Pliny referred to "your histories" when writing to him about his earlier work. Although Tacitus refers to part of his work as "my annals", the title of the work Annals used today was not assigned by Tacitus himself, but derives from its year-by-year structure. Of the sixteen books in Annals, the reign of Tiberius takes up six books, of which only Book 5 is missing. These books are neatly divided into two sets of three, corresponding to the change in the nature of the political climate during the period. The next six books are devoted to the reigns of Caligula and Claudius. Books 7 through 10 are missing. Books 11 and 12 cover the period from the treachery of Messalina to the end of Claudius' reign. The final four books cover the reign of Nero and Book 16 cuts off in the middle of the year AD 66. This leaves the material that would have covered the final two years of Nero's reign lost. Content and style Tacitus documented a Roman imperial system of government. Tacitus chose to start his work with the death of Augustus Caesar in AD 14, and his succession by Tiberius. As in the Histories, Tacitus maintains his thesis of the necessity of the Principate. He says again that Augustus gave and warranted peace to the state after years of civil war, but on the other hand he shows us the dark side of life under the Caesars. The history of the beginning of the principate is also the history of the end of the political freedom that the senatorial aristocracy, which Tacitus viewed as morally decadent, corrupt, and servile towards the emperor, had enjoyed during the republic. During Nero's reign there had been a widespread diffusion of literary works in favor of this suicidal exitus illustrium virorum ("end of the illustrious men"). Again, as in his Agricola, Tacitus is opposed to those who chose useless martyrdom through vain suicides. In the Annals, Tacitus further improved the style of portraiture that he had used so well in the Historiae. Perhaps the best portrait is that of Tiberius, portrayed in an indirect way, painted progressively during the course of a narrative, with observations and commentary along the way filling in details. Tacitus portrays both Tiberius and Nero as tyrants who caused fear in their subjects. But while he views Tiberius as someone who had once been a