The Roman legion was a highly disciplined, well-trained, and heavily armed body of infantry, which, in the first century AD, comprised between five and six thousand men (the exact number is not stated in the classical literature), all of whom were Roman citizens.
Its basic unit was the century, which comprised eighty men, divided into ten sections (contubernia) of eight, who shared either a barrack room or tent. Six centuries of eighty men formed a cohort, and ten cohorts made up a legion. Nine of the cohorts were divided into six centuries. The first cohort, which was the most prestigious, consisted of five double-strength centuries. It was to them that the legionary standard and its gold eagle (aquila) were entrusted. The infantry was supported by one hundred-twenty horsemen (equites), who acted as scouts and dispatch riders. The total nominal strength of the legion, therefore, was approximately 5,240 men, which further was supported by lighter armed auxiliaries (auxilia) and cavalry, as well as specialized forces such as archers and slingers, who were recruited from the provinces and local tribes.
The legion was commanded by a legate (legatus legionis), a senator who already had been a praetor at Rome. He was assisted by six staff officers: a senior military tribune (tribunus laticlavius, so called from the broad purple stripe on the toga), who also was of the senatorial class (but at the beginning of his career) and the legate's second-in-command, and five others (tribuni angusticlavii, narrow stripe) from the equestrian class. These were short-term commissions that offered preliminary experience before the assumption of administrative office.
The senior professional soldier was the camp prefect (praefectus castrorum), who was responsible for the organization of the legionary fortress and the training of the men. In the absence of the legate, it was he who took command of the legion. Ranking below the officers were fifty-nine centurions, each of whom led a century. The five centurions of the first cohort outranked the others and, themselves, comprised a hierarchy. The primus pilus (first spear) commanded the first century of the first cohort. It was he who was charged with protecting the legionary standard and pay-chest.
The legion largely was self-supporting and among its men were specialists, such as engineers, surveyors, and architects, as well as craftsmen. These immunes often were excused from camp duty and earned more pay. Within the century, there also was the guard commander (tesserarius) and centurion-in-waiting (optio), who was appointed by the centurior as his deputy. The standard bearer (signifer) was responsible for the men's pay and savings, which were withheld, and the centurial standard (signum), a spear shaft decorated with medallions and often topped with an open hand to signify the oath of loyalty taken by the soldiers. It was this standard that served as a rallying point for the men in battle, their attention drawn to it by the sound of the horn blower (cornicen). There also was a standard bearing the image of the emperor (imago) carried by the imaginifer as a reminder of their loyalty to him. Cohorts also were detached from the legion to fight under their own banner (vexillum), which displayed their name and badge or insignia.
The centurion was a professional soldier who had risen in the ranks and provided discipline and leadership for the army. He was identified by the transverse crest on his helmet, his mail or scale armor, and shin guards. The centurion also wore his sword (gladius) on the left and his dagger (pugio) on the right, just the reverse of the legionary soldier, and carried a vine stick (vitis) as a badge of his rank and to discipline the troops. He might also wear medals of valor (phalerae). Promotion to the centurionate was by the provincial governor and the pay twenty times that of a legionary.
After twenty-five, or more, years of service, the legionary could be discharged, free to live in a colonia with his fellows or return home. For the auxiliary soldier, who so often was sent first into battle, there was the promise of Roman citizenship for himself and his descendants.