From each of the classes except the youngest they elect ten centurions according to merit, and then they elect a second ten.
All these are called centurions, and the first man elected has a seat in the military council. The centurions then appoint an equal number of rearguard officers (optiones). Next, in conjunction with the centurions, they divide each class into ten companies, except the velites, and assign to each company two centurions and two optiones from among the elected officers. The velites are divided equally among all the companies; these companies are called ordines or manipuli or vexilla, and their officers are called centurions or ordinum ductores.
Finally these officers appoint from the ranks two of the finest and bravest men to be standard-bearers (vexillarii) in each maniple.
It is natural that they should appoint two commanders for each maniple; for it being uncertain what may be the conduct of an officer or what may happen to him, and affairs of war not admitting of pretexts and excuses, they wish the maniple never to be without a leader and chief. When both centurions are on the spot, the first elected commands the right half of the maniple and the second the left, but if both are not present the one who is commands the whole. They wish the centurions not so much to be venturesome and daredevil as to be natural leaders, of a steady and sedate spirit.
They do not desire them so much to be men who will initiate attacks and open the battle, but men who will hold their ground when worsted and hard-pressed and be ready to die at their posts.
Walbank Commentary