The Prince of Wales was seen warmly greeting his stepmother Queen Camilla with kisses on both cheeks as they arrived at the Commonwealth Day service at Westminster Abbey on Monday.
Unlike his uncle, the Duke of Edinburgh, Prince William did not appear to bow to Her Majesty as the royals assembled to process down the aisle to take their seats.
The Duchess of Edinburgh also greeted Camilla with a kiss and curtsey, as captured in the video below.
But there's a logical reason as to why William didn't bow to the Queen. If a member of the royal family isn't seen greeting the King or Queen with a bow or a curtsey, it's usually because they've already seen each other that day.
The Queen and Prince of Wales were seen chatting and smiling together ahead of the national anthem being played. The pair headed up the royal family amid the King and the Princess of Wales's absences from the service.
A pre-recorded message from the monarch was played during the ceremony, in which Charles said he has "been most deeply touched by your wonderfully kind and thoughtful good wishes for my health and, in return, can only continue to serve you, to the best of my ability, throughout the Commonwealth".
The King has postponed most of his public-facing duties as he continues cancer treatment, but is still continuing with his daily red boxes behind palace walls.
Princess Kate is not expected to return to her engagements until after Easter amid her recovery from abdominal surgery.
Kensington Palace released the first image of Kate and her children, Prince George, ten, Princess Charlotte, eight, and five-year-old Prince Louis, taken by William at their Windsor home, to mark Mother's Day on Sunday.
However, the Princess publicly apologised for editing the family photograph as it was pulled by news agencies over claims it had been "manipulated".
Kate said in a statement shared on social media: "Like many amateur photographers, I do occasionally experiment with editing. I wanted to express my apologies for any confusion the family photograph we shared yesterday caused. I hope everyone celebrating had a very happy Mother’s Day. C."
How to greet a member of the royal family
According to the royal family's website, there are no "obligatory codes of behaviour" when meeting the King and Queen or a member of the Royal Family, but many people wish to observe the traditional forms, which for men is a small bow and for women, a curtsey.