Cohort (military unit)
The Roman army replaced the maniple with the cohort during the late second century BC. This shift created a standard unit of 480 soldiers for early imperial legions. Ten cohorts made up a single legion, totaling about 5,000 men by the middle of the third century AD. Each cohort operated independently to offer flexibility and maneuverability on the battlefield. The first cohort held experienced legionaries while the tenth cohort contained less experienced troops. A legionary cohort consisted of six centuries, each containing 80 legionaries. Early in the Republic, these centuries numbered 100 men before shrinking to their later size.
A centurion commanded each century within the cohort structure. Six centurions led the six centuries, increasing in seniority from the sixth to the first. The pilus prior commanded the entire cohort overall. Four junior officers assisted every centurion in daily operations. An optio served as the second-in-command and received double the standard pay rate. A tesserarius exercised command over nightly guard duties and distributed watchwords for the camp. Signifers carried the century's standard into battle and acted as treasurers for the unit. Cornicens equipped with cornu horns served as signallers for communication across the ranks. These officers maintained their own support staff, tents, and mules separate from the common soldiers.
Eight men formed a contubernium group that shared a single A-frame tent together. A decanus led this small group and was likely the longest-serving legionary among them. One to two pack mules carried heavier equipment and the mill for grinding grain rations. Camp slaves tended to the mules and ensured water availability during marches. These slaves also handled cooking, washing, and specialized skills like smithing or carpentry. Younger legionaries fetched fodder, firewood, and water while encamped. A full cohort included 60 to 120 slaves providing logistical support alongside 60 to 120 mules. The system required careful management of resources to sustain marching armies.
The first cohort consisted of five double-strength centuries totaling 800 men instead of 480. This unit held unique honor as its aquilifer carried the legion's eagle standard into battle. Loss of an eagle represented extreme shame and disgrace for any Roman legion. An imaginifer within the first cohort carried a three-dimensional relief image of the reigning Emperor. The commander of the first century in this unit held the title primus pilus. This senior centurion became eligible for promotion to praefectus castrorum or camp prefect. The first cohort ranks were filled with veteran troops or the best recruits available.
Vegetius documented that each century possessed a small torsion-powered ballista called a scorpio. Six scorpios equipped every cohort for use during fixed positions or sieges. A contubernium group operated these weapons when required by commanders. Each cohort also contained one onager stone-throwing catapult transported fully assembled on ox-drawn carts. These machines ensured immediate readiness upon arrival at a siege site. Additional equipment like rams, towers, or larger ballistae appeared as needed for specific battles. A full legion maintained an artillery train of approximately sixty scorpios and ten onagers according to ancient records.
Cohortes urbanae served as law enforcement officers patrolling the capital city under the urban prefect. Cohortes vigilum formed fire brigades led by the prefect of the watchmen within Rome. Nine cohortes praetoriae functioned as imperial guards without grouping into a standard legion structure. Cohors alaria units included allied or auxiliary forces distinct from regular legions. Cohors classica units originally formed from sailors and marines for naval support roles. Some paramilitary corps consisted of one or more cohorts outside the traditional legion framework. The term cohors described large companies of people in looser contexts beyond military definitions.
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Common questions
When did the Roman army replace the maniple with the cohort?
The Roman army replaced the maniple with the cohort during the late second century BC. This shift created a standard unit of 480 soldiers for early imperial legions.
How many men were in a full Roman legion by the middle of the third century AD?
Ten cohorts made up a single legion, totaling about 5,000 men by the middle of the third century AD. Each cohort operated independently to offer flexibility and maneuverability on the battlefield.
Who commanded each century within the Roman cohort structure?
A centurion commanded each century within the cohort structure. Six centurions led the six centuries, increasing in seniority from the sixth to the first.
What was the size of the first cohort compared to other cohorts?
The first cohort consisted of five double-strength centuries totaling 800 men instead of 480. This unit held unique honor as its aquilifer carried the legion's eagle standard into battle.
Which officers assisted every centurion in daily operations within a Roman cohort?
Four junior officers assisted every centurion in daily operations. An optio served as the second-in-command and received double the standard pay rate while a tesserarius exercised command over nightly guard duties.