The ala was composed entirely of cavalry. The name is Latin for wing and derives from the use of horsemen on the flanks of an army, where they gave protection to the infantry centre. When necessary they could deliver flank attacks themselves and thereafter deploy against retreating enemy or attempt to divert pressure on a withdrawal of their own side. The large-scale employment of cavalry serving as auxilia was in large measure due to Caesar following his experience in Gaul. His Gallic units were originally led by their own chiefs and it seems probable that their internal organization was left to the commanders and local custom. Eventually the units became organized into troops (turmae). The alae were normally of a strength of roughly five hundred (quingenaria), but there were a few of the strength of roughly a thousand (millaria).
By the second century the ala quingenaria were arranged into section of sixteen turmae, whereas the ala millaria were partitioned into twenty four turmae. So a turmae was not necessarily always of the same size.
The ala was open to all. Normally only non-citizens were recruited, as the ala was understood to be a auxiliary force. But there was no bar on citizens entering who preferred the life of a horseman to that of a legionary.
At first the commander of an ala would have been a chief of his tribe taking his rightful place at the head of his troops. As the system became rationalized in the first century AD this command became a stepping stone in the career of young equestrians, i.e. men of the Roman knightly class.
The three main military commands (called the tres militiae) in this career were:
In the early empire, the men who aspired to these positions were mostly young equestrians or ex-centurions from the legions. The chief centurion of a legion, the primus pilus, automatically qualified for equestrian status and he could then obtain an independent military command in the auxilia, if he so wished.
The troop commander was the decurio. A man promoted to this position could come from the lower ranks of the legion, since this was an accepted step from the ranks into the legionary centurionate.