There is disagreement about the origin of the term, but general consensus that "cardinalis" from the word cardo (meaning 'pivot' or 'hinge') was first used in late antiquity to designate a bishop or priest who was incorporated into a church for which he had not originally been ordained. In Rome the first persons to be called cardinals were the deacons of the seven regions of the city at the beginning of the 6th century, when the word began to mean “principal,” “eminent,” or "superior." The name was also given to the senior priest in each of the "title" churches (the parish churches) of Rome and to the bishops of the seven sees surrounding the city. By the 8th century the Roman cardinals constituted a privileged class among the Roman clergy. They took part in the administration of the church of Rome and in the papal liturgy. By decree of a synod of 769, only a cardinal was eligible to become pope. In 1059, during the pontificate of Nicholas II, cardinals were given the right to elect the pope under the Papal Bull In nomine Domini. For a time this power was assigned exclusively to the cardinal bishops, but the Third Lateran Council in 1179 gave back the right to the whole body of cardinals. Cardinals were granted the privilege of wearing the red hat by Pope Innocent IV in 1244.
What changed after the sixth century?
By the 8th century the Roman cardinals constituted a privileged class among the Roman clergy