This article relies largely or entirely on a single source. (January 2009)
The origin of the name is very ancient: it used to be the commander of the first maniple of third line (triarii) of early republican Roman armies. A common, but wrong translation of "primus pilus" is "first spear", however a much more accurate translation would be "first pillar" since the word "pilus", sometimes confused with "pilum" or javelin, is a specific term to indicate the weapon of the triarii (a spear, not a javelin).
In the late Roman republic, the cohort became the basic tactical unit of the legions. The cohort was composed of five to eight centuries, each led by a centurion. The senior centurion of the legion and commander of the first cohort was called the primus pilus; he was a career soldier and advisor to the legate. While every normal cohort was composed of five to eight centuries, the one that was led by the primus pilus had about ten centuries, 800 men. It also had around 200 non-combatant staff, such as cooks and clerks. In modern infantry terms, primus pilus would be considered a lieutenant colonel in relation to battalion-size units, though there is no direct equivalent. The primus pilus would remain in command for one year, although after his term was finished he could continue to serve in the army if there was a vacancy in command or if he wished to become an independent commander of an auxilia unit or praefectus castrorum.
During the Roman Empire, emperor Claudius created the office of primus pilus iterum. This officer would be a former tribune in the vigiles, cohortes urbanae, or Praetorian Guard. After he had served as a tribune in those units, he would be transferred to a legion. The primus pilus iterum would hold the responsibility of a Praefectus castrorum but with higher pay.
The primus pilus was a well paid position. He would accumulate enough wealth to become part of the Equestrian class. However, it did not matter whether or not the Primus Pilus would become wealthy enough to qualify as an Equestrian, because a Primus pilus would gain Equestrian status after retiring.[clarification needed]
Only eight officers in a fully officered legion outranked the primus pilus: The legate (l?g?tus legi?nis), commanding the legion; the senior tribune (tribunus laticlavius); the Camp Prefect (praefectus castrorum); and the five junior tribunes (tribuni angusticlavii).
The primus pilus centurion had a place in the war councils along with the military tribunes and the Legatus of the legion.
Centurions commonly had similar homes to the cives Romani (full Roman citizens), but the higher ranked centurions such as the primus pilus often had larger-than-usual homes. Otherwise, they were treated as ordinary Cives Romani. They had the right to vote, own property, marry, and hold office.