But marrying royalty doesn't automatically enter you into the nobility. When Princess Anne, Queen Elizabeth II's daughter, married commoner Mark Phillips in 1973, rumor has it that the queen offered him an earldom. Phillips declined, and the couple's children are commoners, even though they are in the line of succession.
Now that royals are making a habit of marrying commoners, is there anyone they aren't allowed to marry?
Roman Catholics. Pursuant to the 1701 Act of Settlement, royals are removed from the line of succession if they convert to Catholicism or marry a Roman Catholic. The act has been put into use several times. Prince Michael of Kent, Queen Elizabeth II's cousin, lost his place in the succession when he married Catholic Baroness Marie-Christine von Reibnitz in 1978.
There's nothing to stop Prince William from taking a Jewish, Muslim, Hindu, or Buddhist bride. The prince himself must be a Protestant at the time of his accession to the throne, however, because the king is the sworn defender of the faith.
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If the queen really didn't like Kate Middleton, could she stop the wedding?
She could make it difficult. Under the Royal Marriages Act of 1772, the monarch's direct descendants must seek his or her permission to marry. But if a prince or princess can't get the monarch's consent, they just have to wait a year. Unless both houses of Parliament object, the marriage is on.
Royals who elope without the royal blessing aren't removed from succession, but their marriage is void. Any children born to the couple are illegitimate and can't ascend to the throne.
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Explainer thanks Estelle Bouthillier of Concordia University, Steven Frieder of Marquette University, and royal expert Charlie Jacoby.