The 41-year-old, who is known as the Duchess of Rothesay in Scotland, made an appearance in Edinburgh alongside other senior royals to watch Charles be presented with the Crown of Scotland at St Giles' Cathedral.
Surprisingly, during one point in the service, Kate cheekily tapped William on the behind, something she has been seen doing more of lately.
The Princess of Wales previously tapped him on the behind at the Royal Ascot last month, with fans going wild for the gesture, which she also did at the BAFTAs in February.
During the coronation, Kate paid tribute to the late Queen by wearing her pearl choker necklace, which had also previously been loaned to Princess Diana. Kate wore the same necklace to both the Queen and Prince Philip's funerals.
She also wore Diana's pearl drop earrings and a Catherine Walker coat dress, which she has worn on a number of occasions previously.
Roger Federer breaks royal protocol
It comes after Roger Federer broke royal protocol at Wimbledon on Tuesday when greeting Kate, seemingly wanting to give her a kiss on the cheek.
The tennis star, who retired last year, was greeted with a standing ovation when he arrived at Centre Court to watch Elena Rybakina and Shelby Rogers.
As he walked to the Royal Box, he gave his wife Mirka a kiss on the cheek and then touched Kate's back and appeared to start going in for a kiss on the cheek, before stopping as Kate simply gave him a nod.
— Wimbledon (@Wimbledon) July 4, 2023
This may have felt odd for Roger, who has been friends with Kate's sister Pippa for years and even taught Prince George how to play tennis, however, in royal circles a kiss on the cheek may be seen as disrespectful.
Out of respect, a handshake is the most familiar greeting most commoners will receive from a British royal, and this is only if the royal should extend their hand out first, otherwise, members of the royal family should not be touched. Even Kate's children are meant to bow and curtsy to her.
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