Triarii (plural) were one of the elements of the early Roman military manipular legions of the early Roman Republic (509BC – 107BC). 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 and were elite soldiers among the legion …
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:
Res ad triarios venit. It comes down to the triarii.