Tetrarchy comes from the Greek for four (tetra-) and rule (arch-). It was also known as a quadrumvirate. It refers to Emperor Diocletian's four-way division of the Roman Empire in 293. Diocletian ruled the East, while his co-emperor Maximian ruled the West. They were both called "Augustus," signaling that they were emperors. Subordinate to them were the two "Caesars." They were Galerius, in the east, and Constantius, in the west.
Picture: http://tetrarchy.com/coppermine/albums/userpics/DAG_map.jpg
