The Tetrarchy

The Tetrarchy was the first true imperical system of the Dominate, started by Diocletian. He split the Empire into one with a single ruler to one with multiple rulers, for ease of succession and accessible rule. The Tetrarchian Emperors are as follows:

Maximian (293 AD -- 305 AD) Constantius Chlorus (305 AD -- 306 AD) Flavius Valerius Severus (306 AD -- 307 AD) Maximian/Maxentius (307 AD -- 308 AD) Licentius (308 AD -- 313 AD) Maxentius (308 AD -- 312 AD) (Usurper) Domitius Alexander (308 AD -- 309 AD) (African Usurper)

http://www.livius.org/te-tg/tetrarchy/tetrarchy.html