Royal Marriages:
Your monarch has a limited number of sons and daughters which will be born at irregular intervals. Of course a campaigining monarch will have less time for his Queen and thus will have less offspring. The game effects is that sons and daughters may have Royal marriages.
Those nations that do not have a royal family cannot enter into a Royal Marriage.
A Royal marriage between two nations has three effects.
1. The two parties of a royal marriage are entered into a immediate military alliance, which will cause some instability if broken, but the fact remains that it can be broken.
2. Trade relations between the two parties are immediatley boosted, so long as the alliance is kept.
3. If a royal line in one of the two nations dies out, the other may try to claim the throne of the other Kingdom (in effect taking over the nation through Royal Union). The player whose nation has been taking over in this manner would become a native pretender to the throne and would have to try and win the throne back or lose his nation.
While some great benefits are opened up so are some problems, and thus you need to be careful as to who you join in Royal marriages, although a royal line wont die out easily, it is more likely to do in the midst of warfare.
Those nations without monarchs will have the trade relations boost trading with other government types of the same as theirs so as to not disadvantage them by denying them the Royal marriage option. The Pope and Dali Lama are special cases as it is in effect an Elected Monarchy although there is still no Royal family. Thus it has relations boosted with other Catholic nations and Buddhist nations repsectivley.
Royal marriages survive a 10 turn period before becoming obsolete.
I have added the royal offspring to the stats already.