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The Structure and Ranks
of a Roman Legion

Introduction to a Roman legion

The main fighting unit of the Roman army was the legion. During the first three centuries of the empire the army contained between 25 and 34 legions. Each legion was made up of about 5000 men recruited from the citizens. Although the soldiers of the legion were Roman citizens, they were drawn from all walks of life and legions often consisted of natives from conquered countries..

The legion contained within its ranks troops trained and equipped to perform many duties in wartime and peace. Although the vast majority of soldiers served as heavy infantry, other legionaries fought as cavalry, archers or light infantry. Other troops operated artillery such as the ballista, onagar and scorpio The troops were however not solely prepared for combat. Legionaries regularly served as engineers constructing fortifications, roads and bridges. As the legion counted among its complement a vast number of men with special skills, it was in many ways self-supporting. A large part of its military equipment could be produced by craftsmen drawn from the ranks. Soldiers trained as surveyors, engineers and architects ensured that the legion needed little outside help for its building requirements. Administrative duties were performed by other legionaries both within their unit as well as in the community

Each legion carried a number and a name, e.g. Legio X Gemina (the tenth 'twin' or 'double' legion), to which honorary titles like pia fidelis (dutiful and loyal) could be added.

The numbering and naming of units followed no rational pattern. As many of the formations originated in the various armies of the civil wars following the death of Julius Caesar, several legions carried identical numerals or nicknames. Even new legions that were formed were named and numbered according to the fancies and whims of the emperor. The sense of individuality provided by these numbers and titles was reinforced by the use of different unit symbols and signs like bulls, boars or capricorns.

The organization of a legion

Organization of a
Roman Legio Table.
Click thumbnail for full size view.

The strength and organisation of the legions changed over time and was uniform throughout the army. Generally speaking however, the legio was organised in ten cohorts. These cohorts consisted each of three manipuli, literally 'handfuls', which were in their turn subdivided in two centuries or 'hundreds'. These centuries were made up of a number of contubernia or 'tentparties'. Although the name century would seem to indicate a unit of a hundred soldiers, this unit could comprise anything from 30 to over 200 troops. The usual establishment strength however is reckoned to have been 80 men. From the second half of the first century AD in at least some of the legions the first cohort was reorganised in five double strength centuries while the remainder continued to be organised in the old manner.

In addition to the regular organisation of cohortes, manipuli and centuries, there were other sub-units for the equites legionis? ? the legionary cavalry - and the antesignani or lancearii, the legionary light infantry. The exact details of their organisation is still unclear even today. For a variety of duties provisional units known as vexillationes or numeri were formed. The strength and organisation of these provisional units varied greatly and was only in part based on the more regular subdivisions within a legion.

The chain of command within a legion

Each legion was designed to be a self contained team and usually contained 4,200 men; and 300 cavalry. The ranks could increase to 5,000 men if needed. There was a rigid framework and chain of command as shown here.

Legatus legionis (Legate)
First in command

Tribunus laticlavius
Second in command

Praefectus castrorum
Camp commandant and Third in command

Primus pilus
Senior centurion

Centuriones ordinarii
Centurion
There were 10 Centurions
in a legion

Optio
Deputy Centurion
The post of Optio was optional,
depending on the will of the
Centurion to have a deputy

A Cohort (80-100 Troops)
The soldiers
There were 10 cohorts in a legion

This is very much a simplified view of a legion.
For a more detailed breakdown, see the list of ranks below

The sections within a legion

Antesignani	One name for the light cavalry. The other name was Lancearii
Centuriae or Century	A unit of troops with a legion. Mostly 80 strong, but it could actually number anything between 30 to 100 soldiers
Cohort	A unit within the legion. There were 10 cohorts in a legion. The first cohort was twice the size of the other cohorts.
Equites legionis	The light cavalry
Lancearii	One name for the light cavalry. The other name was Antesignani
Legatus legionis (Legate)	The first in command of a legion
Legio	The actual legion itself
Manipuli	A division with a legion. There were three manipuli within a cohort each consisting of two centuries
Numeri	A unit in a legion where the soldiers had special skills. E.g. bridge building, engineering, administration etc.
Vexillationes	A unit in a legion where the soldiers had special skills. E.g. bridge building, engineering, administration etc.

The ranks within a legion

Centurion	A commander of one hundred men. Also know as a Centuriones ordinarii
Centuriones exercitatores	The training officer for the cavalry
Centuriones lanceariorum	The leader of the legionary infantry known as antesignani or lancearii.
Cornicen	The trumpeter. There was one to every century
Cornicularius	An administrative officer within a legion
Discentes signiferorum	A trainee standard bearer
Extraordinarii	Soldiers set aside for special backup services. This force usually contained 20% of the infantry and 33% of the cavalry
Hastaii	These were the best of the best of the legion and stood on the front line in battles
Hastatus posterior	The deputy to a Hastatus prior
Hastatus prior	A high ranking officer within a manipuli
Immunes	A soldier who had special duties and talents and did not perform many of the menial tasks that were expected of an ordinary soldier
Munifex	The lowest rank in a legion. Equivalent to a private in today's armies
Optio	The centurion's second in command
Pilus posterior	The deputy to a Pilus prior
Pilus prior	A high ranking officer witihin a manipuli who also commanded the cohorts
Praefectus castrorum	The third in command in a legion
Primi ordines	The senior centurions in a legion who commanded the first cohort
Primus Pilus	The senior centurion in a legion
Princeps prior	A high ranking officer witihin a manipuli
Principales	A non-commissioned officer
Principes	These were the second best of the best of the legion and stood on the second line in battles behind the Hastaii
Princeps posterior	The deputy to a Princeps prior
Signifer	The standard bearer for the century in a legion
Singulares	The auxiliary soldiers
Trarii	Experienced soldiers, who used the same weapons as the Hastaii and Principes, but instead of a pilum, they had a long spear.
Tribunus	One of six assistants to a legion commander - the Primus Pilus. Usually a young senator learning the basics for his career
Tribunus laticlavius	The second in command in a legion
Velites	The light soldiers. Unlike the Hastaii, they did not have full armour, although they did have a helmet and a shield. They were also armed with a sword, but had a only a short javelin as opposed to a pilum.