Structure of the Legion
The military was a highly organized institution. There was a clear-cut system of rank, and a number of different divisions of the basic unit, the legion. There were about 30 legions. The legions were numbered, but the numbers tended to repeat themselves. At one point, there were 5 legions numbered III. If a legion was destroyed, its number could not be used again, such as the case of the massacre of legions XVII, XVIII, and XIX.
Each legion had about 5,500 men. The legion was subdivided into ten units called cohorts. Nine of the cohorts had 480 soldiers. The cohorts were subdivided into six centuries, of about 80 men each. Each century was commanded by a centurion. Each century also had a tesserarius, a signifer, a cornicen, and an optio. The tesserarius got 1½ pay, and was in control of guard duties. The signifer was the standard-bearer, who also kept track of pay and expenses, and received double pay. The cornicen was a hornblower. The optio was a backup if the centurion fell, and helped with the training of the century. The legion also had about 120 cavalrymen, who were used as scouts.
The first cohort was different. It had about 800 men, and only five centuries. Many of the extra men in the first cohort were specialists, such as blacksmiths or builders. The centurion of the first cohort's first century was the primus pilus , or "first spear," and was the highest ranking centurion in the legion.
Each legion also had an aquilifer and several ranking officers, as well as a legatus. The aquilifer carried the eagle, the standard of the entire legion. Much superstition was attached to the eagle, and if it was lost, the entire legion could be disbanded. The legion carried other standards as well, such as the imago, an image of the emperor, the legionary symbol, and special flags called vexilla, that were used when detachments of the legion were sent away. For this reason the detachments were referred to as vexillations. The legatus was an officer appointed by the emperor, and commanded the legion with a great deal of help from his centurions and the camp prefect, who had been promoted from primus pilus, and could be compared to a quartermaster.