What would happen if a King’s first born were twins?
Who would be the crown prince?
Is there any historical precedent for it?
Well surely one twin is always older, even if it’s only by a few minutes?
I don’t recall any precedent, but I imagine it would be whichever twin is born first.
Presumably, the twin that came out first would be the heir, and the second would be the spare.
I have no idea if there is an historical precedent, but the issue comes up in Lewis’ A Horse and His Boy, where the long-lost elder twin is found and declared the heir. When he protests that his brother and he were the same age, his father replies:
“Nay, lad, art Corin’s elder by full twenty minutes. And his better too, let’s hope, though that’s no great mastery.” And he looked at Corin with a twinkle in his eyes .
One gets imprisoned in the Bastille and has to wear an Iron Mask their whole lives until the 3 musketeers + D’artagnon rescue them.
Just kidding. 
K, back to your question:
Hmm, googling it says it depends on:
- Country where it happened and their succession rules
- If males only were allowed to be heirs
- If the twins were oldest
- Who survived longest (infant mortality was very high back then)
- and, if everything else is equal, which twin came out first.
Oh, and regarding historical precedent, in the examples I found it never came down to #5. Usually, since chance for twins increase because of age, first births were rarely twins. Also, if twins did come out first, they were usually hit by #4: an early death.
I’ll give you the twins Berenguer Ramon and Ramon Berenguer, counts of Barcelona. They were supposed to rule jointly; according to Wiki they divided the inheritance, according to stories I’ve been told they took turns (six months each). In any case the problem was finally solved when Ramón broke his neck during a hunt (or couse, Berenguer got accused of having a hand in the accident, hence the nickname in Wiki).
Please note: although they were Counts, and the Counties of Catalonia and Provence would often pay tribute to a bigger neighbor or other, they were independent rulers.
Unless the first sells his birthright for a bowl of soup.
The late Shah of Iran, Mohammed Reza Pahlavi, was a twin. He was the elder, and his twin was a girl, Princess Ashraf Pahlavi, so there was no question of the succession.
And it never really came up, did it?
King William IV of Britain and his queen, Adelaide, had twins stillborn in 1822.
King Maximilian I of Bavaria had two sets of twin daughters: Elisabeth (queen of King Friedrich Wilhelm IV of Prussia) and Amalie (queen of Johann I of Saxony) born 1801, and Sophie (wife of Archduke Franz of Austria) and Maria Leopoldine (queen of King Friedrich August II of Saxony) in 1805. Sophie narrowly missed becoming an empress, which would’ve made all of Maximilian’s twin daughters queen consorts. She did become the mother of the Emperor Franz Joseph I, however.
The famous Cleopatra had twins, Alexander Helios and Cleopatra Selene, by Mark Antony.
AFAIK there’s never been a case of twin boys being born as the heirs apparent to a throne, at least not in the European countries. There are some cases of boy/girl twins (where typically the boy would take precedence in succession regardless of birth order), boy/boy twins born to families that already have older sons, and twins born to dethroned former royal families.
King William IV of Britain and his queen, Adelaide, had twins stillborn in 1822.
King Maximilian I of Bavaria had two sets of twin daughters: Elisabeth (queen of King Friedrich Wilhelm IV of Prussia) and Amalie (queen of Johann I of Saxony) born 1801, and Sophie (wife of Archduke Franz of Austria) and Maria Leopoldine (queen of King Friedrich August II of Saxony) in 1805. Sophie narrowly missed becoming an empress, which would’ve made all of Maximilian’s twin daughters queen consorts. She did become the mother of the Emperor Franz Joseph I, however.
The famous Cleopatra had twins, Alexander Helios and Cleopatra Selene, by Mark Antony.
AFAIK there’s never been a case of twin boys being born as the heirs apparent to a throne, at least not in the European countries. There are some cases of boy/girl twins (where typically the boy would take precedence in succession regardless of birth order), boy/boy twins born to families that already have older sons, and twins born to dethroned former royal families.
Actually, Sophie, realizing her husband wasn’t up to ruling, convinced him to abdicate his claim to the throne, in favor of their son, Franz Joseph. (She had a great deal of influence over Franz Joseph, however.)
(I’m currently reading a biography of FJ’s wife, Empress Elisabeth, aka “Sisi”.)
In Jeffrey Archer’s great political novel First Among Equals, a pair of twin boys is born to a couple of British aristocrats just a few minutes apart. The eldest eventually becomes duke; as it happens, he’s the dimmer of the two. The younger boy, who is much sharper, goes into politics and does very well for himself.
I cited one in post 7. The fact that their title was “counts” doesn’t mean it wasn’t a throne. They were independent rulers, same as the Princes of Monaco or Luxembourg are independent rulers.