The Portrait of the Four Tetrarchs is a porphyry sculpture group of four Roman emperors dating from around 300 AD. Since the Middle Ages it has been fixed to a corner of the façade of St Mark's Basilica in Venice, Italy. It probably originally formed part of the decorations of the Philadelphion in Constantinople, and was removed to Venice in 1204 or soon after.
The Roman Empire was for a time after 293 ruled by a tetrarchy , instituted by Emperor Diocletian. The tetrarchy consisted of two Augusti and two Caesars . The empire was territorially divided into western and eastern halves, with a senior and a junior emperor in each half. After Diocletian and his colleague, Maximian, retired in 305, internal strife erupted among the tetrarchs. The system finally ceased to exist around 313.
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