After the death of the Roman emperor Theodosius I (347–395), the Roman Empire was divided into a western and an eastern half, each of which was ruled by its own emperor. The eastern Roman emperor reigned in Constantinople (formerly Byzantium, currently Istanbul), while the western Roman emperor resided for a time in Milan, and at other times in Rome and Ravenna.
Map of the boundaries of the western and eastern Roman empires after the death of Theodosius I, in 395 AD, graphic, 2006: creator: Geuiwogbil; source: Wikimedia Commons, http://commons.wikimedia.org/wiki/File:Theodosius_I%27s_empire.png?uselang=de. Creative Commons Attribution ShareAlike 3.0 Germany.