The Scottish composer Erland Cooper has disclosed that the Princess of Wales and Princess Charlotte were already accomplished performers when they sat down for their piano duet, requiring no guidance from him whatsoever.
The Orkney Islands-born musician acknowledged that playing before a film crew and the piece's creator would unsettle most people, yet the royal pair handled it with ease.
Cooper explained: "I didn't need to give direction, they were so good."
"I think I was there as a cheerleader."
Princess Kate invited composer Erland Cooper hear Princess Charlotte play at Windsor Castle
| ITV/BBC STUDIOSCooper was particularly impressed by the 10-year-old princess, describing Charlotte as "very confident" and praising her performance: "She played it really well. She plays it so beautifully."
The mother-daughter performance took place in Windsor Castle's Inner Hall, where the segment was filmed last week ahead of the Together At Christmas concert broadcast on Christmas Eve.
Cooper revealed it came as "a big surprise to be invited to Windsor Castle just last week and hear it performed by a pair of princesses."
The duet saw Kate using only her left hand whilst Charlotte played exclusively with her right, creating a touching collaborative rendition of Cooper's piece Holm Sound.
The composer had spoken with the princess earlier in the year and was aware she enjoyed performing the music with her family at home, though the invitation to witness the recording still caught him off guard.
The piece holds particular significance given a remarkable coincidence surrounding its origins.
Erland Cooper revealed he composed Holm Sound for his own mother
|GETTY
Cooper composed Holm Sound for his own mother, who shares the name Charlotte, and the work explores themes of motherhood and generational bonds.
"This is the wonderful coincidence, it's about how we see our parents in our children's eyes and but then when you look at your mother and you look at granny, you see your children," he explained.
"It's about that sort of full circle moment and it's about motherhood."
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The princess and her daughter have been playing the piece together at home, and during the broadcast, footage showed guests arriving at Westminster Abbey whilst the Prince of Wales and Kate watched their children attach paper chains to a "Connection Tree" outside the venue.
The Together At Christmas carol concert marked the fifth year Kate has organised her festive celebration, having previously surprised audiences at the inaugural event by accompanying Tom Walker on piano for his song For Those Who Can't Be Here.
The princess narrated a letter distributed with every order of service, reflecting on how Christmas reminds us "how deeply our lives are woven together."
Kate using only her left hand whilst Charlotte played with her right during the performance
| ITV/BBC STUDIOSActor Chiwetel Ejiofor recited the classic poem A Visit From St Nicholas, whilst Kate Winslet offered a reflection on love at Christmas, telling attendees it can "light up the world around us."
Family members present included Kate's parents Carole and Michael Middleton, her brother James and his wife Alizee Thevenet, Zara and Mike Tindall, the Duchess of Edinburgh, and Prince Michael of Kent.
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