Prince William Credit: Chris Jackson/Getty

Thursday was a big day for Prince William, who was installed as a Knight of the Order of the Thistle by his grandmother, Queen Elizabeth II during a ceremony in Scotland. The honor is considered the highest available in the country.

"It is our pleasure that His Royal Highness the Prince William, Earl of Strathearn, be installed a Knight of the most ancient and most noble Order of the Thistle," the Queen, 86, announced during the ceremony, which is part of her Diamond Jubilee celebrations in Scotland. (Her engagements in England wrapped in early June.)

The Duke of Cambridge wore a traditional forest green velour cloak adorned with gold embroidery and tassels, plus a blue hat with a giant white plume.

Also in attendance was William's wife, Duchess Kate, 30, who looked stylish and chic in a buttercream yellow coatdress, black heels, a beige hat and a Scottish plaid purse.

The 30-year-old prince joins his father, Prince Charles, his grandfather, Prince Philip and his aunt, Princess Anne as part of the Order's Extra Knights and Ladies. As an "Extra Knight," he will not count toward the Order's 16-member limit (consisting of several politicians, doctors and scholars) but will be recognized as The Earl of Strathearn.

This isn't William's first appointment as Knight. In 2008, he was invested as Royal Knight Companion of the Most Noble Order of the Garter, making him the thousandth to receive the highest British honor of chivalry, which recognizes his seniority within the royal family, since the Order was formed in 1348.