COMMAND STRUCTURE
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SENIOR COMMAND STRUCTURE OF THE LEGIONS (1ST & 2ND CENTURIES A.D.)

Legatus Legionis

The legati Augusti legionis (legionary commanders) were senators, ex-praetors around thirty years of age who typically served a 3
(sometimes up to 7)  year tour of duty.
Legati Augusti pro-praetore governed Imperial provinces. Those provinces with one legion were
commanded by ex-praetors (who also acted as legionary COs) and those with more by ex-consuls. The latter commanded all of the
provincial legates and their troops (i.e., Britain had four legions).

Tribunus Laticlavius

The second-in-command, a tribune and a senatorial designate around the age of 25, served for one year. Although command
‘technically’ defaulted to the
Tribunus Laticlavius in the absence of the Legatus, it is more probable that the Praefectus and Tribuni
Angusticlavii
(see below) assumed control.

Tribuni Angusticlavii

Each legion had five Tribuni Angusticlavii, equestrian tribunes, who served 3-4 years and then assumed command of a cavalry ala.
They were generally young men (former magistrates and/or auxiliary
cohors peditata/equitata commanders). During the Republic
each tribune commanded two cohorts but in the Principate they occupied staff positions with an administrative/judicial focus.
Tacitus reported that they monitored the performance/reliability of the centurions. Their staff (
officium) was comprised of clerks
(
cornicularii and secutores) without military responsibilities who recorded casualties, maintained current lists of men serving,
processed furlough applications and dispensed discharges to retiring veterans. Tribunes often commanded vexillations, detachments
of one or more cohorts on special assignments.

Tribuni Semestres were part of H.Q. staff and served 6 months before resuming civilian life. Following the Flavian era they were placed
in direct command of the legion’s 120 man cavalry contingent (four
turmae of 30 horsemen). The cavalrymen were ranked as non-
combatants along with the headquarters staff. They were attached to specific centuries (as opposed to forming individual units) and
served as messengers and scouts.

Praefectus Castrorum

The third-in-command was an equestrian,  a former head centurion in his late 50s with 30 plus years of experience. He was basically a
quartermaster responsible for choosing camp sites and overseeing their construction inclusive of entrenchments. He inspected tent
lines in temporary camps. During war he supervised the legion’s baggage train, and if siege operations were anticipated he managed
the battering rams and ammunition supplies. In permanent installations he oversaw the construction of barracks-blocks, internal
buildings, wells and aqueducts.

He was the officer in command of the doctors (
medici), the surveyors (mensores) and the horologiarius, the man responsible for the
camp clock. He oversaw the supply of construction and surveying equipment, furnishings, fabrica supplies (inclusive of wood, iron and
coal) and medical equipment for the legion’s hospital (
valetudinarium).

Primus Pilus

The fourth-in-command was the senior centurion and leader of the 1st century of the 1st cohort. He held his post for one year. Parker
believes that there were two primi pili, one who commanded troops in the first cohort and one who served in the Legion’s headquarters.

Princeps

The centurion commanding the second century of the first cohort was next in line, and he was responsible for the headquarters’ staff as
well as  training. Ranked below him were the remaining centurions of the first cohort in the following order -
hastatus, princeps
posterior
and hastatus posterior.

THE CENTURIONATE AND BELOW (1ST AND 2ND CENTURIES A.D.)

Aquilifer

He carried the legion’s silver (subsequently gilded) eagle and ranked just below a centurion. He was entrusted with the legion’s pay
chest.

Principales and Immunes

Below the rank of centurion men were designated as either principales or immunes. Principales assisted the centurions and were
exempt from normal fatigues (
optio, signifer and tesserarius). Immunes, men who were excused from regular fatigues by virtue of
special skills (blacksmiths, carpenters, surveyors, medical assistants, etc.),  comprised almost 20% of each unit.

Centurion

Centurions in cohorts 2-9 were equal in rank with seniority determined by years of service. They were recognized by their transverse
helmet crests, swords worn on the left and their swagger sticks (
vitus) with which they dispensed discipline. They typically rode when a
legion was on the march. Centurions tended to die in service as opposed to retiring to civilian life.

Signifer

The standard bearers handled the paperwork at century level. Vegetius says that the garrison was managed by the signiferi who were
directly responsible for finances. They recorded financial transactions, issued receipts, documented account activity and dispensed pay.

Optio

He was a ‘chosen man’, a centurion-in-training who assumed field command if the unit's centurion was incapacitated or killed. He
carried a staff of office known as a
hastile.

Tesserarius

The tesserarius controlled the daily watchword and was responsible for sentries and other fatigues.

Custos Armorum

He was responsible for centurial equipment, repairs, and weapons.

Decanus

A century was composed of 10 mess units (contubernia) of six to ten men who shared a tent and a mule; each was commanded by a
decanus.

Miles

The basic foot soldier served for 20-25 years (depending upon the time period).