The law states that the firstborn of a monarch shall be anointed regardless of gender. However, the law is biased when it comes to the spouses of royalty.
Queen Elizabeth II's husband, Prince Philip, possesses many titles but 'King' is not one of them. All King's wives have been called The Queen, but why is The Queen's husband not called a King in the British monarchy?
The answer lies in the parliamentary succession law. It determines who's up next for the throne, and also what title his or her spouse will have. The law states that the firstborn of a monarch shall be anointed regardless of gender. However, the law is biased when it comes to the spouses of royalty.
Wives of British kings take the female form of their husband's name and become known as queen consorts. Thus, when Prince William married Kate Middleton and became Duke of Cambridge, Kate became Duchess of Cambridge and is likely to become queen consort if William were to accede to the throne. Like such, when Queen Elizabeth II's father, George VI, became King, his wife's title became "Queen Consort". She held her title until George died, at which point Elizabeth II ascended to the throne, and her mum became "Queen Mother."
On the other hand, when a female in the royal bloodline marries, her husband is not eligible to take the male form of his wife's title. Instead, a Prince Consort is the husband of a Queen who is not himself a King in his own right.
So when Philip Mountbatten married then-Princess Elizabeth in 1947, he did not become Prince. It wasn't until 1957 that Queen Elizabeth bestowed the title of "prince" on Philip by issuing a letters patent. Prince Philip is the consort of Queen Elizabeth II of the United Kingdom and the Commonwealth realms, meaning he is a Prince of the UK.
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. On the eve of the wedding, he was designated His Royal Highness, the Duke of Edinburgh, a title he retains to this day.