For a young man who craves a "normal" life, it was yet another reminder of just how abnormal his existence can sometimes be. Last year, Prince William had to check that his grandmother wouldn't object to him marrying Kate Middleton. It was both a formality and a requirement under British law. The Queen readily gave her consent. William can thank one of his royal ancestors for imposing this hurdle on his path to marriage.
Back in the 18th century, as well as dealing with the challenging issues of losing the American colonies and serious bouts of illness, King George III was also vexed by the behavior of his younger brother. The Duke of Cumberland had married, in secret, Lady Anne Horton. She was said to have "bewitching eyes" and was regarded by the King as highly disreputable. Incensed, he took action and a bill, known as the 1772 Royal Marriages Act, came in to being.
Three centuries on, the Queen has the right to rule a royal marriage invalid, if she objects to an individual who is poised to join her family. It's a power some mothers-in-law would love to have. In reality, the Queen would only say "no" on the advice of the Prime Minister. It's not something she has ever done. It is, though, fascinating to speculate what her response would have been in the late '90s if her son, Prince Charles, had told her he wanted to marry Camilla Parker-Bowles. As it was, he waited until 2005 when public opinion about the one-time royal mistress had softened.
Before he follows in his father's footsteps and gets to the altar, Prince William has to navigate not one, but two ancient laws. The second one is still in force, even though many people see it as blatantly discriminatory. It stretches back once again to the 18th century - this time, to the reign of King William III.
At this stage in history, the monarch was both ruler of England and, thanks to Henry VIII, Supreme Governor of the established Anglican church, the Church of England. But in 1701, William III wanted the law to go further. He was ill, childless, and desperate to ensure his rival James II, an exiled Catholic, wouldn't return to the throne.
So, in 1701, what's known as the Act of Settlement was passed. To this day, a British King or Queen – if they want to stay on the throne – cannot be a Roman Catholic or marry a Roman Catholic. There has been much talk about changing this law - what one critic has called Britain's "grubby little secret" – but no sign of any action.
Having negotiated the hurdles of his grandmother's approval and his fiancée's religion, William is in the clear. There are no requirements for a prince to marry someone of royal or even aristocratic blood. A future bride doesn't have to be a British citizen or come from a Commonwealth country, like, say, Canada. There is no legal bar on them marrying a Muslim, a Jew or, indeed, an atheist.
And the unwritten royal rules have also been relaxed, mirroring changes in British society. When Lady Diana Spencer was catapulted onto the scene, she was very young and she had very little experience of life.
By contrast, her son lived with his girlfriend at university. They've been going out for nearly a decade. Indeed, the second-in-line to the throne has said one of the reasons he waited so long to propose was because he wanted to show Kate what life was like in his family, and, in his words, "give her the chance to back out if she needed to before it all got too much."
Kate Middleton hasn't backed out, and the path is clear for the couple to tie the knot. After the fairytale royal marriages of the '80s which ended in divorce, the fervent hope in royal circles is that this will be a modern union that does endure.
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