"The Tetrarchy is the term adopted to describe the system of government of the ancient Roman Empire instituted by Roman Emperor Diocletian in 293, marking the ...
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end of the Crisis of the Third Century and the recovery of the Roman Empire. The government of the empire was divided between the two senior emperors, the augusti, and their juniors and designated successors, the caesares.
Initially Diocletian chose Maximian as his caesar in 285, raising him to co-augustus the following year; Maximian was to govern the western provinces and Diocletian would administer the eastern ones. The role of the augustus was likened to Jupiter, while his caesar was akin to JupiterĀ“s son Hercules. Galerius and Constantius were appointed caesares in March 293. Diocletian and Maximian retired on 1 May 305, raising Galerius and Constantius to the rank of augustus. Their places as caesares were in turn taken by Valerius Severus and Maximinus Daia." - (en.wikipedia.org 10.01.2021)