"Under Sp. Cassius [B. C. 493], Rome concluded a treaty with
the Latins, in which the right of isopolity or the 'jus
municipi' was conceded to them. The idea of isopolity changed
in the course of time, but its essential features in early
times were these: between the Romans and Latins and between
the Romans and Caerites there existed this arrangement, that
any citizen of the one state who wished to settle in the
other, might forthwith be able to exercise there the rights of
a citizen."
_B. G. Niebuhr,
Lectures on the History of Rome,
lecture 13 (volume 1)._