The Roman legion was the basic military unit of the Roman Army in the period of the Roman Republic and the Roman Empire. It consisted of 6,000 infantry men which was divided into 10 cohorts of 600 men each. Each cohort was made up of three maniples of 200 soldiers, and each maniple was composed of 2 centuries, that is to say that there were 6 centuries in each cohorts, and 60 in a legion. A century was commanded by a centurion; a maniple by a Princeps Prior (experienced centurion); a cohort by a Primus Pilus (highly experienced centurion), or by a military tribune; and the whole legion was commanded by a Legatus Legionis, the overall legion commander, who was usually a senator of military experience.
Prior to the Marian reforms of 107 BC, the Roman legion was subdivided, according to experience, into three separate lines of troops:
1) The hastati (sing. hastatus) consisted of inexperienced soldiers, considered to be less reliable than legionaries of several years’ service. They were young and some of the poorest men in the legion. The hastati stood in the first line.
2) The principes (sing. princeps) were men in their prime, late twenties to early thirties. They were relatively wealthy and could afford decent protective equipment, such as heavy shield and armor. They stood in the second line.
3) The triarii (sing. triarius) were the veteran soldiers, to be used in battle only in extreme situations; they rested one knee down when not engaged in combat. The triarii served primarily as reserves or barrier troops designed to backstop the hastati and principes, and were equipped with long spears rather than the pilum.
In 107 BC, Gaius Marius undertook the task of reforming the Roman legion. Before the reform, each soldier provided their own weapons and protective gear such as shield, helmet, and armor, which were expensive and only proprietors such as farmers and wealthy Roman citizens could afford. Marius changed the Roman Legion into a professional force drawing from the poorest classes, enabling Rome to field larger armies and providing employment for jobless citizens of the city of Rome. The state would provide their weapons and protective equipment. However, this put the loyalty of the soldiers in the hands of their general rather than Rome itself. In this period all Italian regions obtained full Roman citizenship and provided a larger basis for the army, supplemented by poor urban Romans.
Marius granted all Italian soldiers Roman citizenship. He justified this action to the Senate by saying that in the din of battle he could not distinguish Roman from ally. This effectively eliminated the notion of allied legions; henceforth all Italian legions would be regarded as Roman legions, and full Roman citizenship was open to all the regions of Italy. Thus the three different types of heavy infantry were replaced by a single, standard type based on the Principes which were armed with two heavy javelins called pila (sing. pilum), the short sword called gladius, chain mail (lorica hamata) or banded armour (lorica segmentata), helmet and rectangular shield (the scutum).
Legionaries received 225 denarii a year. This basic pay remained unchanged until Domitian, who increased it to 300 denarii. In spite of the steady inflation during the 2nd century, there was no further rise until the time of Septimius Severus, who increased it to 500 denarii a year. However, the soldiers did not receive all the money in cash. From their pay was deducted the money spent by the state for clothing and feeding each soldier, therefore the net salary was not very high. All legionary soldiers would also receive a sizeable sum of money on the completion of their term of service: 3000 denarii from the time of Augustus and a plot of good farmland. Later, under Caracalla, this increased to 5000 denarii.