From Wikipedia

Byzantium, The Lost Empire

John Romer

Episode Two

Heaven On Earth

Byzantium under the Constantinian and Valentinian dynasties was the earliest period of the Byzantine Empire that saw the creation of an eastern Empire from the Roman Republic under the emperor Constantine I in his new capital, Constantinople, created on the site of the old Greek Byzantium. Constantine's successors ruled over portions of the empire in Egypt, Syria, and the Balkans.

The Roman emperors Diocletian and Constantine I both played an important role in reforming the organization of the whole Empire. The Empire in its entirety had become difficult to control, and Diocletian resolved this by creating a tetrarchy that allowed for Augusti to rule in each of the western and eastern halves of the Empire, while two Caesars would be their seconds. In case of the loss of either Augusti, the Caesar would take their place, and a new Caesar would be selected. The only significant change made by Constantine to this system was the replacement of the selection of Caesars with a succession by bloodline.

To alleviate the concerns of territorial administration, Diocletian divided the whole of the Empire into one hundred distinct provinces. Administrative control was brought under the auspices of the Emperor, and the whole of Italia was relegated to the status of a regular province, now also compelled to pay taxes. Each province was subdivided into a diocese, twelve in total. Constantine organized the provinces even further by creating prefectures, each one consisting of several dioceses, and each diocese consisting of several provinces. The Praetorian prefecture of the East(Praefectura praetorio per Orientem) was made up of five dioceses- Aegyptus, Oriens, Pontus, Asiana, and Thracia. This enabled the Empire to harness the control of each prefecture by providing a distinct difference between military and civil administration.