List_of_Roman_army_unit_types

Sub-units of the Roman legion

Before the so-called "Marian reforms", the structure of the legions was as follows:

• Contubernium – The smallest organized unit of soldiers in the Roman Army. It was composed of eight legionaries led by a decanus. When on the march a Legion would often march contubernium-abreast (8-abreast). In the Imperial Legion, ten contubernia formed a centuria.
• Maniple (Manipulus) – The pre-Marian sub-unit of the Roman Legions, consisting of 120 men (60 for the Triarii).
• Legio (Republic) – A legion in the pre-Marian armies consisted of 60 manipuli of infantry and 10 turmae of cavalry. By 250 BC, there would be four Legions, two commanded by each Consul: two Roman legions which would be accompanied by an additional two allied Legions of similar strength and structure. For every Roman Legion there would be an allied Legion.
• Turma – A unit of cavalry in the pre-Marian army, which usually consisted of 32 horsemen.

After the consulships of Gaius Marius, the organisation of the legions would standardise as follows:

• Contubernium – The smallest organized unit of soldiers in the Roman Army. It was composed of eight legionaries led by a decanus. Ten contubernia formed a centuria.
• Centuria – 80 men under the command of a centurion and his optio. Six centuriae formed a cohors.
• Cohors (cohort) – A cohors consisted of 480 men. The most senior ranking centurion of the six centuriae commanded the entire cohors.
• First Cohort (Cohors Prima) – The first cohort was a double strength cohort (consisting of five double-strength centuriae), numbering 800 men (excluding officers). The centurion of its first centuria, the Primus Pilus, commanded the first cohort and was also the most senior centurion in the legion.
• Legio (Imperial) – A legion was composed of nine cohorts and one first cohort. The legion's overall commander was the legatus legionis, assisted by the praefectus castrorum and other senior officers.
• Vexillatio – A temporary task force of one or more centuriae detached from the legion for a specific purpose. A vexillatio was commanded by an officer appointed by the Legatus.

References