You've probably noticed that the husband of Queen Elizabeth II is not the King of England. Rather, he is Prince Philip (also known as the Duke of Edinburgh). Yet when Prince William becomes King, his wife, Kate, will be known either as "Queen" or "Queen Consort" for as long as William lives—after which Prince George is in line to become King, and Kate will take on a new title reflecting that she has become the widow of a king and mother of the reigning monarch (assuming she outlives William).
So, you might wonder: If Kate is eligible to be Queen, then why is Prince Philip not King?
The answer, according to the official website of the British royal family, is found in British Parliamentary law, which determines who's up next for the throne, and also what title his or her spouse will have. In terms of succession, the law looks only to blood, and not to gender. In terms of the spouses of royalty, however, that law treats men and women differently. Here's an easy guide to the current line of succession.
In keeping with this, when Philip Mountbatten married then-Princess Elizabeth in 1947, he did not become Prince. He only became Prince, or more formally, "Prince Consort," when Queen Elizabeth II took the throne upon her father's death. Fun fact: Philip was born a prince to both the Greek and Danish royal families but renounced his right to those thrones and titles to marry Elizabeth, and presumably as an incentive to do so, was designated His Royal Highness, the Duke of Edinburgh on the eve of the wedding, a title he retains to this day.
Prince Philip, who officially gave up his royal duties in May 2017, has been Queen Elizabeth II's "strength and stay" during her reign (and he calls her this adorable nickname). So it seems, a title is just that. You won't want to miss these 21 candid, rarely seen photos of the British royals.