The principes formed the second line of battle in the Republican Roman Army. They were experienced soldiers, positioned behind the hastati and in front of the triarii. Like all soldiers of the Roman Republican Army, they wore what armor they could afford. Like the hastati, the Principe were also equipped with a pilum and a gladius.

In battle, the principes were meant to counter attack if the hastati happen to fail in the initial engagement. Originally, the principes were organized like the hastati, in centuries of 60 under a Centurion. However, by the late Republican era, they contained 80 men like the triarii. Two centuries comprised a maniple and 10 maniples were used in battle line formation.