Kate Middleton and Meghan Markle
Meghan Markle wasn't invited to the Order of the Thistle service which Kate Middleton has attended. Pictured: Markle and Middleton attend the first annual Royal Foundation Forum held at Aviva on Feb. 28, 2018 in London. Getty Images/Chris Jackson

Meghan Markle was not invited to an event that Kate Middleton attended in the past.

According to Francesca Specter, a journalist for Express, the Duke and Duchess of Sussex didn't make an appearance at the Order of the Thistle service. Markle's sister-in-law, Middleton, attended the same event in 2012, a year after she married Prince William.

The Duchess of Cambridge is still on maternity leave, so she was not expected to be at the event. A number of royals, including Queen Elizabeth II and Prince William, were present at the occasion. According to Specter, there's a reason the Duke and Duchess of Sussex were not present: Prince Harry is not a Knight of the Thistle.

The Order of the Thistle is an ancient order of chivalry in Scotland. The service takes place every two years to welcome its new members: the knights and ladies. Prince William and Princess Anne both received the honor, so they attended the event this year.

Middleton was able to make it into the event because of her husband, Prince William. Prince Harry has not received the honor, but it's possible that he may do so in the future. The Duke of Sussex has not received a knighthood for the Order of the Garter either.

In related news, Prince Harry and Markle were last spotted together at a reception in London on Thursday. For the said sighting, the new duchess donned a yellow dress. Many also noticed that the couple had returned to holding each other's hands again.

Prince Harry and Markle made headlines in the previous weeks after the duke reportedly ignored his wife's efforts to hold hands during the Queen's Young Leaders Awards ceremony. According to royal experts, Prince Harry declined to hold hands with Markle in the presence of Queen Elizabeth II, because royals are discouraged to show PDA on formal events.

"Public displays of affection, such a hand holding, may be accepted in private situations and in social spheres, but when out in public Harry and Meghan are on official business and so it is not considered professional behaviour," royal etiquette expert William Hanson explained.