Originally, a single emperor in Rome controlled the Roman Empire. As conflict on different fronts proliferated, one man couldn’t handle it competently himself. This led to a period of chaos that Emperor Diocletian (Gaius Aurelius Valerius Diocletianus Augustus) (245–311) of Dalmatia ended. [See Other Dates for the Fall of Rome.] He realized the need for splitting the job officially and so started the four part division of the empire known as the tetrarchy — from the Greek words for “4” and “rule”. Two of the four were subordinates or “Caesars” and the others, seniors or “Augustuses”, were approximately equal. New administrative centers were set up at Nicomedia, Mediolanum, Antioch, and Trier.
Not only did Diocletian share the rule of the Roman Empire, but instead of dying in office, or being assassinated, he retired, the first emperor to do so voluntarily.
It would be a mistake to think of Diocletian as modest and self-sacrificing, however. From this point on, the imperial system is known as the Dominate, for “lord” — the word for a slave’s master — instead of the Principate which was meant to signify the leader was the first among equals. Court formality, such as you may be familiar with from the Byzantine era grew. Here is what Edward Gibbon has to say about the new regal customs:
“Their principal distinction was the Imperial or military robe of purple; whilst the senatorial garment was marked by a broad, and the equestrian by a narrow, band or stripe of the same honorable color. The pride, or rather the policy, of Diocletian, engaged that artful prince to introduce the stately magnificence of the court of Persia. He ventured to assume the diadem, an ornament detested by the Romans as the odious ensign of royalty, and the use of which had been considered as the most desperate act of the madness of Caligula. It was no more than a broad white fillet set with pearls, which encircled the emperor’s head. The sumptuous robes of Diocletian and his successors were of silk and gold; and it is remarked with indignation, that even their shoes were studded with the most precious gems. The access to their sacred person was every day rendered more difficult by the institution of new forms and ceremonies.”