Watch: Everything We Know About Prince Louis's Christening
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The Duchess of Cambridge has built up quite the collection of beautiful looks over her years as a royal, with her style evolving across tours, events and engagements.
But it was nearly three years ago, at the christening of her third child, that she made one of her biggest fashion shifts.
Prince Louis, who turned three on Friday 23 April, was christened at the Chapel Royal in St James's Palace on 9 July 2018.
Fashion journalist and author Bethan Holt says the outfit Kate wore that day was a "seismic shift" - all because of her hat choice.
Holt, writing in her book The Duchess of Cambridge: A decade of modern royal style, said: "Millinery may appear to be a fashion sector that evolves at a glacial pace but the duchess helped to create a seismic shift when she opted to wear a 'hatband' for the christening of Prince Louis in July 2018.
"Akin to an oversized Alice band and sometimes called a Boleyn band (because the second wife of Henry VIII wore a similar headpiece) the duchess sparked a trend for an accessory that last enjoyed the fashion spotlight in the era of the Sloane Ranger and made a calculated effort to be the heroine of its resurgence that year, wearing a blue floral take to the wedding of her friend Sophie Carter and a pumped up green velvet version at Westminster Abbey."
Holt added: "Kate made sure that the world was reminded that she was queen of the hatband, sporting a new plush red iteration for Christmas at Sandringham and a fabulous feathered coral band for the christening of Harry and Meghan's son Archie in the summer of 2019.
"Later in 2019 the duchess seemed to nod to her own influence by wearing a jewelled hairband by Zara to the Festival of Remembrance - a piece that had probably been made in response to demand she had a part in fuelling."
Before the christening, Kate's hat choices had been focussed on discs, saucers and pillboxes, which added height and flamboyance, but still allowed her to put her hair in an up-do.
Holt said the change was "accessible" and "ladylike", and so was perfect for the duchess.
Kate has sent messages with her clothing through the births of her children, and the all-important first appearance on the steps of the Lindo wing.
When Louis was born, on St George's Day, she managed to pull off a look that was both national and personal, as her red and white dress tied into the colours of the St George's Cross and matched the look Diana had worn on the steps when Prince Harry was born.
And she carried a secret spare dress when she gave birth to George in 2013 - packing a pink one in case he had been a girl.
Prince Louis celebrates his third birthday on Friday 23 April, and it will be his second in some form of coronavirus restrictions.
The toddler was in Anmer Hall in Norfolk with his family in Spring 2020, when the first lockdown to prevent the spread of COVID-19 was announced.
Now they are back in London, with his older brother and sister going back to school at Thomas's Battersea.
While he still may not be able to celebrate his birthday properly, this year Louis will at least be able to see some friends outside, as the rules allow for two households to gather in parks and gardens.
Prince Louis has started at Willcocks Nursery in London, the same place where his sister Charlotte went before joining her big brother at Thomas's.
In 2020, Kate released images of her youngest son as he helped them pay tribute to frontline workers with his rainbow handprint paintings.
It remains to be seen how influential this young royal will be on his mum's style in years to come.
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Watch: Prince William's children pay tribute to the late Princess Diana on Mother's Day