Acies triplex, the triple formation, was a military formation used by the Roman legions of the republic period. It was introduced as a result of the reform of the Roman army in the 4th century BCE (so-called Camillian reform), which transformed the structure of the Roman army from a rigid phalanx to a much more flexible manipulative. Acies triplex was formed according to the quincunx – scheme of a five-element geometric pattern.

Strong and numerous Roman infantry formed a battle line consisting of three ranks:

first and closest enemy lines – hastati

second line – principes

third and last line – triarii, who are often veterans also strong reserve

The basic tactical unit was manipul. The Manipul consisted of 120-150 people (triarii counted half of the size), six ranks of 20-25 soldiers. In the acies triplex array, the manipulators were separated from each other by a gap the width of the manipulate face from the next line. This was to ensure high maneuverability and easy regrouping while maintaining the ability to quickly create a compact line of troops.

Before the first heavy-armed array were velites, i.e. light-armed infantry (javelinists, archers). They were set up in a loose, linear formation. On the flanks there was a ride, grouped in 10 squadrons (turmae), protecting the main army from lap. However, riding was not covered by this tactic.

First, the Vellites went into battle. When velites had already passed the first line, they moved to the second stage, forming a battle line. Then the commander of the rear centuria (posterior) at the head of his people moved first to the left and then forward forming a solid line. The same procedure was also used when velites formed behind the first line to protect the sides of the hastati formation.

At this point, the legion presented a solid, strong battle line ready to fight. When the opponent approached hastati they charged. When they lost to the enemy, the posterior centurii returned to its previous position, creating loopholes. At that time, tired manipulations of hastati withdrew through the gaps for principes. When the first row was hidden behind the second, principes formed, according to an earlier procedure, one strong battle line that charged the enemy. The situation repeated itself, when the second line was losing, which was rare, then the triaria intervened. This situation was described as: “It has come to the Triarii” (res ad triaros redit). When even triaria did not defeat the enemy army, the system allowed to safely leave the battlefield of the entire army.