Of course, that Issa dress promptly sold out, as did thousands of copies created around the world by retailers hoping to cash in on the nascent "Kate effect"; it was the first hint at the remarkable impact she would have on fashion, despite the fact that she wasn't really all that fashionable. That's been a more reluctant process.

The appeal of Kate is that she's provided a style template that could hardly be further from the sexy, pumped-up vibe of the pop and reality stars in whose midst she finds herself on magazine pages, best-dressed lists and social media sites. She's normal apart from being extraordinary, a fact emphasised to us via her penchant for skinny jeans, polite knee-length dresses and Barbour jackets, and she rarely looks anything other than perfectly appropriate and polished, which is probably how most women aspire to be, but until Kate there were few role models for this approach.

"There's nothing trashy or vulgar about her," Anna Wintour, the editor of American Vogue, once told The New York Times. "She dresses her age and never looks out of place."

One of Kate's first great fashion acts was to wear high street labels. Unlike the royals of the past, Diana included, there was a very real possibility that women could buy her look and, consequently, her prowess for boosting the fortunes of retailers soon became apparent.

When UK store Reiss posted healthy profits in the early 2010s, it put the figures down, in part, to the fact that Kate had worn its pieces for some key early moments in her royal life, including a cream dress that starred in the official engagement portraits and a caramel shift dress for a post-honeymoon meeting with Michelle Obama.

Some rushed out to buy the exact same pieces (a phenomenon which would become known as "replikate-ing"), but there was a halo effect, too, which prompted a more general spike in interest: any store in which Kate was photographed browsing on one of her frequent shopping jaunts on the King's Road enjoyed a boost and it's now estimated that her fashion choices alone could bring £152 million ($284.8 million) each year to the British economy.

Kate's trendsetting was tentative at first. She became so synonymous with LK Bennett's Sledge patent beige heels – which some found boring and others appreciated for their understated versatility – that the V&A gallery acquired a pair for its permanent collection.

She supported the new generation of British design talent, like Erdem, Emilia Wickstead and Sarah Burton, who had taken over at Alexander McQueen just months before she was commissioned to create the future queen's wedding gown, but their designs for the Duchess were toned-down versions of the creativity they exhibited on the catwalks.

Even if at times fashion editors might have craved something more fashion-forward from Kate's wardrobe, she's nevertheless always been masterful with her sartorial messaging, whether it was the way she mirrored Princess Diana by wearing blue polka dots for Prince George's debut (as Diana had done introducing William over 30 years before) or how she compliments the countries she visits with thoughtful diplomatic gestures. In France, she wore Chanel and in Poland, she and Princess Charlotte both wore red and white to channel the country's flag.

Then, in 2017, along came Meghan Markle. If the royal fashion soap opera had been sedate and quietly lovely until this point, now it flashed to life.

How could it not when, for her first public appearance as Prince Harry's girlfriend, Meghan wore a shirt known as the "Husband" by her friend Misha Nonoo?

The Duchess of Cambridge may have spent the previous seven years finding a way to wear clothes with meaning, without allowing them to become a distraction, but Meghan was immediately more bold.

Weeks after the "Husband" shirt came the engagement and an upscale set of portraits taken by glossy magazine photographer Alexi Lubomirski, which included a shot of Meghan in a couture Ralph and Russo gown rumoured to be worth £56,000 ($104,960)

In January 2018, Meghan offered a smattering of "Markle sparkle" to beleaguered British retailer Marks & Spencer when she chose a black jumper from its Autograph range for a visit to Brixton. It seemed like a match made in PR heaven – Markle & Spencer, anyone? – but it wasn't long before the new Duchess's fashion intentions had pivoted.

Instead of wearing British labels - why should she when she was American-born and had lived in Toronto until weeks before her engagement?

Meghan mostly sported international designers like Dior, Prada, Givenchy and Valentino and she imbued her outfits with messaging that highlighted her eco-conscious and feminist principles by wearing vintage couture and spotlighting brands like Outland Denim, which helps women who are victims of sexual exploitation, Veja trainers (an eco brand with vegan options) and Maggie Marilyn, a New Zealand designer who puts sustainability at the heart of her designs.

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She's made a point of working with women designers, too, from well-known names like Stella McCartney to under-the-radar labels like Safiyaa, a demi-couture brand founded by Daniela Karnuts that caters to the Duchess's preference for clean lines and minimalist tailoring.

One of the Duchess of Sussex's most powerful fashion choices was the dress she wore for the photocall introducing her son Archie to the world in May this year. It was a simple white trench dress – a style Meghan has made her own and which sets her apart from the bright, eye-catching patterns traditionally worn by royal women – by Grace Wales Bonner, a mixed-race British designer whose collections examine her own heritage and ideas of gender identity.

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If Meghan and Harry have carved out a reputation as the most "woke" of royals, then this was the perfect designer to cement the point. Earlier this month, fashion search engine Lyst declared that Meghan has been the most powerful dresser of 2019.

Maybe it was the arrival of Meghan on the scene, maybe it was a newfound sense of confidence as she and William took on a more senior role in the Royal family or maybe it was a desire for a style refresh after her third maternity leave, but in the past year or so, the Duchess of Cambridge's look has become more exciting than ever.

She's started wearing trousers, which Meghan does but wasn't really the done royal thing before, and has consulted her friend Ginnie Chadwyck-Healey, a former Vogue retail editor who now writes a column for the UK's Sunday Telegraph, as well as her long-time stylist Natasha Archer on ways to spice things up.

Peak New Kate was the day she wore a purple Gucci pussy-bow blouse, wide-legged Jigsaw trousers and an Aspinal box bag in lieu of her old favourite clutches to a children's centre in South London, but there have also been newly adventurous looks for state banquets and she instigated the huge recent trend for headbands even before they appeared on the Prada catwalk; she wore one on Christmas morning in 2018 and there have been several more since, including an embellished Zara version in November.

Tellingly, most of the royal women, from the Countess of Wessex to Princesses Beatrice and Eugenie had got the memo and were sporting their own takes for this year's Christmas morning service at Sandringham, proving the Kate effect is in force close to home, too.

While it's mostly been the Kate and Meghan show, royal fashion hasn't just been all about them.

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The Queen has enjoyed a style renaissance in the 2010s. We loved her for sporting a neon green outfit for her 90th birthday celebrations in 2016 and high street retailer JD Williams reported a 134 per cent rise in sales for items in similar colours the following week.

Her cheerfully colourful looks often go viral on Instagram (the Burberry silk scarf she wore to travel to Norfolk for Christmas last year was especially popular). She went to her first London Fashion Week show in 2018 and showed true fashion leadership this year when it emerged that she is swapping real fur for faux.

Meanwhile, The Crown has reminded us of the fabulous royal fashion of the Forties, Fifties, Sixties and Seventies. Cue a renewed appreciation for the Queen's elegant look as a young woman – those full-skirted ball gowns, neat cardigans and ostentatious millinery being highlights – Princess Margaret's It-girl glamour and Princess Anne's casual chic heyday.

It's no mistake that this new golden age of regal style appreciation has coincided with the rise of Instagram, where royal imagery, whether it's throwback or contemporary, has become catnip.

Fashion has helped the royals to both elevate themselves and make themselves relatable in the delicate climate of this 24/7 digital news era. It's been a tool to revive popularity and exert clever displays of soft diplomacy.

But the royals have helped fashion, too. They make the designers they wear famous, they set trends, boost business and, dare I say it, the fashion moments they create are often more compelling than those on the catwalks. It's been the best decade to be a fashion editor.