Roman Legions: Structure and Ranks

The Roman army’s primary combat unit was the legion, active during the first three centuries of the empire, with 25 to 34 legions in operation. Each legion comprised approximately 5,000 men, all Roman citizens, recruited from diverse backgrounds, including natives from conquered territories. What are the roles and ranks of the Legion?

Legatus Augusti pro paretore

The highest rank an officer could reach was the legatus Augusti proparetore. The commander of two or more legions and the military governor of a province of the empire. The careers of most senators combined military and political duties, so they were prepared for such a role, but with so few posts only a minority reached such heights.

The Legatus Augusti pro paretore commanded an entire province such as Syria or Britain, and led the army occupying that province. He held the post for an average of three years, but it could be a much longer or shorter period, so some armies lacked consistent leadership.

Legate (Legatus Legionis)

Legions were commanded by a legionary legate (legatus). Legates were assigned by the Senate to the consul, and deriving their authority from him. The Legatus would report directly to the Consul. Caesar had one for each of his legions. This was the first time their duty had been made definite. Aged around thirty, he would usually be a senator on a three-year appointment.

Tribunus Laticlavius – ‘Broad-stripe’ Tribune

The second-in-command to the legate was the tribunus laticlavius or ‘broad-stripe’ tribune. Named for the broad striped toga worn by men of senatorial rank. This tribune was appointed by the Emperor or the Senate and was given this position to learn and watch the actions of the legate. He was from a senatorial family, in his early twenties, and was fulfilling his military service to qualify for public office. He would serve for 3–5 years and return home to run for office.

They often found themselves leading their unit in the absence of a legate, and some legions were permanently commanded by a broad-stripe tribune, such as those stationed in Egypt, as an Augustan law required that no member of the Senatorial Order ever enter Egypt.

Equestrian Positions in a Legion

Praefectus Castrorum Legionis – Camp Prefect

The praefectus castrorum was, in the Roman army of the early Empire, the third most senior officer of the Roman legion after the legate (legatus) and the senior military tribune (tribunus laticlavius), both of whom were from the senatorial class. He was responsible for training, equipment procurement and maintenance, and construction of the camp, but he could command the legion when his seniors were absent.

The post was usually held by a soldier promoted from the centurionate, having already served as a chief centurion (primus pilus) of a legion. A Praefectus Castrorum would have been from the Plebeian class to begin with and would have been promoted to the equestrian class after retiring.

The emergence of direct promotion to praefectus castrorum from primuspilus can be seen in the cursus of P. Anicius Maximus.

Tribuni Angusticlavii – ‘Thin stripe’ Tribunes

In contrast to the broad-stripe tribune, the other five ‘thin stripe’ tribunes were lower in rank, and were called the tribuni angusticlavii. These ‘officer cadets’ were men of equestrian rank who had military experience, and yet had no authority: they were allowed to sit on a court martial but they held no power in battle. Most thin-stripe tribunes served the legionary legate, yet a lucky few (such as Agricola) were selected to serve on the staff of the provincial governor.

According to Tacitus, they did not always take their appointment as seriously as they might, contrasting Agricola’s tribune ship to his peers by saying “[Agricola did not], like many young men who convert military service into wanton pastime, avail himself licentiously or slothfully of his tribunitial title, or use his inexperience to spend his time in pleasures and absences from duty”.

The six tribunes were divided into sets of two. Each set commanded the legion for two months, the two tribunes alternating daily. The four tribunes off duty acted much as quartermasters, commissaries or aides de camp do in modern times. All of them served mounted.

To be sure that each legion should not suffer from the divided command, a legate was put in supreme supervisory control. Later Caesar put him in actual command, and under him the two tribunes on duty probably acted as chief of staff and adjutant general of a modern brigade.

Praefecti – Prefect

In Caesar’s time the term Prefect is ill-defined and consequently loosely applied. It usually means the commanding officer of auxiliaries, slingers, archers, cavalry or infantry organized in cohorts. Some were chiefs of countries furnishing the contingents, and some were Romans. The Roman praefects were, like the tribunes, in some cases young men who had seen little military service. These two grades constituted the lowest “commissioned” officers, and like all officers were distinguished from the men by cuirasses made of gilded bronze metal and shaped to fit the contours of the body.

The usual career path followed three steps – praefecti of an auxiliary infantry cohort, tribunus anticlavius, and then praefecti of a cavalry unit.


The real commanders of the men at drill, on the march or in battle were the men in charge of the centuries, the sixty centurions of each legion. They were, in the main, expert swordsmen and professional soldiers, promoted from the ranks by the Imperator as men of conspicuous sobriety, loyalty, courage, and capacity for leadership. The centurion was identified by the transverse crest of feathers on his helmet.  They may be compared to our higher non-commissioned officers and warrant officers in station and education and to our captains in respect to command. There were different grades or classes in their own hierarchy to and from which they could be promoted. As these different roles show, the term centurion covered a range of different ranks in today’s terms, rather than being what we would recognise as a single role.

The centurion in the infantry is chosen for his size, strength and dexterity in throwing his missile weapons and for his skill in the use of his sword and shield; in short for his expertness in all the exercises.

