The simple reason Kate Middleton wore a tiara to the state dinner and Meghan Markle didn't.

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When Meghan Markle stepped out at her first formal event as a member of the royal family earlier this week, there was one questions on everybody’s minds – where is her tiara?

While Meghan attended the Fiji state dinner without a tiara in tow, her sister-in-law Kate Middleton attended a formal banquet on the same night at Buckingham Palace wearing Princess Diana’s famous Lover’s Knot tiara.

But according to the experts, Meghan’s lack of tiara all comes down to royal protocol.

According to Hello!, royal tradition dictates that members of the royal family can only wear tiaras at white tie events.

While the Duchess of Cambridge attended a white tie event, Meghan’s event was black tie, meaning a tiara was not appropriate.

Royal rules for tiara don't stop there – marital status also plays a part.

Meghan, Kate and newly-married Princess Eugenie are all able to wear tiaras while Princess Beatrice can't.

"Any woman may wear one, but ancient tradition has it that they must be a bride or already married," expert Geoffrey Munn told Town and Country.

"The tiara has its roots in classical antiquity and was seen as an emblem of the loss of innocence to the crowning of love."

Black tie and white tie events also determine what members of the royal family should wear to events.

For white-tie events, like the event Kate Middleton and Prince William attended earlier this week, women are required to wear an evening gown and tiara while men are required to wear a white tie evening suit or military uniform.

On the other hand, for black tie events, royal men must wear a white shirt and black tuxedo and women are required to wear a mid or floor-length dress.