When Kate Middleton’s engagement to Prince William was announced on November 16, 2010, she wore a dramatic sapphire blue dress from the brand Issa. Within 24 hours, the dress sold out online, and “the Kate effect” was born. Though social media was still in its early days—Instagram had only been launched one month earlier—Kate immediately became something we were only beginning to understand but now see everywhere: a style influencer.

Bethan Holt, the fashion news and features director at The Telegraph, has been following Kate professionally for the last 10 years, and in her new book, The Duchess of Cambridge: A Decade of Modern Royal Style, she looks back on that dress and attempts to explain why it caused such a sensation. “Would any dress have had the same effect? No,” she writes. “There was something about the Issa. It made Kate look glamorous yet respectful; she wasn’t trying to seem more mature than she was, but nor did the dress depict her as a slave to fashion.”

To Holt, Kate’s style is defined by that high-wire balancing act between formal and casual, youthful and serious, glamorous and down-to-earth. With meticulous research, The Duchess of Cambridge both documents Kate’s style and unearths the behind-the-scenes stories of how it all came together, speaking to designers and learning more about the team of assistants who help her out. “She’s found this circle of people that she obviously really trusts,” Holt said in a recent interview. “If you go to anyone in that inner circle they don’t want to talk about their relationship with her because it is quite a sacred thing.”

One important through line of the book is the way Kate’s wardrobe rewrites the royal rules. Unlike Queen Elizabeth, who has a closet full of custom-designed skirt suits, or Kate’s late mother-in-law, Princess Diana, who defined the style of the age with showstopping designer looks, Kate’s wardrobe is nearly entirely composed from off-the-rack outfits from high-street brands. Though looking relatable was a goal of Kate’s from the early days in the family, Holt points out that her clothing choices also reflect the way that the world has changed since the queen took the throne in 1952.

“Diana was often telling a story with her clothes, and she had a big evolution of her own personal life. Whereas the queen, the whole point of her is to remain basically the same,” she said. “The public mood now is much less towards wanting royals that are very distant in their palaces. People want to be able to relate, and I think the royals realize that clothing is a way that they can do it.”

Last month, Vanity Fair spoke to Holt about the changes she’s seen in Kate’s style over the last decade, her predictions for how she will pay homage to her predecessors when she one day becomes queen, and the philosophical ideas that underpin a royal wardrobe. “The battle for royal women is to show their substance whilst also ticking the box of style, and I think the two go really strongly hand in hand,” she said. “The truth is—and it may be a sad truth for some people but I think it’s an exciting truth—without the style the substance wouldn’t have the same impact.”

Vanity Fair: In the book, you see both the evolution of fashion and watch Kate go from being a young woman to a confident adult. Looking back, I was so struck by how age-appropriate her dressing has always been. Even in formal moments, she always looks serious and youthful at the same time. How does she and her team achieve that?

Bethan Holt: It’s about the small touches that keep her in sync with the things her peers would be wearing. You’ve just made me think back to the picture that was taken the day after they got married. They were leaving Buckingham Palace, and she was wearing a really short Zara dress, which probably any other 29- or 30-year-old might have in their summer wardrobe. It’s above the knee and quite fun and flirty, but then she had a jacket over it, which gives it a slight element of formality. Those small changes actually make quite a big difference to how she looks.

It feels like a gradual change, but there are certain things she no longer wears. She doesn’t wear those big wedges anymore, perhaps because it’s more of a younger thing or doesn’t look quite as current. She has started wearing many more trousers, which lots of women, when they become more professional and confident in themselves, experiment with. She’s adding in interesting new things that signal she’s getting a bit older, a bit more sophisticated, and a bit more senior. Like any of us, we hope in our careers that we become more senior as we get older. She’s becoming more senior in her career as a princess.

Where that growth comes through the most is in the outfits she wears while she travels. In the book, you compare her slightly underwhelming 2016 India wardrobe with the 2019 trip to Pakistan where she hit it out of the park. What changed between those two trips?

Part of it is becoming a little braver. Before the Pakistan trip in 2019, lots of people were comparing her to Diana, when Diana went to Pakistan, and that was a part of it. That was in the air before she went to India in 2016 as well, because there’s obviously some crossover in the dress of those two countries, though there are definitely differences as well. The India trip had the specter of the Taj Mahal moment, which obviously Diana had also had.

