As the family groups expanded, their claim to the land became one of the most important aspects of their forming society. What seems to be the fundamental unit of their social organization was the kin-group. In the Irish [earliest Celtic society to form with a written record], this was called the "derefine". This was a 4-generation descent group through the male line of descent.[patrilineal] They would share a common great-grandfather. They held land in common. They were held responsible for the offences of family members. They sought retribution in common for any perceived grievances. Most importantly, anyone to the forth generation had equal claim to the land and its resources! The family groups that settled in the mountains of what was to become Wales, took things a little bit farther back the family tree.
Two articles by Wendy Davies, present this history in great detail. In her article titled "Wales in the Early Middle Ages", Leicester University Press, 1982, and her article titled "Patterns of Power In Early Wales", Clarendon Press, Oxford, 1990, she describes the six generation Welsh kindred [cenedl] and the Welsh laws that developed around this society. In this tribal setting, one may be called upon to the 4th cousin to take part in tribal activities. The head of the kin was called pencenedl. You can begin to see how important your identify would be among the various tribal groups. You certainly would not want to help pay for a wedding if you did not belong to the family. Thus, the name you were given at birth, took the names of your father, grandfather, great-grandfather...etc. This name essentially became your Social Security number! The elders of the family would know if you really belong to the tribe by giving your name to the 6th generation.
At birth, each male child born to the cenedl was presented to the tribe. The one claimed to be the father had to accept his fatherhood and give the name of the child. The elders would then accept the child into the tribe, and add the name of the family. Thus, if you where given the name Thomas, and your father was named Peter, you would become Thomas ap Peter, who was the son of David...Thomas ap Peter ap David...so on and so on...until the male descent was completed. Thus your Welsh name would become: Thomas ap Peter ap David ab Owen ap Jevan ap Griffith. Your first cousin may have been David ap Jevan ap David ab Owen ap Jevan ap Griffith. With a bunch of youngins you can begin to see how complex the naming would become to anyone outside the tribe. What a living it would be.