The triarii are the third (tri =three) and last line available in the republican legions, organized into ten maniples of 160 men, each divided into two centuries of 80 soldiers.
They are soldiers with heavier armament than simple legionnaires, and of better quality. They possess a long lance, a large curved shield, the ancestor of the scutum, as well as a sword, but are shaped like a phalanx, which makes them suitable for repelling cavalry charges.
They were the oldest and most experienced soldiers, with more financial means to pay for their equipment. They therefore formed the last line of soldiers, and remained in the background most of the time during the battle, sometimes contenting themselves with guarding the camp. The use of the triarii was synonymous with the defeat or failure of the first two ranks of heavy infantry of the republican legion, the hastati and the principles, therefore a difficult if not hopeless situation. The first two ranks then regrouped around the powerful phalanx and attempted a final counter-attack or prepared to delay the enemy to facilitate the retreat.