Their nomenclature was, in fact, the immediate parent of our own.
Every Roman citizen had necessarily two names. The second of these was the important one which marked his hereditary position in the state, and answered to our surname. It was called the nomen, or name, par excellence, and was inherited from his father, belonging also to the entire gens, or tribe, who considered themselves to have a common ancestor, and who, all alike, whether wealthy or otherwise, took the rank of their gens, whether patrician, equitial, or plebeian. The daughters of the gens were called by the feminine of its name, and sometimes took that of the gens of their husband, but this was not always the custom.
Besides these large tribes, there were lesser ones of families. If an ancestor had acquired an additional appellation, whether honourable or ludicrous, it passed to all his male descendants, thus distinguishing them from the rest of their gens, and was called the cognomen. For instance, after Marcus Manlius had saved the capitol, Capitolinus would be the cognomen not merely of himself but of his posterity.
Clients and freedmen took the gentile name of their patron, and when the freedom of Rome was granted to a stranger, he took the gentile name of him from whom it was received, thus infinitely spreading the more distinguished nomina of the later republic and early empire, and in the Romanized countries gradually becoming the modern hereditary surname, the convenience of the family distinction causing it to be gradually adopted by the rest of the world. When the last of a gens adopted the son of another clan to continue his line, the youth received the nomen and one or more cognomina of his new gens, but brought in that of his old one with the augmentative anus. As for instance, Publius Æmilius Paullus being adopted by Publius Cornelius Scipio Africanus, became Publius Cornelius Scipio Africanus Æmilianus, and his daughter was simply Cornelia. Again, Caius Octavius, as adopted into the Julian gens, became Caius Julius Cæsar Octavius; and the emperors being all adopted, arrived at such a multitude of names that the accumulation was entirely useless, and they were called by a single one.
Added to all these family names, each man had his own individual name, which was bestowed in later times, or more properly registered when, at the age of fourteen, he laid aside the childish tunic and bulla, or golden ball, which he had worn from infancy, put off the toga prætextala, and assumed the toga virilis, or manly gown, white edged with purple, which was the regular official Roman dress. In the latter days, the prænomen was given on the eighth day, with a lustratio or washing of the infant. There was a very small choice of Roman prænomina, not above seventeen; an initial was sufficient to indicate which might be intended, nor did ladies receive their feminines in the earlier times. By which name a man might be called was arbitrary; the gentile name was the distinction of rank, and perhaps the most commonly used by his acquaintance, unless the tribe were very large, when the cognomen would be used; and among brothers the prænomen was brought in first as the Christian name is with us. The great Marcus Tullius Cicero was called Cicero by those who only knew him politically, while to his correspondents he was Tullius; his son, of the same name, was termed Marcus Cicero; his brother, Quintus Cicero; and Caius Julius Cæsar figures in contemporary correspondence as C. Cæsar.
In Christian times, the lustratio at the giving of the prænomen became Holy Baptism, thus making our distinction between baptismal and hereditary names. The strict adherence to the old prænomina had been already broken into, especially in favour of women, who had found the universal gentile name rather confusing, and had added to it feminine prænomina or agnomina, had changed it by diminution or augmentation, or had taken varieties from the other gentes to which they were related. Christianity had given individuality to woman, and she was no longer No. 1, or No. 2, the property of the gens. Significant names, Greek names, or saintly ones were chosen as prænomina, and the true Christian name grew up from the old Roman seventeen. Besides these, the numerous slaves, who formed a large part of the Roman population, had each a single name. Some of these were in their own language, disguised by Latin pronunciation; others were called by Greek or Latin words; others bore their masters' names. Many of these slaves were among the martyrs of the Church, and their names were bestowed on many an infant Christian. Others were afterwards formed from significant Latin words, but far fewer than from Greek words, the rigid hereditary customs of Latin nomencloture long interfering with the vagaries of invention, and most of these later not being far removed from classical Latinity.
It should be observed that the original Latin word, especially if descriptive or adjectival, usually ends in us, representing the Greek ος, and in the oblique cases becoming i and o—in the vocative e. When it was meant to signify one of or belonging to this first, the termination was ius—thus from Tullus comes one belonging to Tullus—Tullius, in the vocative i; and again, one of the gens adopted into another, would become Tullianus,—Tullus, Tullius, Tullianus. The diminutive would be illus, or iolus, and in time became a separate name: Marcus, Marcius, Marcianus, Marcellus. In the adoption of Latin by the barbarous nations, the language was spoken without the least attention to declension; the Italians and Spanish used only the dative termination, making all their words end in o; but the former preserving the nominative plural i, and the latter the accusative plural os, while the French stopped short at the simple elementary word, and while finishing it in writing with an e, discarded all pronunciation of its termination. The vocative was their favourite case in pronunciation, and has passed to us in our usual terminal y. The a of feminine names was retained by Italy and Spain; cut off by France, Germany, and England.[[51]]
[51]. Niebuhr, Rome; Arnold, Rome; Smith, Dictionary of Greek and Roman Antiquities; Max Müller.