Royal fans have noticed that King Charles actually has another official title that is less-known, ahead of his official birthday on Saturday (14 June).
It's a big week for the King, despite not having his actual birthday until 14 November, the official birthday of the monarch always falls on the second Saturday of June.
Thousands of spectators are expected to flood the Mall to celebrate the beginning of King Charles' celebrations this weekend.
Trooping the Colour has marked the official birthday of the King or Queen for over 260 years, and it's an event that has numerous moving parts - including more than 1,400 soldiers, 200 horses, and 400 musicians.
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The King will take on a slightly different role during this year's celebrations, given that he is still receiving weekly treatment for his cancer diagnosis.
In previous years, Charles has taken part in the parade on horseback, but he will travel by carriage from Buckingham Palace this year, alongside Queen Camilla.
But while it's widely known that the King is the UK head of state, many people are just discovering another of his official titles.
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During a visit to Lancaster Castle in the north of England on Monday (9 June), the King was pictured greeting fans at what is known as the Ceremony of the Keys.
The ceremony is a long-standing royal tradition that dates back 174 years, which began during the reign of Queen Victoria.
It marks the occasion that the monarch also became the Duke of Lancaster - which is another of King Charles' titles.
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The royal family's official X account posted: "The King, who is also The Duke of Lancaster, has taken part in the traditional Ceremony of the Keys at Lancaster Castle!
"It was wonderful to meet so many members of the local community this morning!"
But it's actually not the only alternative title given the monarch.
Charles holds a number of titles, and is listed officially in the House of Commons Library as: "Charles the Third, by the Grace of God of the United Kingdom of Great Britain and Northern Ireland and of His other Realms and Territories, King, Head of the Commonwealth, Defender of the Faith."
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A mouthful for sure.
He also has a number of unofficial titles, as The Isle of Man has also proclaimed Charles as 'Lord of Mann', but this isn't recognised by the library.
'Seigneur of the Swans' is another of his lesser-known titles, but has been in place for centuries.
This is because the monarch has ownership of all unmarked mute swans in open water.
Topics: Royal Family, King Charles III
Mia Williams