Vegetius – The Military Institutions of the Romans (De Re Militari)

Being plebeians, they were generally ineligible for advancement to the higher commissioned grades, because of lack of social standing, i.e., either that of equestrian rank or senatorial rank.

Centurion Primus Pilus

Also referred to as the Chief or Master Centurion, this is the pinnacle of the career of a Roman soldier. Though socially subordinate to the Tribunes, the Centurion Primus Pilus possessed more power and influence than any, and was in fact third-in-command of the entire legion. He was also the commander of the elite First Cohort in battle. Upon retirement, a Centurion Primus Pilus (and possibly Centurions of lesser ranks as well) was elevated into the Patrician Class of society. He could then stand for public office, and his sons would be eligable for appointments as Tribunes. Even while still serving in the ranks, a Centurion Primus Pilus was allowed to wear the narrow purple stripe of a Patrician on his toga, such was the respect Roman society held for them.

Centurion Pilus prior

The century was the unit men most identified themselves with, but the cohort of six centuries was the basic battlefield unit of a legion. Commander of a cohort of six centuries, the Centurion Pilus Prior was a man of considerable influence and responsibility. He not only had to be able to command a century on a line of battle, but he had to be able to maneuverer his cohort as a single unit. Such men were often given independent commands over small garrisons or on low-level conflicts. A Centurion Pilus Prior could also be tasked with diplomatic duties, such was the respect foreign princes held for them. At this level, a soldier had to focus not just on his abilities as a leader of fighting men, but on his skills at diplomacy and politics.

Centurion Primus Ordo

The elite First Cohort’s centuries were commanded by the Centurions Primus Ordo. The primi ordines consisted of hastatus posterior, princeps posterior, hastatus prior, princeps prior, and primus pilus in ascending order. Though the number of soldiers under their direct command was fewer, these men were senior in rank to the Centurions Pilus Prior. Men were often selected for these positions based on vast experience and for being the best tacticians in the legion. As such part of the duty of a Centurion Primus Ordo was acting as a strategic and tactical advisor to the commanding general. Generals such as Caesar, Marius, Tiberius, and Agrippa were successful in part because they had a strong circle of First Cohort Centurions advising them.

RIB 341 - Centurial stone of Roesius Moderatus

From the sixth cohort the century of Roesius Moderatus, hastatus prior, (built this).


For hastatus prior see Glossary.


While decurions, as their name implies, may have been originally commanders of decuries, or squads of ten horsemen, they were in actual fact commanders of troops or turmae and, as we should say now, captains of cavalry. Their position in the cavalry was similar to that of the centurions in the infantry.

 A turmae consists of thirty-two men and is commanded by a Decurion.

Vegetius – The Military Institutions of the Romans (De Re Militari)


A first century aqulifer. His armour and helmet are covered in a lion skin, and he also carries a small circular shield. The aquilifer takes his name from the type of standard he carries, the aquila (the ‘eagle’). The aquila was the standard of the legion. It was the item which had to be defended at all cost, as it represented the legion’s honour. Bearing the legion’s most prized possession the aquilifer’s position was of high standing. In fact, part of the aquilifer’s was to be in charge of the legion’s pay chest. He would therefore also be the man to whom the legionaries and officers would entrust their savings.

The chief ensign (signum) of the whole legion is the eagle and is carried by the eagle-bearer [aquilifer]. 

Vegetius – The Military Institutions of the Romans (De Re Militari)

The aquilifer’s position was accordingly one of enormous prestige, and he was ranked immediately below the centurions and above the optiones, he was a Duplicarius, receiving twice the pay of an ordinary legionary.

The Principal Ranks – Principales 

Within the century were three senior officers who were the primary assistants to the Centurion. Principales were much like the senior Non-Commissioned Officers of a modern company, the Principals oversaw the day-to-day operations of the Century. Given their administrative responsibilities, it is generally agreed that a soldier wishing to progress into the Principal ranks would need to be able to read and write. It is not known for certain if Decanii and Principal officers wore crests or other devices on their helmets to denote their rank in the same manner that modern Non-Commissioned Officers wear stripes on their uniforms; however it can be reasonably speculated that they did.


An optio, was a position in a centuria (century) of a Roman army similar to that of an executive officer. The main function of an optio was as an optio centuriae, the second-in-command of a century, although there were many other roles an optio could adopt. The name comes from the word optāre, “to choose” as the position was chosen by the centurion himself. Optios carried wax tablets on which they could write passwords and orders of the day.  He is distinguished by two feathers on his helmet. He was a Duplicarius, receiving twice the pay of an ordinary legionary.

The Optiones are subaltern officers, so denominated from their being selected by the option of their superior officers, to do their duty as their substitutes or lieutenants in case of sickness or other accident.

Vegetius – The Military Institutions of the Romans (De Re Militari)


A signifer was a standard bearer of the Roman legions. He carried a signum (standard) for a cohort or century. Each century had a signifer so there were 59 in a legion. Within each cohort, the first century’s signifer would be the senior one.

