At first the roman Army was organized along the greek lines, e.g. as a Phalanx. After time, this became obselete and the army had to evolve again.

They changed their army to a block type of formation. The first ranks were composed of Hastati, followed in order by Principe, Triarii, Rorarii, and Accensii. An individual unit was called a Maniple.

Diagram 1: Order of battle Prior to 2nd cent. BC

 XXXXXXXXXXX Hastati
 DDDDDDDDDDD Principe
 AAAAAAAAAAA Triarii
 GGGGGGGGGGG Rorarii
 WWWWWWWWWWW Accensii

After the 2nd century BC the army was reorganized, collapsing the ranks of the Rorarii and Accensii in to Velites. The Velite, being light infantry, were more mobile than their heavy infantry counterparts, their role was to harry the opponent, and then retreat behind the ranks of the heavy infantry. The new order of battle looked like:

Diagram 2: Revised Order of Battle.

  VVVVVVVVVV Velite
  XXXXXXXXXX Hastati
  DDDDDDDDDD Principe
  AAAAAAAAAA Triarii

After Julius Caesar, the roman army was divided into 28 Legions, each comprising 6000 men. Each legion carried its own standard, called an eagle. To lose it was a disaster. The eagle was carried by an aquifier. The aquifier was the second highest ranking person in the legion, and was also responsible for the pay-chest. The highest ranking officer in the Legion was called the Legatus Legionis.

Diagram 3: Structure of a Legion
1 Contubernium = 8 men
10 Contubernia = 1 Century
2 Centuries = 1 Maniple
6 Centuries = 1 Cohort
10 Cohorts + 120 horsemen = 1 Legion

Diagram 4: a Legion's Order of Battle

-----------------------------------------
 5th  |  4th  |  3rd  |  2nd   |  1st   |
Cohort| Cohort| Cohort| Cohort | Cohort |
-----------------------------------------

-----------------------------------------
10th   |  9th  |  8th  |  7th   |  6th  |
Cohort | Cohort| Cohort| Cohort | Cohort|
-----------------------------------------

Source: http://www.cavazzi.com/roman-empire/army/army.html

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