to nominate an heir (referred to as the Caesar), but the caesar did not have access to the powers of the emperor, nor was he delegated any official authority. It was during the Crisis of the Third Century that the traditional imperial approach of a single imperial magistrate based at Rome became unable to cope with multiple and simultaneous invasions and usurpations that required the emperor to be everywhere at once. Further, it was their absence which caused usurpations to occur in response to a local or provincial crisis that traditionally would have been dealt with by the emperor. Under the Dominate, the burden of the imperial position was increasingly shared between colleagues, referred to as the Consortium imperii. It was Diocletian who introduced this form of government, under a system called the Tetrarchy, which originally consisted of two co-emperors (augusti) and two respectively subordinate junior emperors (caesars), each of whom shared in the imperial power. This original power sharing model lasted from AD 289 through to AD 324, being undone during the Civil wars of the Tetrarchy. With Constantine I's death in AD 337, the empire was again shared between multiple augusti, lasting until AD 350. The model became a permanent feature of the empire in AD 364 with the accession of Valentinian I, who shared the imperial office with his brother Valens. Barring the 3-year period of solitary rule by Theodosius I from AD 392–395, this approach would last until the overthrow of the last western emperor in AD 476. While each augustus was autonomous within each portion of the empire they managed, all laws that were introduced by any emperor were valid across the entirety of the empire. Devaluation of the Consulate During the Roman Republic, the office of Consul was the highest elected magistracy in the Roman state, with two consuls elected annually. With the arrival of the Principate, although all real power was invested in the emperor, the consuls were still in theory the head of state, and the calendar year was identified by the two ordinary consuls who began in office at the start of the year. Throughout the Principate, the imperial consulate was an important position, albeit as the method through which the Roman aristocracy could progress through to the higher levels of imperial administration – only former consuls could become consular legates, the proconsuls of Africa and Asia, or the urban prefect