It’s an important question: Why isn’t Queen Elizabeth’s husband considered the King of England? As with most royal protocol, the reason is a little murky, relying on patriarchal tradition and not always common sense.
Prince Philip is referred to as a prince for a reason.
According to the official website of the British royal family, when it comes to titles, it’s about bloodline, pure and simple. And the laws are applied differently for men and women. (Hat tip to Reader’s Digest for supplying this handy guide to the current line of succession). When a male in the royal bloodline marries, his wife takes whatever is the female form of his title. It’s why when Prince William, Duke of Cambridge, married Kate Middleton, she became Duchess of Cambridge (aka Princess William of Wales).
But when a man marries into the royal bloodline, he’s not allowed to take on his wife’s title. Like in the case of Princess Eugenie, who got hitched last October. Her husband, Jack Brooksbank, remains “Mr. Jack Brooksbank”, however, Eugenie got a title change, becoming Her Royal Highness Princess Eugenie, Mrs. Jack Brooksbank.
“In much the same way, grandchildren born to the sons of the monarch receive the title of prince or princess, but those born to the daughters of a monarch don’t. In the case of the family of Prince and Princess William of Wales: Prince George’s children will automatically be princes and princesses; Princess Charlotte’s kids will not (unless, of course, a tragedy upends the British line to the throne).”
Lucy Hume, an expert on the royal rankings, tells Town & Country that “Royal titles are inherited through sons, so if Princess Charlotte has children they would not automatically inherit the titles ‘HRH,’ ‘Prince,’ or ‘Princess.'”
Another fun fact from Reader’s Digest: Prince Philip was actually born a prince to Greek and Danish royal families, but renounced his title in order to marry Elizabeth. On the eve of the wedding, we was designated His Royal Highness, the Duke of Edinburgh, a title he still retains.