As Prince William enters his 40th year, he finds himself taking on a greater share of royal responsibilities than he ever has done.
The father-of-three not only has the task of bringing up his children alongside wife Kate Middleton in the glare of the royal spotlight, but as Queen Elizabeth II's health and mobility see her drastically reduce her number of public appearances, William is required to step in and step up.
From his birth on June 21, 1982, William has been on a straight path to the British crown and now in his 40s, the end of the path is becoming increasingly closer in sight.
Here, Newsweek looks at the key stages in William's journey from prince to king.
The Heir to the Heir
When Prince William Arthur Philip Louis of Wales was born in 1982 to Prince Charles and Princess Diana he automatically became the second-in-line to the British throne and the third most senior prince in the royal family (behind Charles and Prince Philip).
As the heir to the heir, William became the most famous baby of the 1980s and was joined by brother Harry in 1984.
William's place in the line of succession, unlike those below him, cannot change until Queen Elizabeth II's death and, unlike those below him except for his eldest son, he can only move up the line.
The line of succession is the order in which members of the royal family will inherit the throne and is published by the royal household. The list is currently published to the 24th place and for 40 years, William has remained at position two.
When William himself had children they came after him in the line of succession, meaning that everybody below him, including Harry, moved down the line in accordance with how many children he had.
William's position as second-in-line to the throne comes with no official title or responsibilities, though he has undertaken official duties on behalf of the queen for two decades. The day his father becomes king all this will change.
Heir Apparent
When the queen's reign ends, at the moment of her death Prince Charles will become King Charles III. At the same moment, William will automatically move from second in the line of succession to the first, at which time he will be known as the heir apparent. He does not automatically take his father's old title of Prince of Wales.
In the months following Charles' accession there will be a number of key royal events including the state funeral of Queen Elizabeth II which, in accordance with tradition, is expected to be held at Westminster Abbey in London.
It is also likely that after the funeral, a nationwide tour will be undertaken by Charles to establish himself as king in Scotland, Northern Ireland and Wales as well as England. During this time William and Kate may accompany the king in a dynastic showing of royal power.
The Prince of Wales
Prince William will not automatically become Prince of Wales when his father becomes king as this title is one that is in the gift of the monarch and is not inherited.
Queen Elizabeth II did not create Charles, Prince of Wales until he was nearly 10 years old, six years after her accession to the throne.
The Prince of Wales title is traditionally conferred on the heir apparent during an investiture ceremony held in Wales at Caernarfon Castle. Prince Charles' investiture was held in 1969 when he was 20 years old.
During the investiture of the Prince of Wales, the heir apparent swears allegiance to the monarch in an oath. The oath is the same one which royal peers swear to the monarch upon their coronation and follows a standard form of: "I, [insert name], do become your liege man of life and limb and of earthly worship and faith and truth I will bear unto thee to live and die against all manner of folks."
It is likely that Charles will want William to attend his coronation as the Prince of Wales and so could see the new king bestow the title on his eldest son quickly after accession. When William takes on the title, Kate will become known as The Princess of Wales, a title not publicly held since Princess Diana.
Head of The Duchy of Cornwall
As first in line to the throne, William will also become head of the Duchy of Cornwall and by extent take on the title of Duke of Cornwall.
The duchy is in essence an estate comprising over 52,000 acres of land, the rent of which creates a private income for the monarch's eldest son. The duchy was created in 1337 by King Edward III.
Charles has been Duke of Cornwall since 1952 when his mother became queen, though she ran it in trust for him until he became 21 years old, after which time he became entitled to its full income.
William has taken an increased interest in the duchy over the past decade, regularly meeting with the farming families who work on the estate and learning about its financial management.
The income from the Duchy of Cornwall is published regularly in annual reports and in 2021, Charles received £20 million ($24.5 million).
From this income William will pay his expenses as well as those of wife Kate and of his children and their eventual families.
His Majesty King William V
When King Charles III dies, William will automatically become King William V and his eldest son Prince George moves into the position of heir apparent.
William will be the 63rd English monarch heading an institution thats history covers over a thousand years.
Despite the crown being enshrined in historic symbolism, William has made no secret that he will not stick to antiquated ideals at the risk of jeopardizing the future.
