Augustus’ guardsmen, the Germani corporis custodes, were ‘picked auxiliary horsemen’. At first chosen from tribal cavalry, they were later no doubt drawn from the regular alae-at least that was how the emperors of the second and third centuries chose their horse guard. The army’s tallest recruits, men six feet and over, enrolled in the alae. Horsemen of the auxiliary cohorts, on the other hand, even though they accounted for a third of all Roman cavalry, were barred from the horse guard by their shorter build and lesser skills.