The following list details the different ranks of the Imperial Roman army and what duties they would have performed.
Senior Officers of a Legion
Imperial Legate (legatus Augusti pro praetore): The commander of two or more legions, who also served as the governor of the province in which the legions he commanded were stationed. He was of Senatorial rank and was appointed by the Emperor for a term of 3 or 4 years.
Legion Legate (legatus legionis): The commander of a legion. He was of Senatorial rank and was appointed by the Emperor for a term of 3 or 4 years, although he could serve for a much longer period.
Broad-Stripe Tribune (tribunus laticlavius): He was second in command of a legion. Named for the broad striped tunic worn by men of senatorial rank, he was appointed by the Emperor or the Senate. Though generally young, he was more experienced than the tribuni angusticlavii.
Camp Prefect (praefectus castrorum): The Camp Prefect was third in command of a legion. He was a career centurion soldier, who had previously served as primus pilus. He was used as a senior officer in charge of training a legion. He was of a lower social rank thank the tribunes but outranked them within the army.
Narrow-Stripe Tribunes (tribuni angusticlavii): Each legion had five lower ranking tribunes, who were normally from the equestrian class and had at least some years of prior military experience. They often served the role of administrative officers. This tribunate was often the first step in the cursus honorum for young Romans.
Each legion had 59 or 60 centurions, one to command each century of the 10 cohorts. They were the backbone of the Roman army and were career soldiers promoted from the ranks, who ran the day-to-day life of the soldiers and issued commands in the field.
First Spear (primus pilus): The primus pilus was the commanding centurion of the first century of the first cohort and the most senior centurion of a legion. The primus pilus could be promoted to praefectus castrorum. On retirement, he would most likely gain entry into the equestrian class.
primi ordines: They were the five centurions of the first cohort and included the primus pilus. They outranked all centurions from other cohorts.
pilus prior: A centurion in command of the first century of a cohort, making him the senior centurion of the cohort. During a battle, the pilus prior was in command of his cohort. They would have been veteran centurions, who had been promoted through the cohorts.
pilus posterior: The second centurion in a cohort.
princeps prior: The third centurion in a cohort.
princeps posterior: The fourth centurion in a cohort.
hastatus prior: The fifth centurion in a cohort.
hastatus posterior: The sixth centurion in a cohort.
aquilifier: He was the legion’s standard-bearer, who carried the eagle standard (aquila). It was an enormously important and prestigious position. Losing the aquila was considered the greatest dishonour a legion could endure. This post was therefore given to trustworthy veteran legionary.
cornicen: He was in charge of signalling salutes to officers and sounding orders to the legion with his brass horn.
evocati: They were legionary veterans, who had earned their retirement but had chosen to re-enlist.
imaginifer: He was the standard-bearer of the Emperor’s image, which was carried as a reminder of the legion’s loyalty.
immunes: They were legionaries who possessed specialised skills, which qualified them for better pay and excused them from labour and guard work. Engineers, artillerymen, musicians, clerks, quartermasters, drill and weapons instructors, carpenters, hunters, medical staff and military police are all examples of those who were immunes.
optio: He was appointed by the centurion from within the ranks to act as his second in command. An optio was stationed at the rear of the ranks to keep the troops in order. His duties would include enforcing the orders of the centurion, taking over the centurion’s command in battle should the need arise, supervising his subordinates and a variety of administrative duties.
signifer: He was the standard-bearer of a century, who carried the centurial signum, a spear shaft decorated with medallions and topped with an open hand to signify loyalty, which was a rallying point for the soldiers. The signifer also assumed responsibility for the financial administration of the century.
tesserarius: He organised and had command over the nightly guard assigned to keep watch over the fort when in garrison or on campaign and was responsible for getting the watchwords from the commander and seeing that they were kept safe. The tesserarius also acted as a second in command to the optio and would also have a variety of administartive duties.
Officers of the Auxilia
Cavalry Commander (praefectus equitum): He was in charge of a cavalry wing (ala) of about 480 troopers.
Cohort Commander (praefectus cohortis): He was in charge of an auxiliary cohort (cohors) of about 480 soldiers.
Decurion (decurio): He was in charge of a squadron (turma) of cavalry of about 30 troopers. Although the majority served in the Auxilia, there was a small contigent of cavalry attached to each legion.