How did the Legionaries in a battle line receive their commands?

Once the Legions abandoned the phalanx, they spread heavy infantrymen along lines in maniples. If you’ve ever been in a firefight, you know hearing gets suppressed as you duck, cover, and return fire. While the Marine Corps provided radios, the Roman Legions did not have electronic equipment. Yet, they were famous for executing field maneuvers while in contact with an enemy. It is this battlefield communications that interests me.

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Signal flags, trumpet calls, and mounted messengers were available to deliver instructions from the Legion’s command staff to the Centuries on the combat line. However, all three vehicles only carried the orders to the combat officers at the rear of the fighting formation.

Imagine the 80 Legionaries of a Century placed in a two-row formation. On the front row are 5 contuberniums (squads), composed of 8 heavy infantrymen in each unit, including 5 tent commanders called Decani. That puts 40 shields (scutas) and short swords (gladii) on the shield wall, with 40 more Legionaries behind them. The second rank hold shields and uses spears to keep the enemy from coming over the tops of the Legion shields on the front row. We now have all 80 heavy infantrymen in brutal face to face contact with the enemy.

Directly behind the fighting lines, stands a busy Centurion. When the flags signaled, the trumpets rang out, or a mounted courier dashed up with orders, the Centurion received the instructions. But the order, once again, only reached the rear of the fighting formation.

While the combat officer clearly grasped the order, the reality for his infantrymen was completely different. The Legionaries were in a world of pain, disgust, and desperation. Death lurked a sword stroke or spear thrust away. Men on both sides of the shield wall were screaming war cries, urinating, and letting loose their bowels, while striking and blocking with all their strength. How then did an order filter forward from the Centurion to the distracted Legionaries doing the fighting?

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In my opinion, the tent commanders’ voices were the key. Above the din of battle, the Centurion’s voice might penetrate the fighting formation. However, it wouldn’t necessarily reach every single Legionary. But, dispersed along the fighting lines were 10 Decani, the tent commanders, people that were intimately known to the infantrymen.

The Decanus from each contubernium would react to the distinctive voice of their officer and NCOs. Just as the voice of my Sergeant in the Marines cut through the noise of a firefight, the Legionaries would recognize the unique voices of their Decani. In a short span, every Legionary in the Century would hear the order from a trusted voice and execute the command. Thus, directives traveled quickly from the Legion command staff to the Centurion then forward, the final distance, to the men fighting on the battle line.

Combat communications in the Legions must have depended on identifiable and familiar voices. How else would you explain the Legion’s ability to outmaneuver an enemy in an age when the fight was often less than an arm’s length away?