In later times (from about 350) all who were not Patricians or slaves were called Plebeians.
THE ARMY.
Until the time of Servius Tullius (about 550) the army was composed entirely of patricians.

It was called a Legio (a word meaning _levy_), and numbered three thousand infantry called _milites_, from _mille_, a thousand, one thousand being levied from each tribe.

The cavalry numbered three hundred at first, one hundred from each tribe, and was divided into three companies called Centuries.
During the reign of Servius the demands of the plebeians, who had now become numerous, for more rights, was met by the so called SERVIAN reform of the constitution.

Heretofore only the patricians had been required to serve in the army.

Now all males were liable to service.