a1 Wolfson College, Oxford
For a space of forty years, from 98 to 138, few praefecti occur on named attestation. In fact only six : two registered in isolation, four in collegiate pairs. Apart from seasons of disturbance, the holders of this useful and necessary office tend to evade notice in written history.
It was not until 2 B.C. that Caesar Augustus appointed commanders of the Praetorian cohorts, so Cassius Dio states. The occasion might excite curiosity: perhaps after the grave crisis in the autumn. The historian adds no comment. He had previously put emphasis on the dangers inherent in a single prefect. Not perhaps clear at first—and Seius Strabo was in sole charge when Augustus died. Named associate with his parent by Tiberius Caesar, Aelius Seianus managed almost at once to have him sent away to be governor of Egypt.