Emperor Diocletian introduced a radical reform known as the Tetrarchy (284-305), with a western and an eastern Augustus or senior emperor, each seconded by a junior emperor (and designated successor) styled Caesar, and each of these four defending and administering a quarter of the Empire.
Detailed information on the arrangements during this period is contained in the Notitia Dignitatum (Record of Offices), a document dating from the early 5th century.