• An early case of one emperor, remaining "sole sovereign" in charge but designating one junior and successor, was
Marcus Aurelius (reigned 161–180 AD), who designated
Lucius Verus. • Emperor
Diocletian attempted an elaborate system with four emperors (two seniors styled
Augustus, each with a junior styled
Caesar), called later the
Tetrarchy. Revolutionary was the notion that each was to be simultaneously in permanent charge of one quarter of the empire, not just sharing in central government. The experiment did not live up to its promise, as succession was not smoothened but contention multiplied, so the quadruple emperorship was abandoned—not the quarters, which remained as administrative and military divisions called
praetorian prefecture, as did the lower level, called
diocese, and the smaller size (and larger number) of
Roman provinces. • In 395, the Roman Empire was split for good, but in two halves:
Western and
Eastern, each under a sovereign emperor, in charge of two praetorian prefectures, each with or without a partner in government. • Both the notion of "partnership" in the form of a senior emperor and several junior co-emperors (usually, but not necessarily, his sons), and Diocletian's titulature, but mainly versed in Greek (e.g.
Sebastos for
Augustus, a literal translation), became quite common in the
Eastern Roman Empire, i.e. Byzantium, which lasted a further millennium after the fall of the Western Empire. ==See also==