When Prince William and Duchess Catherine shared a candid clip of their three children clapping for Britain's NHS last week, royal fans couldn't believe how much little Prince Louis had grown up.
Alongside his big brother Prince George, six, and Princess Charlotte, four, the cheeky toddler- who turns two in April - was the picture of cuteness, but one feature of his outfit was decidedly un-royal.
In the video posted to the Kensington Royal social channels, both Princes George and Louis donned trousers.
Young boys born into aristocratic or royal households traditionally wear shorts until the age of around five to eight years old, and it's simply because of tradition.
It seems Kate and William stuck to these rules with Prince George, who wore trousers for the first time in public at his uncle Prince Harry's wedding when he served as a pageboy, ahead of his fifth birthday.
According to etiquette expert William Hanson, it would be considered "suburban" to dress children of the upper class in trousers rather than shorts.
"Trousers are for older boys and men, whereas shorts on young boys is one of those silent class markers that we have in England," he told Harper's Bazaar UK.
However, seeing as the video was filmed at the family's home, the Cambridges may only stick to royal trouser protocol trousers when it's for an official outing or portrait.
The last time we got a glimpse of Prince Louis was in the Cambridge's Christmas card that was leaked on Twitter, where the littlest member of the family wore a pair of blue overalls over a dark T-shirt.
Ahead of that, his last formal appearance was at the 2019 Trooping the Colour celebrations, when he famously stole the show atop the Buckingham Palace balcony.
WATCH BELOW: Prince Louis waves wildly to the crowds during his Trooping the Colour debut. Post continues after video...
Despite being the future King and Queen, though, Prince William and Duchess Catherine don't always follow tradition and royal protocol when it comes to parenting.
George, Charlotte and Louis may have more privilege than average children growing up in Britain, but they also do relatively normal things too like attend school, hang out with their non-royal grandparents and wear hand-me-down clothes as well.
"I think the things that really resonate with me most are the simple things and actually I see that now with my own children," Duchess Catherine said on the Happy Mum, Happy Baby podcast earlier this year.
"Life now is so busy, so distracting and actually sometimes the simple things like watching a fire on a really rainy day provides such enjoyment."