Meghan Markle’s special nickname for Prince Charles has just been revealed. The Duchess of Sussex reportedly calls her father-in-law “sir.”

According to Express, male members of the royal family must first be addressed as Your Royal Highness and then “sir.” As such, Prince Harry’s wife calls Prince Charles “sir” as a clear sign that she respects royal traditions.

The publication reported that the same rule applies when referring to Queen Elizabeth. She is firstly addressed as Your Majesty and then she’s called “ma’am.”

Meanwhile, royal author Andrew Morton previously revealed that Princess Diana was also urged to call Prince Charles “sir” before they tied the knot. In the documentary “The Royal Wives of Windsor,” Morton said that it was only after the Prince and Princess of Wales tied the knot that the latter was able to call the former by his first name.

“On paper, Diana was absolutely perfect as a royal bride. All of these things seemed to be important at this time. Everyone was acting in what they perceived to be the best interests of the institutions but it was doomed … It was not a fairytale; it was a nightmare,” he said.

In related news, Sarah Ferguson also called Prince Andrew “sir” during the first six months of their relationship. In her autobiography “My Story,” the Duchess of York confirmed that she called the Duke of York by the unusual nickname.

At present, Ferguson refers to her ex-husband as the Duke of York or as Prince Andrew in public. The couple tied the knot in 1986 just months after they got engaged. Princess Diana was reportedly the one who introduced the royal couple.

However, the Duke and Duchess of York’s marriage were short-lived. Ten years after they tied the knot, their divorce was finalized. And just months later, Princess Diana and Prince Charles also divorced after the Queen meddled in their relationship.

Meghan Markle, Prince Harry, Prince Charles
Meghan Markle practices yoga and has inspired Prince Harry and Prince Charles in very Markle, Princes Harry, Charles attend Royal Ascot Day 1 at Ascot Racecourse on June 19, 2018 in Ascot, United Kingdom. Getty Images/Chris Jackson