Portrait of the Four Tetrarchs

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.

• http://en.wikipedia.org/wiki/Portrait of the Four Tetrarchs