(193–194) is expressly noted to only be a tyrant because he was defeated by Septimius Severus (193–211). This is also followed in modern historiography, where, in the absence of constitutional criteria separating them, the main factor that distinguishes usurpers from legitimate Roman emperors is their degree of success. What makes a figure who began as a usurper into a legitimate emperor is typically either that they managed to gain the recognition from a more senior, legitimate, emperor, or that they managed to defeat a more senior, legitimate emperor and seize power from them by force. List inclusion criteria Given that a concept of constitutional legitimacy was irrelevant in the Roman Empire, and emperors were only 'legitimate' in so far as they were able to be accepted in the wider empire, this list of emperors operates on a collection of inclusion criteria: Imperial claimants whose power across the empire became, or from the beginning was, absolute and who ruled undisputed are treated as legitimate emperors. From 286 onward, when imperial power was usually divided among two colleagues in the east and west, control over the respective half is sufficient even if a claimant was not recognized in the other half, such as was the case for several of the last few emperors in the west. Imperial claimants who were proclaimed emperors by another, legitimate, senior emperor, or who were recognized by a legitimate senior emperor, are treated as legitimate emperors. Many emperors ruled alongside one or various joint-emperors. However, and specially from the 4th century onwards, most of these were children who never ruled in their own right. Scholars of the later Empire always omit these rulers, but the same is not always applied during the early Empire. For the purposes of consistency, later senior emperors' tenures as co-emperors are not counted as part of their reign. The list also gives all co-emperors their own entry only up to the 4th century. Imperial claimants who achieved the recognition of the Roman Senate, especially in times of uncertainty and civil war, are, due to the senate's nominal role as an elective body, treated as legitimate emperors. In later times, especially when emperors ruled from other cities, this criterion defaults to the possession and control of Rome itself. In the later eastern empire, possession of the capital of Constantinople was an essential element of imperial legitimacy. In the case of non-dynastic emperors after