Praetorian Prefect. Prefect (praefectus) was a term used to describe someone placed in authority through appointment by a higher official. In the case of the Praetorians, an equestrian had been appointed to command Augutus’ elite bodyguard. The Praetorian Prefect was nominated out of personal loyalty rather than military expertise, though in practice most prefects had been equestrian military officers. The prefect was the only official allowed to wear a sword in the presence of the Imperator. The trusted praetorian prefects quickly became more than just a commander of palace guards; they effectively functioned as the chief-of-staff of the imperial bureaucracy. To this was soon added important judicial powers. The Praetorian prefect became the pinnacle of an Equestrian career.
In the late empire the praetorian guards had been replaced with another agency, and the prefects were disowned of their police powers. Instead their members were promoted through the civil branches of the empire. They were charged with administrative, financial and judicial duties. There was now more than one praetorian prefect (often four or five), each in charge of a particular sector of the Empire. The prefects functioned as regional prime ministers of the imperial court. Their deputies (vicarii) served as a link between the provinces and the center.
Vicarius. Under the Principate, a vicarius was a substitute for a deceased or absent provincial governor. Under the Dominate, each Diocese had a Vicarius who was the deputy of a Praetorian Prefect, trusted with supervising the local governors. The Emperor still reserved the right to intervene directly in a province.
Praesides. In the reorganized provincial system, praesides were now the provincial governors, having replaced the proconsuls, propraetors, and procurators of the Principate. The chief duties of the Praesides and their staff officers were to collect taxes and uphold Roman law. They were supervised by the Vicarii of the Praetorian prefects. In most provinces the praesides were stripped of military powers; the military command of a province was now handled by a dux.
Praefectus Urbi. Prefect of the City. In the days of the Republic the praetors had been junior officials to the Consuls. The senior of these, the Praetor Urbanis, was charged with overseeing the judicial affairs of the city of Rome. Beginning with Augustus, urban affairs were now handed by a specially trusted prefect. The Praefectus Urbi was the highest ranking Senator in Rome, the deputy of the Imperator, who presided over the Senate. He commanded the urban cohorts, presided over courts of law, and supervised the various officials charged with the city’s infrastructure needs. Later in the empire Rome ceased to be the effective political capitol, but Constantine appointed a prefect for Constantinople.
Magister Officiorum. The Master of offices, the effective head of the central bureaucracy, ranking just beneath the Praetorian Prefects. The Master of Offices may have been an invention of Diocletian, but does not become paramount until Constantine. The Master of Offices controlled the postal service and the imperial couriers. He supervised the departments responsible for maintaining the empire’s various legal and financial documents. He formally commanded the scholae, the new imperial guards who had replaced the praetorians. He controlled state-run weapons factories, and was in charge of inspecting border defenses. Finally, he seems to have had supervised all the other affairs of the imperial court ranging from the necessary (diplomats and interpreters) to the trivial (doorkeepers and ushers of the imperial palace).
Comites. “Companions.” Singular: comes. When the Princeps found it necessary to leave Rome and tour the provinces, the leading members of the retinue had been termed companions. In the later empire, the imperial court (comitatus) was often outside Rome and attached to a strategic center closer to the frontier. Headquartered within these strategic centers was the mobile field army (comitatenses), whose purpose was to deal with threats that had broken through the garrison defenses (limitanei). Diocletian had begun using comites to describe the cavalry officers in his mobile field army. Later, Constantine officially bestowed the title of comes (Count) in three grades to the senior members of his court, and was used especially for those who commanded small detachments of the mobile field army. The three grades of “Count” quickly replaced the long-standing distinction between Senator and Equestrian as a practical ranking system.
Magister Militum. “Master of Soldiers.” Constantine enlarged the mobile field army from the forces left by Diocletian. While small detachments of these units were formally under the command of a comes, larger detachments were under the control of magistri militum. The magistri militum was a collective term for two different offices: the magister peditum (infantry commander) and magister equitum (cavalry commander). Splitting the command in this manner not only recognized the increasing importance of the cavalry, but also ensured there was no single all-powerful general to threaten the Dominus. Nonetheless, weak emperors could find themselves under the thumb of an ambitious Magister Militum.
Praefecti. Praefectus, one who has been appointed, was the generic title used for military commanders of defensive installations and their attachments. It was also the title of naval fleet commanders.
Dux. “One who leads” whence the English “Duke” is derived. Once referring to a general who commanded a detachment of troops, Diocletian used it to refer to military commanders of frontier provinces. Constantine further stripped the praesides of their military command (with a few exceptions), and the dux was now the senior commander of any military forces stationed in a given province.
These articles were written by forum member Ursus
Hornblower and Spawforth. The Oxford Classical Dictionary.
Peter Heather. The Fall of the Roman Empire.
Christopher S. Mackay. Ancient Rome: A Political and Military History.