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 The praefectus praetorio was in origin the commander of the Emperor's bodyguard. This corps d'élite, which even in the Republic had grouped itself round a commander in the field, was given a definite existence and organisation in the year 28 , and became the police of Italy, the selected home force composed, unlike the legions, mainly of Italian citizens, and the protector, often the transmitter, of the throne. Its praefects at this early stage represent the military character of the despotism perhaps more purely than any other officials, and even the reign of the second Caesar could show in Sejanus one of the most formidable of those praefects who were almost partners of the throne. The danger threatened by the office illustrates its power, and this was recognised when Vespasian sought security by giving the praefecture to his own son Titus, or Severus married his elder son to the daughter of his praefect Plautianus. A more favourite method was to increase the number of its holders. Two were frequently appointed, and three are found on two occasions since the time of Commodus. Gradually the military functions of the office ceased to be the most important, although its military history had determined its character. The praefect of the guard had always been the man who stood next the throne; he was a truer alter ego of the Princeps than the praefect of the city, for his activity was not confined to Rome and Italy. It was he who issued rapid injunctions for the organisation of the army or for the guidance of the civil service throughout the Empire, and at times we find two praefects, such as Adventus and Macrinus in the reign of Caracalla, representing respectively the military and civil spheres. But jurisdiction, the most constant of the Emperor's cares, and the framing of legal decrees, also demanded the attention of the praefect, and hence it was necessary to entrust the office to the first jurists of the Empire. Papinian, Ulpian, and Paulus were all praefects of the guard. The change in the character of the office perhaps began with Hadrian; it was carried on during the reigns of the Antonine Emperors, and finally achieved in that of Septimius