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)