Whether you’re a king or an ordinary citizen with some property, you can’t take it with you. When you die, your money and your goods and your land have to go to somebody. But who?
I’m sort of lumping two things together here, inheritance and succession, because they resemble each other pretty closely; after all, a title is in many respects just a special case of a thing to be inherited. The specialness of that case does matter — some kinds of inheritance can be divided, but a title isn’t one of them — but since many of the same assumptions underpin both topics, we’ll look at them together.
Let’s start with that notion of division. Formally, this is known as “partible inheritance,” and it’s common in the modern day: in your will, you divvy up your property between various heirs. Those can be pretty much anybody you choose, but in some societies this is or was required to be equal shares to all legitimate offspring, with perhaps a double share to your eldest son, or some other codified arrangement.
Partible inheritance works great with money, and acceptably well with belongings like furniture and such. It works miserably with land. If you’ve got a farmer with a hundred acres of land and two sons, dividing up that estate leaves each son with fifty acres. If each of them has two sons, now every member of the third generation has twenty-five acres. It doesn’t take many generations before nobody has enough land to support their family — a danger real enough that in the eighteenth century, the Parliament of Ireland passed an act which mandated that Roman Catholics had to pass down their estates through gavelkind (a form of partible inheritance), while Protestants could use male primogeniture. This was explicitly a way to break the power of Catholic landowners, or else to pressure them into conversion for the sake of their estates.
The aforementioned primogeniture is a way out of this trap. Under that system, a person may specify bequests of various kinds in their will, but the bulk of their property and wealth (as well as their title, if any) is inherited by a single heir. Most of the time this heir is selected by age, and most of those setups award everything to the eldest living child — hence primogeniture, “first-born.”
The main alternative to this is ultimogeniture, inheritance by the youngest, which is far rarer. (There’s also secundogeniture, but that’s a slightly different arrangement where the second son of an elite family is given a sizable amount of land and wealth to form an official cadet branch of his original house.) Where ultimogeniture exists, the idea seems to be that the eldest children will have already moved away and started their own lives, but the youngest has borne the burden of caring for their elderly parents, and therefore inherits their estate. But that presumes the youngest is of a useful age, which won’t always be the case. Primogeniture at least increases the chance that the heir is an adult, and won’t need some kind of guardian or regent to handle things for years to come.
Gender also plays into this, of course. Because most societies have been patriarchal, most inheritance has been male primogeniture, or partible inheritance for all male heirs, etc. Even in a matrilineal society (not the same as matriarchal), it’s the sister’s sons who most often inherit, not her daughters. The more flexible version of this is male-preference primogeniture, where a daughter can inherit if she has no living brothers and any deceased ones left no living sons. The alternative solution to that problem is agnatic primogeniture — agnates being your relatives traced only through common male ancestors; cognates are related to you through at least one female ancestor, and enates are traced only through those — so that an uncle or a male cousin inherits instead of a daughter. Under agnatic-cognatic succession, you have to go through the entire family tree and verify that all patrilineal descendants are dead before someone can inherit through a female line.
Gender doesn’t have to be a concern, though, or it can be flipped on its head from the usual pattern. Absolute primogeniture discards gender entirely; you see this in some modern monarchies, and it dodges the problem of having a perfectly competent adult daughter set aside in favor of an infant son. You can even have enatic or uterine inheritance, where males are the ones excluded, and only daughters of the matriline can inherit — though unsurprisingly, given patriarchy, this is not very common at all. One of the few examples I know of is the Rain Queen of the Balobedu people in South Africa.
But wait! There’s more! (This stuff can get really complicated.) Sometimes direct lineal descent isn’t the most important factor, but rather seniority. Under agnatic seniority, the oldest male of the relevant patriline inherits, meaning that a man might be succeeded by his younger brother or a cousin or even an uncle, rather than his own son. A specialized version of this was the Russian rota system, where the throne went from brother to brother before passing to a son — but only the sons of men who had previously held the throne. If your father died before taking power, you were excluded from the succession. And finally, there’s “proximity of blood,” where you wind up doing complicated math about who counts as being the most closely related to the deceased, with elder siblings counting as closer than younger, and men as closer than women.
All of this complication happens because deciding who gets the land, the title, the money, or any other thing belonging to the deceased is really important — land and titles most especially. Money is a fluid thing; if you give someone a cash bequest, they can invest that or spend it to jump-start a business or otherwise use it to beget more money. But land doesn’t beget more land, and for it to be useful, you need a usable amount of it, which means a lot of concern has been poured into making sure estates remain intact. And titles, of course, can’t be divided at all. Disputes over who’s supposed to inherit have started everything from fist-fights to lawsuits to multi-generational international wars.
But there’s one wrinkle I should note here, which is that while you would think it’s better for a monarch to make it crystal clear who their heir is . . . sometimes that isn’t the case. If you’re allowed to choose, keeping it open keeps the contenders from getting complacent (though it may also encourage them to off one another, in order to narrow the field). It also means you don’t risk getting pushed aside. More than one elderly, ailing monarch has discovered their influence withering away, as the court gravitates toward currying favor with the throne’s future occupant. Not naming an official heir can be a way of ensuring power remains in your own hands.
You’d just better hope you don’t drop dead without warning. For the average citizen, dying without an heir means tedious legal problems for the family they leave behind — but for a hereditary monarchy with an unclear line of succession, it can mean civil war.