During the Roman Republic a "Polybian" legion (c. 300–88 BC) of citizen-levies had a cavalry complement of 300 horse, divided into 10 turmae (squadrons) of 30 men each.
[1] In the Imperial Roman army of the Principate (30 BC – AD 284), a decurion also commanded a cavalry turma of c. 30 men, but now without colleagues.
In common with all soldiers in the imperial army, decurions were long-service professionals, the majority volunteers.
In the imperial period, decurions were no longer conscripted Romans, commoners who were often promoted from the ranks, but could also be members of native tribal aristocracies.
The latter were not only Roman citizens, but also aristocrats, whereas auxiliary decurions were mostly commoners and non-citizens (until AD 212, when all imperial subjects were granted citizenship).