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
It was in thorough consistency with constitutional principles that the burgesses, just as being the sovereign power, should not on ordinary occasions take part in the course of public business. So long as public action was confined to the carrying into execution of the existing legal arrangements, the power which was, properly speaking, sovereign in the state could not and might not interfere: the laws governed, not the lawgiver.
But it was different where a change of the existing legal arrangements or even a mere deviation from them in a particular case was necessary; and here accordingly, under the Roman constitution, the burgesses emerge without exception as actors; so that each act of the sovereign authority is accomplished by the co-operation of the burgesses and the king or -interrex-. As the legal relation between ruler and ruled was itself sanctioned after the manner of a contract by oral question and answer, so every sovereign act of the community was accomplished by means of a question (-rogatio-), which the king addressed to the burgesses, and to which the majority of the curies gave an affirmative answer. In this case their consent might undoubtedly be refused.
Among the Romans, therefore, law was not primarily, as we conceive it, a command addressed by the sovereign to the whole members of the community, but primarily a contract concluded between the constitutive powers of the state by address and counter-address.(14)
14. -Lex- ("that which binds," related to -legare-, "to bind to something") denotes, as is well known, a contract in general, along, however, with the connotation of a contract whose terms the proposer dictates and the other party simply accepts or declines; as was usually the case, e. g. with public -licitationes-. In the -lex publica populi Romani- the proposer was the king, the acceptor the people; the limited co-operation of the latter was thus significantly indicated in the very language.
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Reference address : https://www.ellopos.net/elpenor/rome/1-05-original-constitution-rome.asp?pg=24