The emperors listed in this article are those generally agreed to have been 'legitimate' emperors, and who appear in published regnal lists. The word 'legitimate' is used by most authors, but usually without clear definition, perhaps not surprisingly, since the emperorship was itself rather vaguely defined legally. In Augustus' original formulation, the "princeps" was selected by either the Senate or "the people" of Rome, but quite quickly the legions became an acknowledged stand-in for "the people." A person could be proclaimed as emperor by their troops or by "the mob" in the street, but in theory needed to be confirmed by the Senate. The coercion that frequently resulted was implied in this formulation. Furthermore, a sitting emperor was empowered to name a successor and take him on as apprentice in government and in that case the Senate had no role to play, although it sometimes did when a successor lacked the power to inhibit bids by rival claimants. By the medieval (or Byzantine) period, the very definition of the Senate became vague as well, adding to the complication.