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The princeps senatus (plural principes senatus) was the first member by precedence of the Roman Senate. Although officially out of the cursus honorum and possessing no imperium, this office conferred prestige on the senator holding it.
The princeps senatus was not a lifetime appointment. He was chosen by every new pair of censors (that is, every 5 years). Censors could, however, confirm a princeps senatus for a period of another 5 years. He was selected from patrician senators with consular rank, usually former censors. The successful candidate had to be a patrician with an impeccable political record, respected by his fellow senators.
The office was established around the year 275 BC. Originally, the position of the princeps was one of honor: he had the privilege of speaking first on the topic presented by the presiding magistrate. This gave the position great dignitas as it allowed the princeps to set the tone of the debate in the Senate. Across the 3rd and 2nd centuries BC, the office gained the prerogatives of the presiding magistrates and additional privileges, namely:
By 80 BC, it is believed that the status and function of the office was changed by the constitutional reforms of Lucius Cornelius Sulla. Although the term remained, reflecting the senator who was named first in the roll of the Senate issued by the Censors, the prerogatives of the office were restricted. In particular, the honor of speaking first on any topic debated in the Senate, a measure of their political clout, was removed from them and transported to the consul designate.
After the fall of the Roman Republic, the princeps senatus was the Roman emperor, and during the period of the Principate, no other individual is believed to have held the office (see also: princeps). However, in the emperor's absence, it is possible that a Senator was granted the privilege of holding this role when the Senate met; the notoriously unreliable Historia Augusta claimed that during the Crisis of the Third Century, some others held the position; in particular, it stated that the future emperor Valerian held the office in 238, during the reigns of Maximinus Thrax and Gordian I, and he continued to hold it through to the reign of Decius. The same source also makes the same claim about Tacitus when the Senate acclaimed him emperor in AD 275.