King Charles III appointed his wife, Queen Camilla, to one of the highest honors in the U.K. this week.

On Wednesday, Prince William joined his father and stepmother in Scotland for the Thistle Service at St. Giles’ Cathedral in Edinburgh during Royal Week, an annual official visit by the monarchy.

The Order of the Thistle is the highest order of chivalry in the country, recognizing 16 knights (men and women) who "have held public office or have contributed significantly to national life," along with additional royal knights, according to the royal family’s official website.

King Charles awarded the honor to Queen Camilla and his brother, Prince Edward, who holds the title of Duke of Edinburgh, the same one their father, Prince Philip, held.

Prince William and Princess Anne are counted among the knights and ladies in the order, along with the 16 selected for their public duties. 

On social media, the royal family shared a group photo of Prince William (known as the Duke of Rothesay when in Scotland), King Charles, Queen Camilla, and Prince Edward in the full regalia for the occasion.

The Order of the Thistle’s ceremonial garments include green velvet robes, insignia and white plumed hats. 

Per the royals’ website, the date of the founding of the Order of the Thistle is not exactly known, but according to legend, it dates back to 809 when King Achaius made an alliance with Emperor Charlemagne. 

King James II of England established the Order with new rules in 1687, to focus on rewarding "Scottish peers who supported the king’s political and religious aims."

As sovereign, only King Charles III can bestow the honor. This year, the honor went to recipients like anthropology professor Dame Sue Black, lawyer Baroness Helena Kennedy and Scotland's first black professor, Sir Geoff Palmer, per the BBC. 

The procession begins at the Signet Library in Edinburgh Town Center, and following the ceremony, a reception is held there, followed by lunch at the Palace of Holyroodhouse, the official residence of the monarch in Scotland.

Royal Week has been shortened this year due to Britain’s general election, but Charles and Camilla also attended a celebration marking the 900th anniversary of the city of Edinburgh, toasting the gathering with a drink.

On Tuesday, the king and queen hosted 8,000 guests at a Garden Party at Holyroodhouse.

Video shared on the official Instagram for the royal family showed the gathering, with the caption, "Guests at the Garden Party represent all walks of life and have all made a positive impact in their community."