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
Plebian Tribunes and Plebian Aediles
In addition to temporary enactments, particularly for remedying the most urgent distress occasioned by debt, and for providing for a number of the rural population by the founding of various colonies, the dictator carried in constitutional form a law, which he moreover --doubtless in order to secure amnesty to the burgesses for the breach of their military oath--caused every individual member of the community to swear to, and then had it deposited in a temple under the charge and custody of two magistrates specially appointed from the plebs for the purpose, the two "house-masters" (-aediles-).
This law placed by the side of the two patrician consuls two plebeian tribunes, who were to be elected by the plebeians assembled in curies. The power of the tribunes was of no avail in opposition to the military -imperium-, that is, in opposition to the authority of the dictator everywhere or to that of the consuls beyond the city; but it confronted, on a footing of independence and equality, the ordinary civil powers which the consuls exercised. There was, however, no partition of powers.
The tribunes obtained the right which pertained to the consul against his fellow-consul and all the more against an inferior magistrate,(4) namely, the right to cancel any command issued by a magistrate, as to which the burgess whom it affected held himself aggrieved and lodged a complaint, through their protest timeously and personally interposed, and likewise of hindering or cancelling at discretion any proposal made by a magistrate to the burgesses, in other words, the right of intercession or the so-called tribunician veto.
4. Cf. II. I. Collegiate Arrangement
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Reference address : https://www.ellopos.net/elpenor/rome/2-02-tribunate-plebs-decemvirate.asp?pg=12