PRAEFECTUS PRAETŌRĬO, was the commander of the troops who guarded
the emperor’s person. [PRAETORIANI.] This office was instituted by
Augustus, and was at first only military, and had comparatively
small power attached to it; but under Tiberius, who made Sejanus
commander of the praetorian troops, it became of much greater
importance, till at length the power of these praefects became only
second to that of the emperors. From the reign of Severus to that
of Diocletian, the praefects, like the vizirs of the east, had the
superintendence of all departments of the state, the palace, the
army, the finances, and the law: they also had a court in which
they decided cases. The office of praefect of the praetorium was
not confined to military officers; it was filled by Ulpian and
Papinian, and other distinguished jurists. Originally there were two
praefects; afterwards sometimes one and sometimes two; from the time
of Commodus sometimes three, and even four. They were, as a regular
rule, chosen only from the equites; but from the time of Alexander
Severus the dignity of senator was always joined with their office.