If she wears it, women want it. The "Kate Effect" was in evidence again this week with news that the Duchess of Cambridge had created huge demand for earrings by Catherine Zoraida.
The Edinburgh-trained designer says her business has gone "crazy" and that she has been inundated with orders for the gold-plated silver double-leaf earrings the Duchess wore recently to church at Sandringham. Her patronage has helped to turn Zoraida's eponymous jewellery line from a relatively obscure brand to one lauded by the likes of Vogue.
Dresses, shoes and coats worn by the Duchess often sell out within hours of photographs appearing of her wearing them. She has, on occasion, caused retailers' websites to crash - and their profits to soar.
She is the ultimate "brand ambassador", although, unlike celebrities who endorse products, she is not paid for her patronage.
Ever since her earliest days in the public eye, women have sought to emulate her look. In 2004, when she was still Kate Middleton, she posed for a photograph in front of Blenheim Palace wearing tweed clothing from the Really Wild Clothing Company (for which she was working as a sales girl at the CLA Game Fair), and instantly boosted the appeal of country style.
Suddenly, green tweed was no longer the preserve of gamekeepers.
A decade on, there are even websites - such as whatkatewore.com - dedicated to identifying her clothes, shoes, jewellery and handbags, then advising copy-Kates where they can buy them, or something similar.
LK Bennett, Reiss and Issa are just some of the brands worn by the Duchess that have rocketed in popularity, particularly in the US.
She wore a royal-blue draped jersey dress by Issa, for example, during the public announcement of her engagement to Prince William in 2010. Costing £385, it sold out within 24 hours and spawned many cheaper imitations.
When she met US President Barack Obama and his wife, Michelle, in 2011, she wore the demure but form-fitting Shola "bandage" dress by Reiss. The £175 dress sold out almost instantly and the company's website was temporarily unable to cope with demand.
Likewise, on a visit to Belfast the same year, a £650 Burberry trench coat she wore quickly sold out. She has also had an invigorating effect on sales of Madderson and Seraphine maternity dresses, J Brand coral jeans, and a cerise Mulberry coat.
So why does the Kate Effect have quite such a huge impact? Yes, she is slim and beautiful, but so are many other high-profile women whose clothes are not so slavishly copied. What makes her different?
Bay Garnett, stylist and Vogue contributing editor, believes her appeal lies in the fact that she is "aspirational but in a completely approachable way. She's got elegance, but it's 'next-door'. It's all about accessibility; it's not avant-garde, it's not eclectic, it's nothing radical, it doesn't take imagination - it's normal. You can do it." In other words, despite being a duchess, she has everywoman appeal.
Kate Moss might be more revered as a style icon by the fashion pack, but for most women, whose lives are a mixture of work and family, it's the other Kate whose look is more adaptable: jeans that are fitted without being prohibitively tight; skirts in which you can safely bend down to talk to a toddler; knee-length dresses that you can wear to the office; colourful coats; walking-friendly wedges; nude shoes that go with everything.
Perhaps her greatest sartorial gift is showing us how to do formal wear without being either too frumpy or too risque. For the average woman, a wedding invitation dropping on to the doormat detonates a hand grenade of sartorial panic.
Photographs of race-goers at Ascot and Aintree show us each year how easy it is to get it wrong. But the Duchess, whose life is one long round of formal occasions, shows us again and again how to get it right, without having to spend thousands.
Although she wears clothes by high-end British designers such as Jenny Packham and Sarah Burton of Alexander McQueen, she is also a faithful patron of high street shops such as Reiss, Jigsaw and LK Bennett. The double-leaf earrings cost a relatively modest £140.
While some might sneer at the copy-Kates, one fashion insider points out: "There's nothing wrong with copying - everyone in fashion copies each other; high street brands copy haute couture. That's simply what fashion does and always has done. Besides, she gives such a massive boost to the British fashion industry."