When the army was marshalled according to this arrangement, the spearmen first commenced the fight.
If the spearmen were unable to repulse the enemy, they retreated leisurely, and were received by the principes into the intervals of the ranks.

The fight then devolved on the principes; the spearmen followed.
The Triarii continued kneeling behind the ensigns, their left leg extended forward, holding their shields resting on their shoulders, and their spears fixed in the ground, with the points erect, so that their line bristled as if enclosed by a rampart.

If the principes also did not make sufficient impression in the fight, they retreated slowly from the front to the Triarii.

Hence, when a difficulty is felt, "Matters have come to the Triarii," became a usual proverb.

The Triarii rising up, after receiving the principes and spearmen into the intervals between their ranks, immediately closing their files, shut up as it were the openings; and in one compact body fell upon the enemy, no other hope being now left: that was the most formidable circumstance to the enemy, when having pursued them as vanquished, they beheld a new line suddenly starting up, increased also in strength.