An ala was a cavalry unit consisting of 500 or 1,000 riders. In order to become a member of an ala, one had to be appointed to the equestrian order, i.e., become a member of the »ordo equester.« Men who owned 400,000 sesterces and who could look back on three generations of free-born ancestors were entitled to membership. However, the Roman Emperor had the last word on this decision, and quite often the aforementioned prerequisites were generously waived. We estimate that the equestrian order counted about 20,000 equites at the time of Emperor Augustus.
When an eques became a member of an ala, the Roman government provided him with a horse, which made him an »eques equo publico,« that is, a »knight with a government-owned horse.« There also existed an inofficial knighthood of »secondary rank« that served the cavalry with their private horses (»equus privatus«), which did not meet all prerequisites. For example, knights of secondary rank were not reserved seats in the first two rows of Rome’s theaters. Access to those seats was the privilege of knights of »proper« rank.
As early as at the time of Caesar, the existing Germanic equestrian units led by Germanic allies had been recruited, but they were not integrated in the Roman army yet. After Augustus’ military reform this changed: Now the Germanic equestrian units were an integral part of the standing army. The alae had a strict hierarchy, and they were commanded by a prefect, often the former centurion of a legion. Those alae, whose members were recruited from allied peoples, often served under the command of their own leaders, as was the case with Arminius and the Cheruscans. Their romanization, brought about by their service in the Roman army, was a desired effect.