I also think it’s the scrutiny that these outfits are under, and I’m sure they read some of the feedback as well. One of the outfits that I really remember from that India trip was a £50 dress from a brand called Glamorous, and it looked like the kind of thing that you could have bought at a market in India. But obviously it was from a British brand. Because it was very inexpensive, thus possibly not with the best labor standards, there was a bit of an uneasy feeling about that.

She was always really good, on that India trip, at incorporating Indian patterns, but I think the whole conversation in fashion moved on. We talk about cultural appropriation so much more in fashion now. it’s called out a lot more, which is a really good thing…. By wearing pieces designed by Pakistani designers, it was a very safe bet that actually looked very brilliant.

How far in advance does she begin to plan for a trip like that, with so much narrative to consider in the outfits?

I interviewed the owner of the boutique where they source a lot of the clothes, and the trip was in October, but the owner of the boutique only heard from Natasha [Archer, Kate’s assistant] in September. It was a really quick turnaround. When I interviewed Anita [Dongre, a Mumbai-based designer], she said, “It was just those few weeks, and it was such a rush working on it constantly.” I had always thought that these things were really planned far in advance!… They’re kind of like swans, aren’t they? They just make everything look so easy while they’re scrambling to pull a wardrobe together.

Despite their differences, that effortlessness in style is one thing that Kate shares with the queen, Diana, and even Meghan Markle. But more than anyone else, Kate’s wardrobe has always been really relatable and down-to-earth. Do you think that is intentional?

Part of the appeal of Kate—and the royal family was obviously very conscious of this—is that she’s not a blue-blooded princess. I just love that her surname is Middleton—you know, like middle class. It couldn’t be more perfect.

Her wearing high-street is a way for her to consistently remind people of that, and to show that she is one of us…. There’s no better way than saying, hey, you can buy the same Zara jeans I’m wearing right now for £30. She’s had a lot of those [relatable] experiences. She went on a gap year, and she was a girl about town just after university. We saw those pictures of her having a job, and I think lots of women can relate to those things. She’s talked about mum guilt as well, and about the box sets they watch and what takeaway they get.

I’m always so curious about how she dresses when she’s just running those types of errands or sitting around the house. How different do you think it is from what we see?

Some of the pictures I’m most obsessed with are those ones. There was one where she was shopping at Lululemon, and I thought, I wonder what Lululemon she wears! What leggings does she have? Having said that, I do think that there is some crossover. You know, we often see her doing something that not many royals do, we see her dressing down for lots of her engagements. She’ll be wearing outdoor coats, like Barbour coats, or there’s that Perfect Moment red puffer jacket that she has, hiking boots, or trainers.

There has even been some crossover. There’s a Dubarry jacket she wore in Ireland, and I think she was photographed wearing [something similar] at the supermarket last fall. Obviously she’s going to have lots of things that she just keeps for work but I do think what we see her wear on a casual engagement is a really good inkling of what she’s wearing when she’s just being a mum taking her kids to the supermarket or to buy craft supplies or whatever.

How do you think her style is going to change when she is queen? Of course, it’s not clear when that will happen. I like to think that it’s never going to happen and the queen is going to live forever.

That would be nice! It probably depends on when it is, but when you think back, she has a lot of these plain block-colored dresses. Most of them were made by Emilia Wickstead, and there’s a lavender one that she’s worn quite a few times. The one I’m thinking about recently hasn’t been identified, but it was a blue dress that she wore to meet the Ukrainian ambassador in London in October. What you can see coming through is this very defined silhouette, kind of taking a lesson from the queen, who has that colorful coat that she wears. Kate you can see doing it with a nipped-waist, knee-length dress in a block color. I can imagine her having a whole wardrobe of those, which she wheels out for different occasions.

I don’t think she’s ever going to go for the matching umbrellas, though.

No, that could be a step too far! Or the hat either. I think hats will become a little bit rarer, sadly. But I can see her taking that idea from the queen, of having a uniform for sure. I hope that she still keeps those experimental fashion moments in there—it keeps everyone excited. Obviously when she becomes queen, there will possibly be more of those really formal moments as well, which would be really interesting to see. I’m excited to see it…but I also don’t want to see it!

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