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
Powers of the Consuls
If we are left in ignorance of the historical connections of this important event, we are fortunately in possession of clearer light as to the nature of the change which was made in the constitution. The royal power was by no means abolished, as is shown by the very fact that, when a vacancy occurred afterwards as before, an "interim king" (-interrex-) was nominated.
The one life-king was simply replaced by two year-kings, who called themselves generals (-praetores-), or judges (-iudices-), or merely colleagues (consules).(3) The principles of collegiate tenure and of annual duration are those which distinguish the republic from the monarchy, and they first meet us here.
3. -Consules- are those who "leap or dance together," as -praesul- is one who "leaps before," -exsul-, one who "leaps out" (ὁ ἐκπεσών), -insula-, a "leap into," primarily applied to a mass of rock fallen into the sea.
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Reference address : http://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=7