The Princess of Wales, 41, and the Duchess of Edinburgh, 58, are known to be very close and stood side by side as they watched Prince Edward, 59, and Prince William, 40, partake in the Order of the Garter procession.
Kate and Sophie could be seen laughing and giggling as they waited for their husbands to walk past.
Judi James, a body language expert, spoke exclusively to Express.co.uk to explain what Kate and Sophie were likely laughing about during the ceremony.
She said: “Kate and Sophie’s body language is so nicely relaxed, synchronised and mirrored here that they look as though they are close mates.
“Their conversation prompts a lot of shared laughter and when they stop smiling to hold onto their hats in the breeze their poses still look identical.
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“A test of like-minded friendship is always when the mirroring occurs when two people are out of sight of each other.
“It can be simple to deliberately mirror or mimic someone to flatter them when you are face to face, but when you perform subliminal mirroring, as Kate and Sophie do when they stand side by side facing front rather than each other and with their hat brims blocking the view, then the suggestion is one of shared thoughts and emotions.
“Kate and Sophie always have fun when they share carriages and appear together and it was clear from Sophie’s royal visit with William last week that the affection and friendships involve the husbands as well.”
Judi suggested reasons as to why the two royal women could have been laughing during the ceremony.
She claimed: “So what was making the two women laugh so much? My money would be on the outfits their husbands wear for this annual event.
“While some outfits, like the uniforms William and Edward wore for the Trooping of the Colour on Saturday, bring out the dashing side of both men, the big floppy hats and feathers worn for this Garter Day service tend to throw a hint of bashful embarrassment up from their body language and facial expressions as they walk through the grounds at Windsor.
“It has always seemed to be something of a shared joke that the men try to keep a straight face as they parade past their wives like this and the sight of the wives grinning or laughing is quite a common response.
“For Charles though, the sight of his brother Edward and his son William in a line with their wives side-by-side between them and all four looking genuinely happy and relaxed together as the King’s carriage rode past must have been gratifying and rewarding.”
The body language expert explained whether Sophie, Kate, William and Edward have become the new “Fab Four” since the departure of the Duke and Duchess of Sussex from the Royal Family.
“If there is a genuine ‘Fab Four’ in the royal firm it is probably these four.
“Not only are they the chosen stars of the slimmed-down Firm, along with Anne, but they always seem to provide the kind of body language signals together that suggest low drama, authentic affection and actual happiness.
“They look like the kind of support that Charles will need and be grateful for, offering a sense of calm, solid continuity after the last few years of rifts and stress.”
