A 'Decurion' is a cavalry officer in command of a troop or Turma of thirty men in the army of the Roman Empire. The Roman Empire is the term conventionally used to describe the Ancient Roman polity in the centuries following its reorganization under the leadership of Octavian (better known as Caesar Augustus). ...

There were 120 cavalry in an Ala (auxilary cohort) of a legion. There were 4 decurions, each commanding thirty men. (adhering to 43 a.d under Emperor Claudius)

In the infantry, the rank carried less prestige. A 'Decurion' only lead a squad called a Contubernium or "Tent Group" of 8 men. But they were still allowed to carry a Pugio or dagger as a sign of their rank. A pugio is a dagger used by Roman soldiers. ...