Kate Middleton will have to follow this royal rule with her outfit at Prince Harry and Meghan Markle's wedding....

There's no denying Kate Middleton is known for her immaculate dress sense.

She truly is the royal family’s current fashion icon.

And after the Duchess of Cambridge WOWED us on her own wedding day, we’re all dying to know what she will be wearing for Prince Harry and Meghan Markle's royal nuptials.

While we can currently only speculate as to what the mother-of-two's ensemble will be come May... There is one accessory we know for sure she’ll be wearing.

Royal dress code etiquette states that every female monarch must wear a hat for formal events.

Why hats you ask?

According to Oriole Cullen, curator of a Victoria & Albert Museum 2009 hat exhibit, "hats have long denoted wealth and status."

Plus, word around Buckingham Palace is that the Queen sticks to this dress code religiously as she wants to stand out in the crowds so people can see her if they've been queuing for hours, perhaps miles away.

We look forward to seeing her gorgeous head decor!