07/10/2018 12:46 EDT | Updated 08/05/2018 12:07 EDT

Meghan Markle's Gorgeous Dior Dress Just Broke A Royal Fashion Rule

It's a rule Princess Diana was known to break, too.

Doug Peters/EMPICS Entertainment
The Duke and Duchess of Cambridge arrive with the Duke and Duchess of Sussex to the RAF Centenary at Westminster Abbey, London.

You probably won't be surprised to learn that the two royal duchesses formerly known as Meghan Markle and Kate Middleton looked gorgeous at a royal event today.

But did you know the Duchess of Sussex's black Dior dress may have broken a Royal Family fashion rule?

On Tuesday, the day after Prince Louis' christening, Catherine, Duchess of Cambridge and Meghan, Duchess of Sussex joined their husbands, along with Queen Elizabeth, Prince Charles and Duchess Camilla, at the 100th celebration of the Royal Air Force.

Meghan wore what's quickly becoming her signature style: a simple, elegant boatneck dress. But by choosing to wear black, she was flouting a royal fashion rule: royals are not supposed to wear black to events unless they're in mourning.

Doug Peters/EMPICS Entertainment
Meghan, Duchess of Sussex during the RAF Centenary at Westminster Abbey, London.

But those rules might be loosening up a bit, a British etiquette expert told InStyle.

As a fashion icon, Princess Diana was considered an exception to the rule, Alexandra Messervy, founder of The English Manner, explained to the site. And Meghan has in fact worn black before, and hasn't faced a huge amount of outcry, except from the most staunch traditionalists.

(UPDATE: It's worth noting that some outlets are calling the colour of the dress "deep navy," although Harper's Bazaar and InStyle both called it black. Without an official statement from the palace or Dior, it remains a matter of debate, and, possibly, screen settings.)

Meghan Markle channeled Audrey Hepburn once again in @dior 🖤

A post shared by Meghan Markle (@hrhofsussex) on

Besides, it's hard to deny that she looked stunning in the black Dior dress, which some commentators noted was reminiscent of her stylish but restrained Givenchy boatneck wedding dress. It seemed to continue her Audrey Hepburn-inspired streak, too — the outfit she wore to the Royal Ascot last month was pure Eliza Doolittle in "My Fair Lady."

Meghan topped her recent look off with a Stephen Jones fascinator and nude pumps.

Doug Peters/EMPICS Entertainment
Catherine, Duchess of Cambridge during the RAF Centenary at Westminster Abbey, London.

Kate's coat dress was by Alexander McQueen, the label that also designed her wedding dress. Her hat — you didn't think she'd attend a royal function without a fascinator, did you? — was by Sean Barrett.

She also had a particularly significant accessory: the diamond brooch she wore on her left lapel. It's called the Dacre Brooch, and it's an honour given to high-achieving female cadets in the Royal Air Force. Kate received it in 2015, when she became Honorary Air Commandant of the Air Cadets, a title that had previously been held by Prince Philip.

Doug Peters/EMPICS Entertainment
Catherine, Duchess of Cambridge during the RAF Centenary at Westminster Abbey, London.

The Queen was noticeably absent from Prince Louis' christening on Monday, but she seemed in good spirits at the celebration, and from what we could see, wasn't particularly upset about Meghan's dress choice.

The Queen gave a short speech, and presented a new flag to the Air Force. The ceremony was followed by a reception at Buckingham Palace.

Later on Tuesday, the Duke and Duchess of Sussex — that's Harry and Meghan, if you have trouble keeping up — will visit Dublin, Ireland for their first international trip since getting married. They'll attend museums, go to a Gaelic sports festival, and meet with Prime Minister Leo Varadkar.

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