What is "triarii"

Triarii (singular: Triarius) were one of the elements of the early Roman military Manipular legions of the early Roman Republic (509 BC – 107 BC). They were the oldest and among the wealthiest men in the army, and could afford high quality equipment. They wore heavy metal armor and carried large shields, their usual position being the third battle line. They were equipped with spears.

During the Camillan era, they fought in a shallow phalanx formation, supported by light troops. In most battles triarii were not used because the lighter troops usually defeated the enemy before the triarii were committed to the battle. They were meant to be used as a decisive force in the battle, thus prompting an old Roman saying: 'It comes down to the triarii' (res ad triarios venit), which meant carrying on to the bitter end.