I have elsewhere defined nobiliary law as “national legislation, or international or national customs, regulating nobiliary issues. In many cases this is not codified, but rather a set of rules and traditions having gained acceptance” (see my book at http://www.findyournobleancestors.com).
Examples of some of the more important issues regulated by nobiliary law are:
– claims to nobility (surname, coat of arms, title) by non-noble persons. This could, but must not, include: children with one or two noble parents but born out of wedlock; stepchildren to noble parents; children to a noble lady in an agnatic family, etc.
– claims to nobility by noble persons, where the claims cannot be automatically verified. This could be e.g. the inheritance of a noble title in a junior line of the family when the senior line becomes extinct.
– borderline cases, such as which among the ancient patrician families were, and were not, to be numbered among the nobility. Or the reactivation of a family’s nobility after some time of voluntary or involuntary loss of nobility (usually because one or all of the nobiliary qualities has not been used for two or more generations).
– the naturalisation of foreign nobility, that is the assimilation of immigrant nobility into the domestic nobility, usually with the purpose of ensuring the foreign nobility the same privileges as the domestic.
– heraldry, and more specifically the use of certain symbols usually reserved for the nobility, such as coronets of nobiliary rank, the use of supporters, etc. Also marshalling of arms, that is the proper combination of two or more coats of arms due to marriage between two noble families, and similar issues may be regulated.
In some countries the nobility is a subject of public law (Belgium, Finland, Netherlands, and in Spain only regarding the titled nobility). In other countries this is not the case, and then the nobility may have organised itself in one or more associations in order to have an institution to handle nobiliary issues such as those mentioned above. It is therefore of the utmost importance for every noble family to define and clarify under which legislation, or under which set of rules or regulations whether codified or not, they are a subject.
Nobiliary law is a complex and multi-faceted subject. It is often necessary to do extensive research in order to establish which rules apply to a specific noble family. A starting place can be to collect relevant literature from (or about) the country where the family is known (or believed) to have been ennobled (or first recognised as noble). This may be done by searching the many antiquarian bookshops available on the Internet, for keywords such as “nobility” or “nobiliary” in the book title. Sometimes a specific Internet site will be dedicated to nobiliary law (such as the Italian http://www.dirittonobiliare.com).
Perhaps the most important thing to remember about nobiliary law is that it is not the same as public law. It may well be possible, according to national legislation, for a non-noble person to assume a noble surname, but this does not make them members of the nobility. A person can only be a member of the nobility if they are so according to nobiliary law, whether this is in harmony with the public law or not.