LONDON, April 21, 2011 -- Forget the dress. It's all about that cherry on the top: the hat.
The Brits are obsessed with hats at the moment. Headwear is most important at society events, which includes the upcoming wedding of the century.
So, there's no one better to speak to that than London's king hatmaker, Philip Treacy. After all, Kate Middleton has chosen Treacy to design hats for the royal-wedding party.
Actress Sarah Jessica Parker, singer Lady Gaga and princess-to-be Middleton have worn Treacy's creations. And everybody at the April 29 wedding will undoubtedly be wearing a hat.
"You have to wear a hat," Treacy said. " "Because it's tradition. That's what England's about."
Treacy makes hats for Middleton, so he knows what sort of thing she likes.
"She's worn some hats... [that are] very smart," the hatmaker said.
The royal wedding will be a sort of fashion show for hats. "A royal wedding happens like this only every 30 years," Treacy said. "So [it] is the ultimate hat moment."
He has some thoughts on why the British are generally more infatuated with hats than Americans.
"Well, the British have an in-built understanding of hats I make," Treacy said. "My customers are very conservative but they think my hats are normal."
ABC News' Barbara Walters and Brian O'Keefe contributed to this report.