The primus pilus or primipilus was the senior centurion of the first cohort in a Roman legion, a formation of five double-strength centuries of 160 men each; he was a career soldier and advisor to the legate. The primus pilus would remain in command for one year. They could continue to serve in the army after their term ended if there was a vacancy in command or if they wished to become an independent commander of an auxilia unit or the praefectus castrorum.