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A cohort (from the Latin cohors, plural cohortes, see wikt:cohors for full inflection table) was a standard tactical military unit of a Roman legion, though the standard changed with time and situation, and was composed of between 360-800 soldiers. A cohort is considered to be the equivalent of a modern military battalion. The cohort replaced the maniple following the reforms attributed to Gaius Marius in 107 BC. Shortly after the military reforms of Marius, each legion formed 10 cohorts. The cohorts were named "first cohort," "second cohort" etc. The first cohort gathered the most experienced legionaries, while the legionaries in the tenth cohort were the least experienced. Until the middle of the third century AD, 10 cohorts (about 5000 men total) made up a Roman legion.
Originally, a cohort consisted of six centuriae, each commanded by a centurion assisted by junior officers. At various times prior to the reforms, a century might have 100 men. The cohort had no permanent commander; it is assumed that in combat the most senior centurion of the six would have commanded the entire cohort. In order of seniority, the six centurions were titled hastatus posterior, hastatus prior, princeps posterior, princeps prior, pilus posterior and pilus prior. The first centurion of the first cohort was called primus pilus.
During the reforms in the 1st century AD, the command structure and make-up of the legions was formally laid down, in a form that would endure for centuries. Standard centuriae consisted of 80 men each. The first cohort was made up of five double-strength centuries (160 men). The centurion of its first century automatically was the most senior in the legion was known as the primus pilus. The primus pilus could be promoted to praefectus castrorum, (camp prefect). The praefectus castrorum was in charge of the daily running of a legion.
These ranks followed the order of seniority in the earlier manipular legions, where the youngest and least experienced units were termed hastati, next principes, and the oldest and most experienced triarii (pilus was a rare alternative name for triarius, the singular of triarii).
Some paramilitary corps in Rome consisted of one or more cohorts, though none were part of a legion:
Furthermore, the Latin word cohors was used in a looser way to describe a rather large "company" of people (see, for instance, cohors amicorum).
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