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 facade of St Mark's Basilica in Venice, Italy. It probably originally formed part of the decorations of the Philadelphion in Constantinople. The Roman Empire was for a time after 293 ruled by a tetrarchy (a group of four rulers), instituted by Emperor Diocletian. The tetrarchy consisted of two Augusti (senior emperors) and two Caesars (younger emperors). The empire was territorially divided into western and eastern halves, with a senior and a junior emperor in each half.