Roman Emperor's legal status and succession

A set of related questions about the Roman emperors:

  1. What was the legal basis for the Emperor’s status? I understand that the first emperor, Augustus, exercised power through a combination of republican forms, primarily by personally holding the offices of tribune and (sometimes) consul and by issuing decrees through the senate. His immediate successors also clothed their authority in republican forms. Was there some point at which the Roman state began recognizing an office of princeps or imperator (or, later, in the Eastern Empire, basileos) as the supreme magistracy, independent of any other title?*

  2. What was the legal basis for imperial succession – that is, exactly what act by what body conferred imperial status upon a claimant? Augustus and his immediate successors “adopted” their heirs, whose claim to the throne therefore derived at least partly through nomination by and inheritance from his predecessor. The senate, the Praetorian guard, and the army also played a role in acknowledging or acclaiming the early emperors: was consent from one of them legally necessary or sufficient? (I understand how the tetrarchy operated, where each augustus served with a caesar as his designated successor, who then nominated his own caesar when he succeeded to the throne as augustus in his own right. I’m really interested in the succession before and after the tetrarchy.)

2A. By the fall of the Eastern Empire, the throne had become almost purely hereditary. Was there some point after which the succession was purely hereditary, and did not depend at all on accession to a title or acclamation by some governmental body?

I realize that I am asking about a period of a millenium and a half, and that all these practices evolved over time. I’m interested in general or particular information about them, from any point in the principate or the imperium, in the Western or Eastern Empire.


  *The analogy that comes to mind is the British prime ministership, which as a distinct office was unknown to the law until the 20th century. Earlier prime ministers held their status by virtue of serving as First Lord of the Treasury.

I rather suspect that the Emperor’s legal status was more or less zilch. There was no “law to create the Emperor” position. Rather, with an overwhelming support of the military and civilian foprces, the Emperor ascended. Heck, some of them were chosen instead by the Imperial guard, which sort of messes with the whole idea of succession…

Augustus became Emperor through force of arms and his wits. Successive Emperors held the position - or lost it - similarly. As the Western Empire descended into chaos it was increasingly common for generals to contest the throne.

From the Wikipedia (