appointing promagistrates to serve, either by random casting of lots or by senatus consultum (advice of the Senate); however, these appointments were not formally binding on a legal basis and could be nullified by Roman assemblies. The governor's level of authority was determined by what type of imperium he possessed. Most provinces were governed by propraetors who had served an annual term in the praetorship the year before. The provinces governed by propraetors were usually the more tranquil ones, where chances of revolt or invasion were small, but in some cases propraetors would be given command of more troubled provinces. Provinces that lay on the empire's borders, thereby requiring a permanent military garrison, were governed by proconsuls who had served a term as consul (the highest rank of magistrate) the year before their governorship. They were given the authority to command provinces with actual Roman legions, rather than just using the militia. These promagistrates held equality with other magistrates with the same level of imperium and were attended by the same number of lictors. Generally speaking, they had autocratic power within their provinces. A provincial governor almost possessed unlimited authority and often extorted vast amounts of money from the provincial population—but, though he retained immunity from prosecution as long as he held his imperium, once he left office he became vulnerable to prosecution for his actions during his term. Imperial governors Imperial provinces After Augustus established the Principate, the Emperor himself was the direct governor of Rome's most important provinces (called imperial provinces) and even in the provinces he did not directly govern, was senior to other provincial governors through holding imperium maius, or supreme imperium. In imperial provinces, the Emperor would appoint legates to govern in his name. The Emperor had sole say in the appointing of these legates, who were lower in rank than other provincial governors, as officially they were only representatives of the province's true governor, the Emperor. The Principate did not totally do away with the system of selecting proconsuls and propraetors. In provinces with one legion, a legate bearing praetorian imperium, thus being a propraetor, not only governed the province in the Emperor's name but also controlled the legion himself. However, in provinces with more than one legion, each legion was commanded by its own legate with praetorian imperium, while the province as a whole was commanded by a legate with consular imperium, who