During these reforms, the legions were also organized into permanent cohorts for the first time. Prior to this cohorts had been temporary administrative units or tactical task forces of several maniples, even more transitory than that of the legions of the early republic themselves. Now the cohorts were six to ten permanent units, composed of five to eight centuries each led by a centurion assisted by an "Optio", a soldier who could read and write. These came to form the basic tactical unit of the legions. The senior centurion of the legion was called the Primus pilus, a career soldier and adviser to the legate; he was generally 50 years of age or older. There were also additional officers assigned to each legion, an "Aquilifer", "Imaginifer" (Imperial Rome only), a "Tesserarius", and a "Cornicen". The "aquilifer" was in charge of the legion standard, so there was only one per legion. The legion standard created a sense of unit loyalty. The soldiers' loyalty to their unit also created rivalries between some units in the same army.