men, equally split between legionaries and auxiliaries (25 legions and c. 250 auxiliary regiments). The numbers grew to a peak of about 450,000 by 211 (33 legions and c. 400 auxiliary regiments). By then, auxiliaries outnumbered legionaries substantially. From the peak, numbers probably underwent a steep decline by 270 due to plague and losses during multiple major barbarian invasions. Numbers were restored to their early 2nd-century level of c. 400,000 (but probably not to their 211 peak) under Diocletian (r. 284–305). After the empire's borders became settled (on the Rhine-Danube line in Europe) by 68, virtually all military units (except the Praetorian Guard) were stationed on or near the borders, in roughly 17 of the 42 provinces of the empire in the reign of Hadrian (r. 117–38). The military chain of command was relatively uniform across the Empire. In each province, the deployed legions' legati (legion commanders, who also controlled the auxiliary regiments attached to their legion) reported to the legatus Augusti pro praetore (provincial governor), who also headed the civil administration. The governor in turn reported direct to the emperor in Rome. There was no army general staff in Rome, but the leading praefectus praetorio (commander of the Praetorian Guard) often acted as the emperor's de facto military chief-of-staff. Legionary rankers were relatively well-paid, compared to contemporary common labourers. Compared with their subsistence-level peasant families, they enjoyed considerable disposable income, enhanced by periodic cash bonuses on special occasions such as the accession of a new emperor. In addition, on completion of their term of service, they were given a generous discharge bonus equivalent to 13 years' salary. Auxiliaries were paid much less in the early 1st century, but by 100 AD, the differential had virtually disappeared. Similarly, in the earlier period, auxiliaries appear not to have received cash and discharge bonuses, but probably did so from Hadrian onwards. Junior officers (principales), the equivalent of non-commissioned officers in modern armies, could expect to earn up to twice basic pay. Legionary centurions, the equivalent of mid-level commissioned officers, were organised in an elaborate hierarchy. Usually risen from the ranks, they commanded the legion's tactical sub-units of centuriae (c. 80 men) and cohorts (c. 480 men). They were paid several multiples of basic pay. The most senior centurion, the primus pilus, was elevated to equestrian rank upon completion of his single-year term of office. The senior officers of the army, the legati