Rules of noble succession by Jan-Olov von Wowern
Let me first say that rules of noble succession, as they apply to a specific noble family, can normally not be changed. They are determined either by:
1) rules of succession laid down in letter patent which was given to family member who was first ennobled (for patent nobility)
2) rules of succession in use at time and place where family was first recognised as noble (for original nobility)
It follows that in case of patent nobility rules of succession could only be changed by conferring authority, reigning Monarch or his or hers successors (if constitution would allow for such changes).
In case of original nobility in my opinion rules of succession cannot ever be changed, not even by a successor of reigning Monarch who once recognised family as noble, because rules of succession were in most cases not at Monarch's disposition to change. Hence, for all practical purposes and certainly for study of genealogy, we may assume that noble families are bound by certain rules of succession that must be adhered to.
When we discuss succession, we should define whether we mean succession to nobility (that is, which family members in next generation will inherit name, arms and quality of continuing family line), and succession to headship of a noble family. As implied by title, in this article I will mainly discuss succession of nobility in general and collective sense.
Noble succession is either agnatic or cognatic. In my genealogy book "Find Your Noble Ancestors!", I have defined those terms as follows: "Agnatic succession: succession to nobility ... continues only on spear (male) side, from father to son." "Cognatic succession: in many countries this was original form of succession among ancient nobility, meaning nobility ... continued on both spear (male) and distaff (female) side in parallel lines".