Royal weddings require a dire planning process from the venue, food, and especially the royals' clothing. Before Prince William tied the knot with his wife Kate Middleton, he had an argument with Queen Elizabeth; what did they disagree about?

According to Express UK, the Duke of Cambridge previously shared to the public about a disagreement he had with the longest-reigning monarch.

The two senior members of the royal family were arguing about what to wear during the Prince's special day.

Speaking to ABC News in 2012, the Duke said he "really wanted to wear" the Irish Guards frock coat.

Queen Elizabeth reportedly didn't like her grandson's decision to wear "a frock coat" that he previously wore to royal engagements and events.

"We had a couple of discussions on this matter but, as I learned growing up, you don't mess with your grandmother. What she says goes," he said.

After the argument, Prince William had no choice but to obey his grandmother's orders as he later wore the red tunic during the ceremony.

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His outfit for the day became iconic, as shown in numerous photos, postcards, memorabilia, souvenirs, and the likes.

As the Prince had no choice but to wear the outfit that his grandmother wanted him to wear, both he and Kate Middleton were able to break a few royal traditions during the wedding of the century.

The Duke and Duchess of Cambridge were permitted by Her Majesty to draft their own guest list.

As Prince William donned a simple red attire for his special day, Kate Middleton's outfit is much more complicated and had several interpretations from the public.

According to Town and Country Magazine, the Alexander McQueen-designed gown was inspired by the Victorian tradition of "corsetry." It featured a narrower waist and had a little padding below the midsection.

The dress featured almost nine feet of train, and the lace details were all created by hand.

Middleton followed the "something borrowed" and "something blue" tradition. The first one was the Queen's Cartier Halo tiara that she wore throughout the ceremony. King George VI initially purchased the crown for the Queen Mother in 1936.

The second one was a blue ribbon which was reportedly sewed in the interior of her dress.

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