Knighthood is not inherited; it must be earned. Theoretically, one need not already be noble in order to be knighted -- even the most base-born peasant on Kethira may be knighted under the right circumstances, usually for exemplery military service to the crown. But knighthood grants nobility and at the instant of his dubbing the new knight goes from being a commoner to being noble. And, because of the conventions of Hârnic feudal society, the knight's nobility is inherited by the first generation of his or her offspring.
The offspring of a knight will be born gentle but this status will lapse for the next generation unless another knighthood is acquired.
HârnWorld, HârnPlayer, and Encyclopedia Harnica #7
This basic pattern of inherited nobility can be illustrated with a simple diagram.
But that illustration ignores the advantages gentle birth offers the knight's children. At the very least, the knight will make every effort to get his oldest son squired off to some other knight and, unless he should prove unsuited to the role, the son very likely will himself be knighted about the time he turns 21 years of age. In so doing, nobility is passed down another generation while the son makes himself eligible to inherit his father's fief. This more typical pattern of inherited nobility associated with knighthood can be illustrated with this diagram.
While the children of a knight have every advantage in obtaining knighthood for themselves, it is never guaranteed. As a result, it is possible for the chain of nobility to be broken in any generation that fails to obtain a knighthood. For example, in the clan depicted above, should person M and person N not obtain knighthood, they will be the last in the line of nobility within that clan.
Limited opportunities and the high costs associated with squiring off their offspring limit most manor knights to providing that advantage to only one of their children. As a result, most knightly clans include a large number of families of common-birth.
In the clan illustrated above, for example, by the time Person I inherits the manor, the clan will consist of about six families*, and only two of those families (Person I and Person J) will have a member who is noble.
Chybisa's great nobles are better positioned to give all of their offspring the opportunity to obtain knighthood, but even in those clans, there are a number of common-born families.
*These six families will be the households descended in father-son direct lines from Person C and Person D; female descendants will produce offspring into the clan of their husbands and not clan A. So, of the families in this generation descended from Person C (families I, J, K, L, W, X, Y, and Z), only about one-third of those (2.6, rounded to 3) will carry the clan name. Assuming a similar number and distribution of descendants from Person D gives us about six families in the clan when I inherits the manor.