I do not see the difference between 'commoner' and nobility.
If the father is titled but the mother isn't why should the child be classified as nobility?
If the mother is titled but the father isn't why should the child be classified as commoner?
What makes a person a commoner? A member of the nobility?
Good questions. Basically, in those countries where nobility has
official recognition, the law clearly defines who is noble and who is not.
In the UK, strictly speaking there is no such thing as a "nobility", but rather a "peerage", i.e the set of all individuals who bear the titles of duke, marquess, earl, viscount, or baron and who, until 1999, were entitled to sit and vote in the House of Lords. With a few exceptions, these titles are normally inherited only in male line by the first-born son. Wives and daughters of a peer, as well as their sons (including the firstborn before he inherits the title) may use courtesy titles or honorific predicates, but, strictly speaking, they are "commoners" in the sense that they could, already before 1999 vote and stand as candidates in elections to the House of Commons. In the broader sense and in common usage though, people normally identify the immediate family of a peer (for example, an earl's daughter like Lady Diana Spencer) as "nobility".
In continental Europe, on the other hand, the concept of "nobility" in somewhat different. In the Netherlands, for example, there are 3 main categories of "nobility": (1) individuals from native Dutch families who were ennobled by a sovereign (e.g. the Holy German Emperor or the French King)
prior to the creation of the Kingdom of the Netherlands and whose nobility status was
recognized by a Dutch monarch; (2) individuals who belong to the nobility of another country and were
incorporated into the nobility of the Netherlands by a Dutch monarch; and (3) individuals who were
elevated to the nobility of the Netherlands by a Dutch monarch. In all cases, nobility may come with a title (in the Netherlands, usually baron or, more rarely, count) or not (there is something then called "untitled nobility" . e.g. a
jonkheer or jonkvrouw). In the case of recognized (and sometimes also incorporated) nobility, the nobility status , including titles like count or baron, normally extends from birth to all male and female descendants of a family, but only in male line, meaning that fathers can transmit nobility (and titles if any) to all their sons and daughters, but mothers do not transmit nobility to any of their children. In the case of elevated nobility, many times titles are transmitted only to the first-born son (or the "head of the family"), but other descendants in male line remain "untitled nobility".
In Scandinavia, the Kingdom of Norway doesn't have a nobility (just a royal family). In Denmark, the nobility is still legally recognized and, in Sweden, it ceased to be officially recognized in 2003 (?), but individuals who belong to the nobility may still use their titles. From what I understand, Sweden was similar to the Netherlands in the sense that there were titled nobles (counts and barons) and untitled nobility. Moreover, the old nobility from the pre-Bernadotte era (i.e. dating prior to 1810) is also inheritable by all male and female descendants in the family, but again only in male line,whereas the new titles of nobility created after 1810 are normally inherited by the first-born son only.
Spain, on the other hand, seems to have a system also similar to the UK. There are ranking titles of nobility like duke, marquis, count, viscount, etc. inheritable by the firstborn, except that in Spain, daughters can now also inherit titles if they are the first child (or, at least, I believe they can, but I'm not very knowledgeable about Spanish rules). Then there is a separate, special status of "grandee" of Spain, which can be awarded by the king to a noble person, e.g. a duke or a marquis, but can also be awarded to an untitled individual.