A tribe was not an indication of common ancestry; the tribes were distributed geographically and a man belonged to the tribe in which his main residence was located. The tribe was an essential part of citizenship, since voting was often carried out by tribe. By the Middle Republic the abbreviation for tribe in which the person was enrolled was added to the person's name.
Usage in names
The tribe is inserted between the nomen and the cognomen, e.g. M. Tullius Cor. Cicero (meaning "Marcus Tullius Cornelia tribu Cicero", or "Marcus Tullius, of the Cornelian tribe, Cicero").
Often the tribe is included together with the filiation, in which case it appears after the filiation and before the cognomen, thus:
M. Tullius M. f. M. n. Cor. Cicero
Development of tribes
According to tradition there were originally three tribes, created by king Romulus; the tribes were reorganized by king Servius Tullius and greatly increased in number. New tribes were added from time to time until N. Vibulano T. Capitolino cos.‡ (CCCXXXIII a.u.c.) it was decided to fix the number at 35.
The tribes in ancient times were fundamental social units. Each tribes had its own officials, and many important activities were organized on a tribal basis including the census, collection of taxes, and voting in the Comitia Populi Tributa and the Comitia Plebis Tributa. They were so important that a Roman's full name included the name of his tribe along with the name of his father and grandfather.
Four of the tribes (Collina, Esquilina, Palatina, and Suburana) are urban tribes; the rest are rural tribes. In ancient times citizens who owned land outside the city of Rome were enrolled in rural tribes, while those who lived exclusively in the city belonged to urban tribes. The result was that the urban tribes had much less power in the voting assemblies than the rural tribes had.
By 242 BCE the number of tribes was fixed at 35.