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1 hour ago, Lady Blizzardborn said:
Useful is not all it's cracked up to be. I'd love the details. I'm a total history nerd. Feel free to send them in a PM if you like.
If I were smart, I would have just started at Plantagenet England, realized that it fit pretty well, and have been done. But instead…
The most famous cadet house traditions are the early modern HRE ones, I'm pretty sure GRRM didn't borrow from there, because the reason they're famous is all the the wacky results that still persist to modern times, like the British royal house being a cadet branch of a cadet branch of a Wettin duchy and the German imperial house being a cadet branch of the County of Hohenzollern. There doesn't seem to be anything like that in Westeros.
So, next, medieval France, which is the original basis for most other European systems (since they had strict primogeniture before anyone else, so they needed it). The heraldic rules fit, and the few descriptions we get of cadet houses seem to work, but there's a huge problem: In France, if you can't acquire a new territory through marriage or conquest to give to your second son, you almost always split off a new fief within your main holdings for him. In Westeros, acquiring territory through marriage seems to be actively discouraged rather than encouraged. Conquest may be about as common as in France (like House Lannister of Darry) but that's not very common. And we don't see subdivision every generation—it's rare enough that it's presumably something you only do if you really like your second son or are really worried about him rebelling, rather than it being expected.
Norman England directly brought over the French appanage traditions, but William immediately ran into the problem of his heir trying to overthrow him twice, so he ended up leaving everything to his second son, William II. Meanwhile, William II and Henry I were trying to flatten out the aristocracy and shrink all the old and new earldoms down, so nobody wanted to subdivide their domains either, because that would just make it easier for the kings to turn everyone into little more than barons. But when it did happen (often after acquiring new territory—e.g., Edward II conquered north Wales and part of Scotland, then his son gave York and Lancaster to his cadets), they followed the French rules. Which is basically the same thing we see in Westeros.
And if you assume that GRRM borrowed this system for cadet houses, everything works, including the heraldry.
What about bastard houses? They weren't very common in Plantagenet England, but they were common a few centuries later. And if you look at the Westerosi heraldic pattern—the father's arms quartered on a plain field with a baton sinister—that was invented by the King of Arms standardizations in the late 16th century. At that time, it was clear that bastards were not considered cadets—this was explicit with Charles II's bastards. Appanage was pretty much dead at that point, but if it had still existed, I don't think it would have applied to bastards. At that time, appanage was pretty much dead anyway, but also, Charles II's bastards were explicitly not called cadets.
So, my guess is that Westeros's cadet house system is borrowed from Norman to Plantagenet England, but its bastard system is borrowed from Stuart England.
As you can see, there's a lot of guessing here, but I think it all fits.