Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Thus there grew up by the side of the burgesses a second community in Rome: out of the clients arose the Plebs. This change of name is significant. In law there was no difference between the client and the plebeian, the "dependent" and the "man of the multitude;" but in fact there was a very important one, for the former term brought into prominence the relation of dependence on a member of the politically privileged class; the latter suggested merely the want of political rights.
As the feeling of special dependence diminished, that of political inferiority forced itself on the thoughts of the free μέτοικοι; and it was only the sovereignty of the king ruling equally over all that prevented the outbreak of political conflict between the privileged and the non-privileged classes.
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Reference address : https://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=11