Although it never was addressed in the series, it would be interesting to determine if primogeniture had a direct influence in Fire Lord succession protocols. Before I go any further, primogeniture refers to the right of inheritance granted to the firstborn. It's obvious that this form of system was in effect in the Fire Nation, but which type exactly? There are two major types of this: male-preference (a.k.a. Agnatic), and absolute (a.k.a. Absolute Cognatic). The former is traditionally how primogeniture is viewed; the oldest male child gets the inheritance, regardless if he has older sisters. The British Royal Family traditionally used this form of succession, with the notable exceptions being few (including Queen Elizabeth II). The latter refers to succession by the oldest child, regardless of gender. This form was very rarely used until around 30 years ago (especially with regards to the monarchies), when Sweden passed an amendment to change from male-preference to absolute. This is why the current King of Sweden (Carl XVI Gustaf)'s eldest daughter (Victoria) is now the heir apparent (her younger brother, Carl, had it for a few months, until the amendment). The British monarchy also recently passed an amendment to change from male-preference to absolute primogeniture, which would benefit Prince William and Princess Catherine should their eldest child be a girl.
In terms of Avatar, I would assume that the Fire Nation (and the Avatar world in general) uses male-preference primogeniture when deciding their lines of succession. Here however, some assumptions and liberties must be distinguished due to lack of information from the series. Firstly, with the notable exception of Ozai, none of the Fire Lords (or purported successors) had more than one child (Ozai obviously had Zuko and Azula). Secondly, every child borne by the known rulers were male (except Azula). Thirdly, it can be assumed that both Sozin and his son, Azulon, had no siblings (at least none mentioned in the series or the continuity' yes, this is contradictory to what I mentioned as the first assumption, but it's still relevant nonetheless).
The only reason why Ozai got the throne and not Iroh was that the former asked his father to extricate the latter from the line of succession (making this occurrence the only time that an heir apparent was disavowed and removed of his title and replaced by a younger sibling; it nearly happened again when Azula was almost crowned Fire Lord).
In conclusion, I believe that male-preference primogeniture was used in the Fire Nation, as well as the rest of the Avatar world. Now that I've parlayed my reasons for this, what are your opinions on the matter? Entia non sunt multiplicanda praeter necessitatem 05:19, December 5, 2011 (UTC)