Roman Emperor (Dominate)

The accession on November 20, 284, of Diocletian, the lower-class, Greek-speaking Dalmatian commander of Carus's and Numerian's household cavalry, marked a major departure from traditional Roman constitutional theory regarding the Emperor, who was nominally first among equals during the Principate. Whereas before Emperors had worn only a purple toga and were greeted with deference, Diocletian wore jewelled robes and shoes, and required those who greeted him to kneel and kiss the hem of his robe. In many ways, Diocletian was the first monarchical Emperor, and this is symbolised by the fact that the word dominus ("Lord") rapidly replaced princeps as the favoured word for referring to the Emperor. In short, the Dominate represents a time when the emperors unabashedly showcased their status and authority compared to the earlier Principate.

The Dominate also featured a shift in the Empire's "center of gravity" from the west to the east, particularly after the establishment of Constantinople; neither Diocletian nor his co-Emperor Maximian spent much time in Rome after 286, establishing their Imperial capitals at Nicomedia and Mediolanum (modern Milan), respectively.

The Tetrarchy was a system established by Diocletian to facilitate effective government of the Empire.

After acceding to power in 284, Diocletian decide to share the load of government with his friend Maximian, whom he appointed co-regent in 286. Maximian was to use the title Caesar, while Diocletian alone was Augusti. Diocletian focused on the eastern parts of the Empire while Maximian focused on the west.

In 293, this system was further developed into the Tetrarchy: Maximian was elevated to the rank of Augustus and both Augusti appointed junior sub-emperors with the title Caesar.

There were two senior emperors (titled Augusti), one for the West and one for the East, and two junior sub-emperors (titled Caesares), one for each senior emperor. When the Augusti left office for whatever reason, the Caesares would become Augusti and appoint their own Caesares; the retired Augusti took the title senior augustus and were styled Patres Imperatorum et Caesarum ("Fathers of the Imperators and of the Caesars").

Note: In 307, the augustus Severus was murdered by mutinous soldiers while attempting to suppress the rebellion and usurpation of Maxentius, who had invited his father Maximian to return from retirement and reassume the purple as augustus with him. Maxentius and Maximian reigned in the West as augusti co-operating with Constantine as caesar until the Imperial conference at Carnutum in November 308, whereat Constantine confirmed as caesar, Maximian deposed, and Licinius appointed augustus in his place. Maxentius continued to hold power as a rival Emperor until 312; his father Maximian (the first Emperor to be restored) committed suicide after an attempt to don the purple a third time in 310.

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