Kate Middleton and Meghan Markle have to follow a certain rule on Princess Beatrice’s royal wedding day.

In September, Buckingham Palace announced Princess Beatrice’s engagement to Edoardo Mapelli Mozzi. With a royal wedding on the way, the Duke and Duchess of Cambridge and the Duke and Duchess of Sussex are expected to receive invites to the event.

If Middleton and Markle will attend, they have to follow a certain dress code that is true at all times when it comes to weddings.

“The only rule is the same at all weddings. Wear something chic, royals are expected to be chic and it goes without saying that no one should upstage the bride,” royal commentator Richard Fitzwilliams told Insider.

Many noticed that Middleton usually recycles an old dress when attending a wedding. She also tends to sport a frock designed by Alexander McQueen, the same designer who created her wedding dress. The duchess does this to keep the attention away from her dress and to not upstage the bride. Meanwhile, Markle did not recycle an ensemble but also paid tribute to her wedding dress designer by wearing a Givenchy frock when she attended Princess Eugenie’s big day.

“Tradition may be overstated, but it’s something that both Kate and Meghan have been known to do, most recently at Eugenie’s wedding,” Kristen Meinzer said. “It’s a way to show some nostalgia for their own days without overshadowing the bride. After all, it’s just them wearing their favorite designer again.”

However, according to an insider, if Princess Beatrice would have her way, she would not invite Prince Harry and Markle. Sarah Ferguson and Prince Andrew’s daughters are reportedly at odds with the Duke and Duchess of Sussex.

An insider said that there has been tension between Prince Harry and Markle and Princess Beatrice and Princess Eugenie for months. It reportedly started when Markle debuted her baby bump at Princess Eugenie’s royal wedding. They felt that the Duchess of Sussex stole the bride’s thunder by doing so.

There were also rumors that Princess Beatrice would not invite her own father Prince Andrew due to the controversy plaguing him. The Duke of York has been in hot water for his connections with pedophile Jeffrey Epstein and Princess Beatrice doesn’t want any drama on her wedding day.

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Kate Middleton, on the left at St Luke's Community Centre on March 22, 2018 in London, and Meghan Markle, at Cardiff Castle on Jan. 18, 2018 in Cardiff, Wales. Ben Bircall/Paul Edwards/WPA Pool/Getty Images