It's Prince Charles 71st birthday, and royal family members are now sending their warm wishes to the Prince of Wales through social media. Meghan Markle and Prince William's greeting is far different from the rest after they called the heir to the throne three different names, "Sir, Pa, and Grandpa." So, why do the Sussexes addressed the future king as "Sir?"

The Duke and Duchess of Sussex posted a never-before-seen photo of Prince William, Prince Charles, and Archie Harrison Mountbatten-Windsor from the kid's christening in July. It was a black-and-white photo that saw how the newly-father cuddled his son, while proudly showing baby Archie to his grandfather.

Royal photographer Chris Allerton captured the sweet scene, showing the love among the three. "Happy birthday to His Royal Highness The Prince of Wales - Sir, Pa, Grandpa!" the caption read.

Fans were in awe to see the snap, but they were quite puzzled with the three names the Sussexes used. According to Express, the "Pa" pertained to Prince Charles being the father of Prince Harry, while the "Grandpa" meant being Archie's grandfather. So, what about the "Sir?"

Despite the rumors of Meghan Markle's strained relationship with the royal family, the former actress is said to be close to Prince Charles. He even calls her "Tungsten" because of her "hardy attitude." However, Prince Harry's wife addresses him as "Sir," which is part of the royal etiquette.

In the royal rules, male members of the royal family should be called "Your Royal Highness" and later on "Sir." Hence, Meghan Markle's use of the word is an evident sign of her respect for the royal tradition. The same thing applies to Queen Elizabeth, who should be called "Your Majesty" and subsequently "Ma'am," although only pronounced with a short "a."

Aside from Meghan Markle and Prince Harry, Prince William and Kate Middleton also greeted Prince Charles on Instagram. They, too, shared a photo of Prince Louis while playing with his grandfather. "Wishing a very happy birthday to The Prince of Wales," the caption read.

The photographer Chris Jackson took the snap in 2018 at his 70th birthday. At the time, Prince William, Kate Middleton, their three children, Prince Harry, and Meghan Markle joined Prince Charles and Camilla Parker-Bowles for a portrait to mark his very special day.

Prince Charles is celebrating his 71st birthday away from his family. He is currently in India for a royal tour, per People. Anyhow, he celebrates his special day with the schoolchildren in Mumbai with a cake and a garland given to him.