The standard had a number of phalarae (disks or medallions) along with a number of other elements mounted on a pole. The pole could be topped with a leaf-shaped spear head or a manus (open human hand) image denoting the oath of loyalty taken by the soldiers. It sometimes included a representation of a wreath, probably denoting an honour or award.

The task of carrying the signum in battle was dangerous, a soldier had to stand in the first rank and could carry only a small buckler. It was that banner that the men from each individual century would rally around. A soldier could also gain the position of discentes signiferorum, or standard bearer in training. If the signifer was lost in battle, the whole unit was dishonoured.

When a detachment was sent out they took a vexillum, a flag type standard. This showed which Legion they belonged to. It was carried by the vexillarius.

Cornicen (Horn Blower)

They worked hand in hand with the Signifer drawing the attention of the men to any new order being signalled by the Centurial Signum and also issuing the audible commands of the officers. The cornicines played the cornu (making him an aeneator). Cornicines always marched at the head of the centuries, with the tesserary and the signifer. The cornicines were also used as assistants to a centurion (like an optio). The cornicen was a duplicary or a soldier who got double the basic pay of the legionary.

The cornets are used only to regulate the motions of the colours; the trumpets serve when the soldiers are ordered out to any work without the colours; but in time of action, the trumpets and cornets sound together.

[post_id=”9505″ anchor=”53-22-the-difference-between-trumpeters-hornblowers-and-the-bugle-call” type=”link”]Vegetius,  De re militari, Book 2, Chapter 22[/link_post]


Octavian Augustus introduced a new military position. – imaginiferi. Imaginiferi, belonging to signiferi (bearing the signs). He carried the Standard bearing the image of the Emperor as a constant reminder of the troop’s loyalty to him. The imago itself was most often three-dimensional and presented the ruler in the form of a bust. It ranged from 25 to 30 cm and was hung on a pole, showing the emperor usually in armour. Imaginiferi brought to the battlefield the image of the one for whom the soldiers fight and die. Also during all military ceremonies, they carried the image of a ruler before whom military oaths were taken.


A tesserarius was a watch commander in the Roman army – the name from from tessera, a small tile or block of wood on which watchwords were written. They organized and had command over fatigue duties and the nightly guard assigned to keep watch over the fort when in garrison or on campaign. He maintained the duty roster and was also keeper of the watch word and seeing that they were kept safe. There was one tesserarius to each centuria. They held a position similar to that of a first sergeant of a company in modern armies and acted as seconds to the optiones. He was a Sesquiplicarius, receiving one and a half times the pay of an ordinary legionary.


The decanus was the first man of an eight man contubernium unit. A normal soldier without formal command he was most likely in charge of various tent or barracks duties. In everyday army life his was most likely a post of no immediate importance, but when the army was on the march and a marching camp needed erecting, the decani would more than likely be a great importance as team leaders in getting the tents unloaded and erected. This insignia of the decanus were the two feathers on his helmet.


Immunis was a soldier immune from common drudgery due to specialist skills such as engineers, artillerymen, musicians, clerks, quartermasters, drill and weapons instructors, carpenters, hunters, medical staff and military police (c. 600 per legion). Attested from the second century AD onwards, the late first century opera vacans may have been an earlier designation for the same position. They could apparently be granted this status both indefinitely and temporarily as one inscription lists an immunis perpetuus.

Immune status within the army was achieved either through selection or through promotion. If not possessing the specialist skills that could see a soldier chosen to become an immune, the legionary who wished to become one would have to undergo a period of specialist training, during which time they would be known as discens. The discens received the same basic pay and board as the non-specialists until he qualified for immune status.


The legionaries were equipped with a pilum (javalin), a gladius (sword), probably a pugio (dagger), and protected with a scrutum (shield), a Montefortino type helmet and a suit of armour, chain armour.

These men were the soldiers that made up the bulk of the legions, liable to perform guard duties, labour work and other less than desired duties.  They were miles gregarius (also known as munifex), a non-specialist regular soldier. Milites would usually have to serve for several years before becoming eligible for training to become immunes.

A tiro was a recent recruit. He was not yet subjected to full rigours of military discipline until he passed out and was registered as a real soldier, no regular pay so presumably living of his enlistment bounty or viaticum.


At the request of the commander, certain courageous and experienced soldiers who had earned distinction because of exceptional ability and loyalty, voluntarily re-enlisted for further active duty after their twenty-year retirement. Such men were known as Evocati. They were of course treated with marked consideration and accorded many special privileges, including a mount while on the march and exemption from all fatigue duty. They were given extra pay and could be promoted directly to the grade of centurion. They helped sustain the morale of the troops and were in general held up as models for all enlisted men to follow.

Engineers and Artificers

In the early Roman army there had been a separate corps of engineers (Fabri), but this organization had been abandoned before Caesars time. Ordnance and engineer work was performed by specially trained legionaries under the direction of men detailed to these tasks because of their skill. Their work included the construction of camps, fortifications, siege works and materiel, bridges, ships and ship repair; the repair of equipment, armor, weapons and artillery, and the building of winter quarters