Perusal of over a thousand years of the fasti of the Romans' eponymous magistracy is sufficient to demonstrate that Roman onomastic practice did not stand still. Why, then, is there a tendency to see the system of three names (tria nomina, i.e. praenomen, nomen gentilicium, and cognomen) as the perfection and culmination of the Roman naming system rather than as a transitory stage in an evolutionary process? The simple answer is probably that usage of the tria nomina happens to be typical of the best documented class in one of the best documented, and certainly most studied, eras of Roman history — the late Republic and early Empire. This perspective tends to pervade discussion of post-classical developments, the basic outline of which is clear from a glancing comparison of the Prosopographia Imperii Romani, which catalogues eminent persons of the first to third centuries A.D., with the Prosopography of the Later Roman Empire, covering the fourth to seventh. The difference in their very organizational structure betrays the change since, while the entries in PIR are classified alphabetically by nomen, those of PLRE are arranged by last name, usually cognomen. The major problem requiring explanation is why the nomen gentilicium, the central element of the classical tria nomina, should have been displaced by the cognomen as the one most consistently attested element
