Wearing colours brighter than the Queen
Yes, you read that correctly. Queen Elizabeth II's wardrobe of bright and bold ensembles isn't for nothing—she actually wears them in order to ensure she stands out from the crowd.
"She needs to stand out for people to be able to say 'I saw the Queen,'" her daughter-in-law, Sophie of Wessex said in the documentary, The Queen at 90.
But in 2012, there might have been a miscommunication between royal stylists. While attending a barge trip alongside Duchess Camilla and The Queen for her Diamond Jubilee, Kate wore a bright red Alexander McQueen dress which made her outfit pop far more than her royal acquaintances'.
Wearing black when not in mourning
It's common knowledge that black is the colour of mourning, with the royal family having conformed to the tradition for centuries. Per The Express, the family also have a protocol that stipulates they carry a black outfit with them whenever they travel in case they need to attend a last-minute funeral.
With this in mind, it's very rare to see a royal wear black in any other circumstance—unless Kate is involved. Indeed, the royal has been known to wear black from time to time, with her striking Alexander McQueen gown worn to the The Sun Military Awards in 2011 being a standout.
Carrying a large handbag
While royals are frequently spotted carrying small clutch bags (Kate keeps a spare lipstick and plasters in hers at all times), it's far less common to see them carry large handbags.
In fact, according to Editor-In-Chief of Majesty magazine Ingrid Seward, the royals are actively discouraged from carrying a hefty bag.
That said, Kate made an exception in 2011 when she was pictured boarding a plane at London's Heathrow Airport. We can't blame her—she was travelling to the United States at the time and no doubt required some extra trinkets to last her the long-haul flight.
Not wearing her engagement ring
Kate's engagement ring is almost as famous as her engagement to Prince William in itself. The stunning blue diamond sapphire once belonged to Princess Diana—and it's easy to spot sitting daintily on Kate's ring finger as she attends her various royal engagements.
While it's unconfirmed as to whether there's a specific rule around royals wearing their engagement rings, Kate has sporadically been seen without it for good reason—during the COVID-19 pandemic she frequently chose not to wear it due to hygiene reasons.
Per The Sun, royal tradition usually dictates that women are not allowed to bare their shoulders.
It obviously a heavily outdated rule, and we're happy to report that Kate appears to be aware of that. At the 2019 BAFTA Awards, the Duchess wore one of our favourite gowns to date—a stunning one-shoulder design by Alexander McQueen.