Birkenstock shares started trading on the New York Stock Exchange on Wednesday. The German footwear company priced its IPO at $46 per share, giving it a valuation of $8.6 billion.

In the center of Eastern Berlin, near Hackescher Markt, you have all the hip brands — including a Birkenstock store. There are often queues of achingly hip young people waiting to get into the store.

So how did the humble Birkenstock go from fashion faux-pas to fashion must-have?

“It’s supermodels like Gigi Hadid and Kendall Jenner that are walking around wearing them, making them look incredibly cool,” said Claire Langhammer, founder of Fake PR, which works with German fashion brands. “I also think that there was the recent ‘Barbie’ movie that certainly, you know, opened a door to the mainstream and made everyone much more excited to wear them again.”

It wasn’t always this way, however. Birkenstocks have long been derided by critics as “uncool” or even “just ugly.” The sandal beloved by physicians made its way to the States in the ’60s, gaining a foothold in hippie circles. Decades later, the sensible shoe has become mainstream — and a global fashion superstar.

“I think they have a really brilliant marketing department to be honest with you,” Langhammer said. “Also, there is whispers that there will be a collaboration between Gucci and Birkenstock. We saw Sarah Jessica Parker while she was filming ‘And Just Like That’ wearing a pair. So, that looks like there’s going to be another exciting collaboration in the works.”

Birkenstock isn’t just reliant on being in fashion, noted David Dubois, an associate professor of marketing at INSEAD Business School in France. It also has a practical appeal.

“They really started off working their functional benefit. It was about the durability, it was about comfort. And so, by nature, the company is really much more sustainable than most shoeware companies,” he said.

The brand’s success is shown by the number of fakes on the market, Dubois added. “The fact that now you increasingly see Birkenstock counterfeits showcases that the brand has really become fashion staple.”

Customers at the Birkenstock store in East Berlin also expressed their love for the arch-supporting sandals.

“My heart always was beating strong for Birkenstock when it was like this family company and stuff, and now I’m a bit ‘hmmm,'” said one customer, who admitted to having a couple of pairs at home. “But at the same time, it’s really cool what they’re doing. And I find them really, really comfortable, their shoes.”

“I mean, I’m happy because I remember my mom was always like, ‘Wear Birkenstock.’ It’s like, ‘Ew’ — you know, it wasn’t cool,” she added, chuckling. “And now it’s like, ‘Yeah, I’m cool. And I’m wearing good stuff for my feet!'”

Birkenstock is hoping by selling its shares publicly on the New York Stock Exchange, it can raise the cash to make its comfortable fashion even more of a global success.