prefects ruled directly the diocese in which their seats were located.