I Get This Impression That House Of The Dragon Doesn't Get That "named" Heirs Aren't Really The Norm

I get this impression that House of the Dragon doesn't get that "named" heirs aren't really the norm in Westeros. If it were that easy for someone to just give everything to their favorite child, Randall Tarly wouldn't have needed to force Sam to go to the Wall and Tywin could have simply chosen Cersei over Tyrion as heir of Casterly Rock.

If we look at the history Westeros borrows from, the concept of "naming" heirs wasn't really a thing in medieval England. Landed gentry didn't have direct say over the order of succession until the Statute of Wills in 1540. Before then, land and subsequent titles could only be inherited through agnatic primogeniture.

Agnatic primogeniture prioritized the living, eldest, trueborn son. Claims can only be passed on patrilineally. This means that a grandaughter can inherit a claim of her grandfather's titles through her father, but a grandson cannot be given the same through his mother. However, if his mother finally does have land and titles under her own name (not under her father's), only then does her son and other children enter the line of succession.

The reason it was like this was because it kept land and titles under one family. Daughters are less preferred because when they are married, they become part of their husband's family — meaning that any titles they receive will be inherited through a new line. This wouldn't be an ideal situation because it gives two families claims to the titles. The more claimants there are, the more unstable the hold the owner has.

In other words, agnatic primogeniture was practiced for stability. Because back in the day, titles weren't just property or land. They came with governorship over a people, so a stable and predictable transfer of titles was necessary to avoid civil conflicts and questions of legitimacy.

A landed lord or lady wasn't given the right to designate heirs for a few reasons:

Most of them were vassals who oversaw the land in the name of someone higher up. It technically isn't even theirs to give away (see: feudal land tenure).

The wishes of a human being are less predictable than having a determined line of succession based on birth order. What if he becomes incapable of declaring an heir either through illness or disability? What if he's captured and a bad actor forces him to name this person heir under threat of violence?

People died unexpectedly all time. This was before germ theory and modern medicine — child mortality was extremely high. With no refrigeration technology, a single poor harvest could mean dying from starvation. Bandits, cutthroats, and raiders were a constant threat. They could not afford to rely on a person choosing a different heir every time the old heir drops dead, because the landed lord/lady could die just as suddenly.

Even 21st century families stab each other in the back over who gets grandma's house — so imagine having an uncertain line of succession in the middle ages over a life-defining lordship and without a modern-day court system to mediate.

Going back to HotD, whenever Targaryens did go against the established line of succession, they could only have done it by consolidating the support of their vassals. Only royalty seemed to have the power to bend agnatic primogeniture, but even then they were beholden to it.

When Jaehaerys I ascended the throne over Aerea, it was mainly because there were those who saw Maegor the Cruel's act of disinheriting Jaehaerys as null and void. This restored Jaehaerys place in the line of succession above Aerea.

And when Rhaenys was passed over for Baelon, Jaehaerys had to convene his lords and offer compelling reasons as to why — her young age, her lack of an heir, her Velaryon last name, etc. It wasn't a given that just because she was a woman that she was ineligible. If he was doing it purely out of misogyny, he still had to legally justify his misogyny in order to strip away her rights.

Even after consolidating support, the book mentions Jaehaerys I and Viserys I's respective hold on the crown was still weakened. Even though their claims were backed by reasons cosigned by a powerful majority, they still had to ensure the security of their rule through other means. There were people who doubted their right to rule, and those people had to be placated with gifts (by Viserys) or intimidated into submission (by Jaehaerys).

So we come to Viserys I who never gave his vassals a reason why Rhaenyra should supercede his three sons other than, "I said so." Had he convened with his lords and maybe made the argument that a first marriage takes precendence over a second one, then maybe he could have set a new precedent and gathered support.

But no, he didn't. He relied on the power of his own words and the lords' personal oaths — oaths that he didn't exactly plan how he would enforce posthumously.

And the Realm did not choose to adopt a different succession law after Jaehaerys's designation of Baelon in 92 AC or the Council of Harrenhal choosing Viserys on 101 AC. If those two events did change anything, it was that now women were exempt from the line of succession for the crown and only the crown. It did not set the precedence that monarchs could freely choose heirs. It did not upend the whole system; it only made a tweak, as most lawful policy-changes do, by carving out at an exception. It was a committee, not a revolution.

Before and after the Dance, no other monarch, lord, or lady "declared" an heir that went against agnatic primogeniture, save for Dornish who have cognatic (equal-gender) primogeniture instead. Ramsay had to get rid of Roose Bolton's living trueborn son AND be legitimized by the crown in order to be recognized as heir (only a crowned monarch can legitimize baseborn children which is another world-building pillar a lot of people miss). Randall basically had to force Sam to abdicate because he wanted his younger brother to inherit instead. And of course, Tywin despite his intense hatred of Tyrion is forced to acknowledge him as his heir.

The rigidity of the line of succession is a major and constant source of conflict in the series, so it baffles me that people really thought that characters could just freely choose their heirs. That's why we have a civil war. It wasn't a misunderstanding. It's the expected consequences of someone carelessly going against a foundational tenent of the society they inhabit.