Roma Victrix: The Principes & Triarii

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Principes (Singular: Princeps) were spearmen, and later swordsmen, in the armies of the early Roman Republic. They were men in the prime of their lives who were fairly wealthy, and could afford decent equipment. They were the heavier infantry of the legion who carried large shields and wore good quality armour. Their usual position was the second battle line. They fought in quincunx formation, supported by light troops.  Principes carried swords, or gladii, instead of spears. Each princeps also carried 2 pila, heavy javelins that bent on impact to prevent them being removed from the victim or thrown back

Triarii (Singular: Triarius) were one 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 good quality equipment. They wore heavy metal armour and carried large shields, their usual position being the third battle line. 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. In battle, triarii formed 10 maniples of 60 men each and made up the third line in the legion behind the front line of hastati and the second line of principes.

Once the battle commenced, velites would gather at the front and fling javelins to cover the advance of the hastati. If the hastati failed to break the enemy, they would fall back on the principes, who now carried swords rather than spears. If the principes could not break them they would retire behind the triarii, who would then engage the enemy —hence the expression rem ad Triarios redisse, "it has come to the triarii"—signalling an act of desperation.

Info paraphrased from Wikipedia