late antiquity. The distinction between a usurper and a legitimate emperor is a blurry one, given that a large number of emperors commonly considered legitimate began their rule as usurpers, revolting against the previous legitimate emperor. True legitimizing structures and theories were weak, or wholly absent, in the Roman Empire, and there were no true objective legal criteria for being acclaimed emperor beyond acceptance by the Roman army. Dynastic succession was not legally formalized, but also not uncommon, with powerful rulers sometimes succeeding in passing power on to their children or other relatives. While dynastic ties could bring someone to the throne, they were not a guarantee that their rule would not be challenged. With the exception of Titus (79–81; son of Vespasian), no son of an emperor who ruled after the death of their father died a natural death until Constantine I in 337. Control of Rome itself and approval of the Roman Senate held some importance as legitimising factors, but was mostly symbolic. Emperors who began their careers as usurpers had often been deemed public enemies by the senate before they managed to take the city. Emperors did not need to be acclaimed or crowned in Rome itself, as demonstrated in the Year of the Four Emperors (69), when claimants were crowned by armies in the Roman provinces, and the senate's role in legitimising emperors had almost faded into insignificance by the Crisis of the Third Century (235–285). By the end of the third century, Rome's importance was mainly ideological, with several emperors and usurpers even beginning to place their court in other cities in the empire, closer to the imperial frontier. Common methods used by emperors to assert claims of legitimacy, such as proclamation by the army, blood connections (sometimes fictitious) to past emperors, wearing imperial regalia, distributing one's own coins or statues and claims to pre-eminent virtue through propaganda, were pursued just as well by many usurpers as they were by legitimate emperors. There were no constitutional or legal distinctions that differentiated legitimate emperors and usurpers. In ancient Roman texts, the differences between emperors and "tyrants" (the term typically used for usurpers) is often a moral one (with the tyrants ascribed wicked behaviour) rather than a legal one. Typically, the actual distinction was whether the claimant had been victorious or not. In the Historia Augusta, an ancient Roman collection of imperial biographies, the usurper Pescennius Niger