Prince William, Princess Kate and Prince Harry took part in a special service on Monday at Westminster Abbey with their granny, Queen Elizabeth.
The service honors the nations in the Commonwealth, a voluntary organization of 53 countries, most of which were once under British rule. (In addition to the U.K., there are 15 other countries in the group that hold the Queen as their Sovereign.)
The presence of William, Kate and Harry shows "the importance of maintaining the Commonwealth link," experienced royals commentator Robert Lacey tells PEOPLE. Interestingly, the Commonwealth doesn’t have to vote for a royal to lead it when the Queen ends her involvement.
"Having William at the service is another way of explaining to others that here, in the House of Windsor, are the people they should turn to for future leadership and commitment," says Lacey.
Kate, in a chic tiled grey hat by John Boyd and a matching grey coat dress by Erdem, and husband William were back at the place they were married nearly five years ago, Westminster Abbey.
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The Queen, in light blue coat and matching hat, and husband Prince Philip were the last of the royals to arrive at the grand Abbey for what is the largest annual inter-faith gathering in the U.K., where she highlighted the theme of "an inclusive Commonwealth" in her annual, pre-recorded address.
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The Queen's message was played at the service. "Being inclusive and accepting diversity goes far deeper than accepting differences at face value and being tolerant," she said. "True celebration of the dignity of each person, and the value of their uniqueness and contribution, involves reaching out, recognizing and embracing their individual identity.
"Let us give it practical effect by supporting those in need and those who feel excluded in all walks of life. By doing so, we will continue to build a truly representative Commonwealth community."
Singer Ellie Goulding – who recently spoke about feeling nervous during her performance at Will and Kate's wedding – also sang at today's proceedings.
When historians examine the Queen's record-breaking reign, "one achievement will be keeping the British Monarchy on the road," says Lacey, "but she also gave life to the Commonwealth and created this unique thing that no other imperial power possesses: Being on good terms with former colonies."
After the service, William, Kate and Harry also attended a reception.
William's father, Prince Charles and his wife Camilla, Duchess of Cornwall are touring the Balkans, beginning in Croatia today.
William and Kate are set to visit the Commonwealth country of India next month. Harry has his own tour coming up – his trip to Nepal starts next weekend.