Prince Philip was the longest serving consort, spending 73 years alongside Queen Elizabeth II and carrying out thousands of engagements.
Despite all his service, he was never known as King.
He died in April 2021, his main title being 'Duke of Edinburgh'.
When Elizabeth II acceded to the throne in 1952, she immediately became Queen.
For her husband Philip, himself of Greek and Danish royal background, a title was more complicated.
Philip and Elizabeth married in 1947, when her father was on the throne, and he became His Royal Highness, the Duke of Edinburgh. He was also given the titles Earl of Merioneth and Baron Greenwich, though these are rarely used.
Watch: Prince Philip and the Queen's marriage
Before their marriage, he had been Prince Philip of Greece and Denmark, but renounced his former royal titles completely to marry Elizabeth.
When she acceded, he was not given any new titles. At her coronation a year later, he was the first to swear an oath of allegiance, in which he said:
“I, Philip, Duke of Edinburgh
do become your liege man of life and limb,
and of earthly worship;
and faith and truth I will bear unto you,
to live and die, against all manner of folks.
So help me God.”
Five years after she’d been on the throne, the Queen bestowed the additional title of Prince onto Philip. Unlike some who went before him, he was not titled as Prince Consort, though his job was as a consort to the Queen.
Read more: Why does the Queen have two birthdays?
In the London Gazette, it was announced: “The Queen has been pleased by Letters Patent under the Great Seal of the Realm bearing date 22nd February, 1957, to give and grant unto His Royal Highness the Duke of Edinburgh, K.G., K.T., G.B.E., the style and titular dignity of a Prince of the United Kingdom of Great Britain and Northern Ireland.”
It added: “The Queen has been pleased to declare her will and pleasure that His 'Royal Highness the Duke of Edinburgh shall henceforth be known as His Royal Highness The Prince Philip, Duke of Edinburgh.”
Why wasn’t Prince Philip a king?
The Act of Succession, which secured the throne for Protestants and ensured no Catholic or anyone married to a Catholic could accede, is usually given as the reason for the Philip remaining a prince, not a king.
The act places gender as part of the line of succession, which until recently meant that those women born in the line would find themselves overtaken by younger brothers and their offspring, as has happened to Princess Anne.
It also means titles bear different significance. While wives of Kings become Queens, more typically fully known as Queen Consorts, men married to Queens are not King Consorts.
While Queen can mean ‘wife of a monarch’, King never means ‘husband of a monarch’ and so it implies ruler, which Philip is not.
In 1960, the Queen issued a decree which allowed her descendants, if they were not styled His or Her Royal Highness, or with a title of Prince or Princess, to take the surname of Mountbatten-Windsor.
Most recently this means Prince Harry’s son is called Archie Mountbatten-Windsor.
In 2013, before the Duchess of Cambridge had her first child, various changes were made to the Act of Settlement which brought it up to date.
Read more: Who was Prince Philip's other royal family?
Any child born after October 2011 retains their place in the line of succession regardless of their gender. This was put in place to ensure that if they had a daughter first, her place would not be lost to a future younger brother.
While it didn’t immediately matter because George was their oldest, it does mean Charlotte keeps her place despite having a younger brother.
The 2013 amendments also end the banning of Catholics acceding to the throne.
As her “strength and stay” Philip became the longest serving royal consort in 2009, and continued to carry out engagements with and for the Queen for another eight years.
He’d clocked up some 22,000 engagements before he retired from public duties in 2017.
Philip died on 9 April 2021, at Windsor Castle, about three weeks after returning there from his longest ever hospital stay.
What happens to his title?
The title of the Duke of Edinburgh is hereditary and so passes down to Prince Charles, his oldest son.
A notice from the College of Arms, which manages heraldry in England, Wales and Northern Ireland, noted: "These peerages are hereditary and on the death of His Royal Highness have passed to his eldest son, HRH The Prince of Wales. In the event of the Prince of Wales or any subsequent holder of these titles succeeding to the Crown, these titles and all others held will merge with the Crown."
However, it won't stay with him forever.
Another change will happen when the Queen dies, as Prince Edward, her youngest son, will become the Duke of Edinburgh.
Edward and his wife Sophie are known as the Earl and Countess of Wessex, titles which were bestowed on them as a wedding gift from the Queen.
At the time of their marriage in 1999, the Palace said that Prince Philip and Prince Charles had agreed Edward would inherit the title, but on the death of both Philip and the Queen.
A title held by someone who becomes the monarch then merges with the Crown so technically ceases to exist.
It can then be recreated for someone else.
Watch: Prince Philip death: The Duke of Edinburgh dies, aged 99