How to become the British Monarch:
Historically, the crown sat upon your head mostly because you had the biggest army.
When you died usually your eldest son kept control over that army and so the crown relocated
to his head, though, of course, someone with a bigger army could change the political landscape
quite abruptly.
As time marched on and the world grew less violent eventually in 1701 Parliament established
a set of rules to transfer the crown from one head to another -- hopefully with less
turmoil than before.
So here's how the 1701 rules work:
Frist: don't be Catholic.
The British Monarch is also the head of the Church of England to which the monarch much
convert if not already a member.
Except that if you're Catholic, no crown for you.
The history of the royal family and how this rule came to be is a story for another time,
but suffice it to say that bigger-army diplomacy was involved.
And, BTW, no you can't cleverly get around this rule by converting from Catholicism to
something else then to Church of England.
In the eyes of the crown, Catholicism is transitive.
Second: don't be a bastard.
Sometimes it's good to be the king, but it's never good to be the illegitimate children
of the king -- who are out of line for the crown literally from the moment of their conception.
If you're related to the monarch but are either a Catholic or a bastard or both, the crown
has the delightful term 'Naturally Dead' to refer to you and your lack of right to succession.
Third spouses don't count.
While people often think of kings and queens as a pair: that's not the way it works here.
Spouses of Monarchs are known as Royal Consorts.
They may be called 'prince' or 'queen' but as far as the crown is concerned, they're
not in line for the throne, they're just the matching 23 Chromosomes needed for the creation
of the real heir.
Fourth and Finally: Male Primogeniture (whatever).
This is the algorithm of inheritance.
When the Monarch dies -- or abdicates -- but usually dies -- the crown goes to the eldest
son who isn't 'naturally dead'.
If there happens to be an elder daughter tough luck to her: baby brother gets the crown.
It's Simple enough, but there are non-obvious cases: take a king with two sons: if the eldest
dies before the king does, obviously the crown goes to the youngest (now oldest) brother.
But what if the eldest son gave the king a grandson before death?
Where does the crown go then?
Well, the crown basically pretends that everyone -- except the naturally dead -- is alive:
so upon the death of the king the crown goes to his eldest son -- who is now sort of the
king who just really happens to be dead -- so the rule kicks in again, and the crown goes
to *his* son, not as seems obvious now, his brother.
But if this 1701 rule means that eldest sons get the crown, how did queens ever come to
be?
Basically, daughters were the last choice of the crown, which is why there have been
so few.
To get the crown, a daughter had to be either the only child of the monarch or the eldest
child without competing brothers.
So pregnant mothers must have made any daughters with queenly aspirations quite nervous.
Now sometimes the branch of a family tree die out: be it from war or plague or whatever
so the crown's contingency plan if it's at a dead end is to back up one level, and then
apply the rules forward again looking for a living head to sit upon.
If no luck, back up again, and repeat and repeat until a living heir is found.
And there will always be an heir.
The first king of England was over a thousand years ago and the mathematics of human reproduction
backed up by DNA evidence reveals that just about every European alive is distantly related
to him.
So the crown will eventually find a way.
So from the first king through the new millennium, the various rules when along, making monarchs,
though with a gender biased result, that no one seemed too bothered about until suddenly,
in 2013 for no particular reason at all, everyone decided that the rules needed to be updated
*right now*.
So, Parliament and the Monarchy got together and made some changes: most notably striking
the male part of rule #4.
From 2013 on the crown views all royal sons and daughters with equal favor.
The only thing that matters is the order of their birth.
So prior to 2013 the boy in a set of fraternal twins in development could sit back and relax
-- secure that the crown would be his no matter what happened on delivery day, but in the
post 2013 gender-equal world it's now a race for the door to win the crown.