The distinguishing feature of Roman nomenclature was the use of both personal names and regular surnames. Throughout Europe and the Mediterranean, other ancient civilizations distinguished individuals through the use of single personal names, usually dithematic in nature. Consisting of two distinct elements, or "themes", these names allowed for hundreds or even thousands of possible combinations. But a markedly different system of nomenclature arose in Italy, where the personal name was joined by a hereditary surname. Over time, this binomial system expanded to include additional names and designations.
The most important of these names was the nomen gentilicium, or simply nomen, a hereditary surname that identified a person as a member of a distinct gens. This was preceded by the praenomen, or "forename", a personal name that served to distinguish between the different members of a family. The origin of this binomial system is lost in prehistory, but it appears to have been established in Latium and Etruria by at least 650 BC. In written form, the nomen was usually followed by a filiation, indicating the personal name of an individual's father, and sometimes the name of the mother or other antecedents. Toward the end of the Roman Republic, this was followed by the name of a citizen's voting tribe. Lastly, these elements could be followed by additional surnames, or cognomina, which could be either personal or hereditary, or a combination of both.
The Roman grammarians came to regard the combination of praenomen, nomen, and cognomen as a defining characteristic of Roman citizenship, known as the tria nomina. However, although all three elements of the Roman name existed throughout most of Roman history, the concept of the tria nomina can be misleading, because not all of these names were required or used throughout the whole of Roman history. During the period of the Roman Republic, the praenomen and nomen represented the essential elements of the name; the cognomen first appeared among the Roman aristocracy at the inception of the Republic, but was not widely used among the plebeians, who made up the majority of the Roman people, until the second century BC. Even then, not all Roman citizens bore cognomina, and until the end of the Republic the cognomen was regarded as somewhat less than an official name. By contrast, in imperial times the cognomen became the principal distinguishing element of the Roman name, and although praenomina never completely vanished, the essential elements of the Roman name from the second century onward were the nomen and cognomen.
Naming conventions for women also varied from the classical concept of the tria nomina. Originally Roman women shared the binomial nomenclature of men; but over time the praenomen became less useful as a distinguishing element, and women's praenomina were gradually discarded, or replaced by informal names. By the end of the Republic, the majority of Roman women either did not have or did not use praenomina. Most women were called by their nomen alone, or by a combination of nomen and cognomen. Praenomina could still be given when necessary, and as with men's praenomina the practice survived well into imperial times, but the proliferation of personal cognomina eventually rendered women's praenomina obsolete.
In the later empire, members of the Roman aristocracy used several different schemes of assuming and inheriting nomina and cognomina, both to signify their rank, and to indicate their family and social connections. Some Romans came to be known by alternative names, or signia, and due to the lack of surviving epigraphic evidence, the full nomenclature of most Romans, even among the aristocracy, is seldom recorded.
Thus, although the three types of names referred to as the tria nomina existed throughout Roman history, the period during which the majority of citizens possessed exactly three names was relatively brief. Nevertheless, because most of the important individuals during the best-recorded periods of Roman history possessed all three names, the tria nomina remains the most familiar conception of the Roman name.
For a variety of reasons, the Roman nomenclature system broke down in the centuries following the collapse of imperial authority in the west. The praenomen had already become scarce in written sources during the fourth century, and by the fifth century it was retained only by the most conservative elements of the old Roman aristocracy. Over the course of the sixth century, as Roman institutions and social structures gradually fell away, the need to distinguish between nomina and cognomina likewise vanished. By the end of the seventh century, the people of Italy and western Europe had reverted to single names. But many of the names that had originated as part of the tria nomina were adapted to this usage, and survived into modern times.