It is a very special day in the Cambridge household as Prince Louis is celebrating his fourth birthday.
It was on this day exactly four years ago in 2018 that mum Kate gave birth to her youngest son at the same hospital she welcomed older siblings, Prince George and Princess Charlotte.
Several days after his birth it was revealed that Prince William and Kate would be naming their young son Louis Arthur Charles.
Louis is a popular name in the royal family and William and George have it as a middle name.
It is thought to be in honour of Prince Charles's mentor Lord Louis Mountbatten, who died in an IRA bombing in 1979.
Charles is a nod to little Louis's grandfather, the Prince of Wales, who said he was delighted to have another baby in the family when he was born.
His official title is rather grand and much like George and Charlotte's. He is officially known as His Royal Highness Prince Louis of Cambridge.
But his official name and title are only possible because his great-grandmother the Queen stepped in before he was born to overwrite a rule put in place by her grandfather King George V, reports the Express.
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Back in 1917, he set out new guidance on which royals were allowed to have titles - and who was too far down the line of succession to qualify for one.
He decided that all of the sovereign's children would automatically become a Prince or Princess, as well as any grandchildren born through the male line.
However, great-grandchildren weren't included on the list.
This caused a bit of an issue when George was born, as despite being our future king he shouldn't have had the title.
The Queen stepped in and said that George would get a title, and decided to extend the change to all of Kate and William's children.
This means that even though Charlotte and Louis, who are further down the line of succession and are unlikely to ever sit on the throne, are also a prince and princess.
If the Monarch didn't make the change, it's likely that Louis would have been Master Louis Cambridge or Master Louis Windsor instead.
Even though he made his public debut on the steps of the Lindo Wing with his parents at just a few hours old, Louis has been growing up away from the spotlight.
It's partly down to the Covid-19 pandemic, which has seen large-scale Royal Family events such as Trooping the Colour, cancelled in recent years.
And this has also played a part in causing him to fail to hit one major royal milestone - going on an overseas tour.
Louis has yet to go on a royal tour, which is mainly due to the travel restrictions that were in place for large parts of 2020 and 2021.
His older siblings were both considerably younger than their little brother when they went on their first overseas tour.
But who knows, maybe Louis will get to hit this milestone soon now travel has opened back up again?
