Prince Harry and Meghan Markle’s recent birthday greeting for Prince Charles on Instagram reveals that the Duchess of Sussex actually calls her father-in-law “Sir.” The post had many royal followers wondering why Meghan needs to address the Prince Charles that way, when the Duchess of Sussex and Prince of Wales are reportedly close even behind closed doors.

As Prince Charles celebrated his 71st birthday this week, Prince Harry and Meghan shared on Instagram a lovely photo of the Prince of Wales with Harry and Baby Archie. While Prince Charles was not able to spend his special day with his family as he is currently on his Indian royal tour, he was able to receive a touching tribute from the Duke and Duchess of Sussex.

In a previously unseen photo of Baby Archie Harrison, Prince Harry was shown holding the youngest royal baby in his arms with Prince Charles looking over at them. Taken by royal photographer Chris Allerton on the day of Archie’s christening, the black and white photo captures Prince Charles’ love for his grandson.

“Happy birthday to His Royal Highness The Prince of Wales—Sir, Pa, Grandpa!” The heartwarming tribute sent their royal followers into a spin, as it left many of them wondering why Meghan has to call Prince Charles “Sir.”

While the caption of the photo makes it clear that Prince Harry calls the Prince of Wales “Pa” and Archie calls him “Grandpa,” it’s a puzzle for many why Meghan cannot call him “Pa” just as Prince Harry does. This is especially since it is no secret to many that Prince Charles and Meghan share a very close daughter-in-law and father-in-law relationship.

Despite their close relationship, Meghan reportedly needs to call Prince Charles “Sir” in formal settings as it is in keeping with royal etiquette. As a rule, male members of the royal family are addressed as “Your Royal Highness,” and subsequently “Sir.”

Royal experts have explained that Meghan’s use of “Sir” when addressing Prince Charles in formal settings is a mark of respect for royal traditions. It remains unknown if Meghan still calls him that way when they are in casual settings, but it was previously revealed that Prince Charles fondly calls her daughter-in-law “Tungsten” because of her hardy attitude.

Prince Charles Prince Charles, Prince of Wales, at a Prince's Trust International reception at the Coral Reef Club Hotel on March 19, 2019, in Bridgetown, Barbados. Chris Jackson/Getty Images