Organization of the Roman Legion
A full strength legion was officially made up of 6,000 men, but typically all legions were organized at under strength and generally consisted of approximately 5,300 fighting men including officers. It is difficult to determine whether non-combatants like field surgeons and clerks were included in the 5,300 or helped bring the total number of men up to the official 6,000.
The basic structure of the army is as follows:
Contubernium: (tent group) consisted of 8 men.
Centuria: (century) was made up of 10 contubernium with a total of 80 men commanded by a centurion
Cohorts: (cohort) included 6 centuriae or a total of 480 fighting men, not including officers. In addition the first cohort was double strength but with only 5 centuriae instead of the normal 6.
Legio: (Legion) consisted of 10 cohorts.
Additionally each Legion had a 120 man Alae (cavalry unit) called the Eques Legionis permanently attached to it possibly to be used as scouts and messengers.
Therefore the total fighting strength of a Legion:
The First Cohort totaling 800 men (5 double-strength centuries with 160 men each) 9 Cohors (with 6 centuries at 80 men each) for a total 4,320, and an additional 120 man cavalry for a grand total of 5,240 men not including all the officers.
The basic designation of the 10 cohors was the same throughout all the Legions. They were arranged in battle so that the strongest and weakest units would be mixed throughout the formation maximizing moral and effectiveness
Cohort I: Was made up of the elite troops. Its direct commander was the Primus Pilus, the highest ranking and most respected of all the Centurions.
Cohort II: Consisted of some of the weaker or newest troops.
Cohort III: No special designation for this unit.
Cohort IV: Another of the four weak cohorts.
Cohort V: Again, no special designation.
Cohort VI: Made up of “The Finest of the Young Men”.
Cohort VII: One of the four weak cohorts and a likely place to find trainees and raw recruits.
Cohort VIII: Contained “The Selected Troops”.
Cohort IX: One of the four weak cohorts and a likely place to find trainees and raw recruits.
Cohort X: Made up of “The Good Troops”.
In general battle order, the Cohors would be arranged within 2 battle lines as follows, again to maximize the effectiveness of the strongest and weakest units:
First Line - Cohors - V - IV - III - II - I
Second Line - Cohors - X - IX - VIII - VII - VI
Roman Legionary Ranks
The following list indicates ranks from highest command to lowest common soldier:
Senior Officers of the Roman Legion
Legatus Legionis: The overall Legionary commander. This post was generally appointed by the emperor, was a former Tribune and held command for 3 or 4 years, although could serve for a much longer period. In a province with only one legion, the Legatus was also the provincial governor and in provinces with multiple legions, each legion has a Legatus and the provincial governor has overall command of them all.
Tribunus Laticlavius: Named for the broad striped toga worn by men of senatorial rank. This tribune was appointed by the Emperor or the Senate. Though generally quite young and less experienced than the Tribuni Angusticlavii, he served as second in command of the legion, behind the Legate.
Praefectus Castrorum: The camp Prefect. Generally he was a long serving veteran who had been promoted through the ranks of the centurions and was 3rd in overall command.
Tribuni Angusticlavii: Each legion had 5 military tribunes of equestrian (knight) class citizens. They were in many cases career officers and served many of the important administrative tasks of the Legion, but still served in a full tactical command function during engagements.
Primus Pilus: The “First File” was the commanding centurion of the first cohort and the senior centurion of the entire Legion.
Mid-Level Officers in the Roman Legion
Centurions: Each Legion had 59 or 60 centurions, one to command each centuria of the 10 cohorts. They were the backbone of the professional army and were the career soldiers who ran the day to day life of the soldiers as well as issuing commands in the field. They were generally moved up from the ranks, but in some cases could be direct appointments from the Emperor or other higher ranking officials. The cohorts were ranked from the First to the Tenth and the Centuria within each cohort ranked from 1 to 6, with only 5 Centuria in the First Cohort (For a total of 59 Centuria and the Primus Pilus). The Centuria that each Centurion commanded was a direct reflection of his rank. (Command of the First Centuria of the First Cohort was the highest and the 6th Centuria of the 10th Cohort was the lowest). The 5 Centurions of the First Cohort were called the Primi Ordines, and included the Primus Pilus. Additional ranks are highlighted here:
Pilus Prior: The commander of the first cohort of each Centuria, with the following six titles for the Centurions in sequence throughout each Centuria.
Low-Level Officers in the Roman Legion
Princepales: The Princepales would be the equivalent of modern day non-commissioned officers and had the following rank structures from highest to lowest.
Aquilifer: A single position within the Legion. The Aquilifer was the Legion’s Standard or Eagle bearer and was an enormously important and prestigious position. The next step up would be a post as a Centurion.
Signifer: Each Centuria had a Signifer (59). He was responsible for the men’s pay and savings, and the standard bearer for the Centurial 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 banner that the men from each individual Centuria would rally around. A soldier could also gain the position of Discentes signiferorum, or standard bearer in training.
Optio:One for each Centurion (59), they were appointed by the Centurion from within the ranks to act as his second in command.
Tesserarius: (Guard Commander) Again there were 59 of these, or one for each Centuria. They acted in similar roles to the Optios.
Cornicen: (Horn blower) They worked hand in hand with the Signifer drawing the attention of the men to the Centurial Signum and issuing the audible commands of the officers.
Imaginifer: Carried the Standard bearing the image of the Emperor as a constant reminder of the troop’s loyalty to him.
The Rank and File of the Roman Legion
Immunes: These were trained specialists, such as surgeons, engineers, surveyors, and architects, as well as craftsmen. They were exempt from camp and hard labor duties due to the nature of their work, and would generally earn slightly more pay than the Milites.
Discens: Milites in training for an immunes position.
Milites Gregarius: The basic private level foot soldier.
Tirones: The basic new recruit. A Tirones could take up to 6 months before becoming a full Milites.
I found a nice site with ranks of the Roman Legion: