Prince Harry and Meghan, Duchess of Sussex, along with Prince William and Princess Kate of Wales appeared together outside Windsor Castle Saturday.
It was the first time the four had appeared together in public since March 2020, PA reports.
Both couples were dressed in black as they looked at flowers and messages left in front of the castle's gates. They then spent more than 30 minutes shaking hands and meeting with mourners.
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William and Harry could be seen walking next to each other at one point, a striking show of unity as they mourn the loss of their grandmother, after a public rift between the brothers.
The event is the first time Prince William, who was appointed to fill his father's role as Prince of Wales upon his accession to the throne, met with the public following the death of Queen Elizabeth II.
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The royal couples received flowers, shook hands and spoke with members of the public following the accession ceremony of King Charles III, William and Harry's father.
William released his first public statement on the passing of his grandmother Saturday and reflected on the time he got to spend with the Queen.
"On Thursday, the world lost an extraordinary leader," he said. "I, however, have lost a grandmother.
"While I will grieve her loss, I also feel incredibly grateful. I have had the benefit of The Queen's wisdom," he continued. "My wife has had twenty years of her guidance and support. My three children have got to spend holidays with her and create memories that will last their whole lives."
Neither William and Kate nor Harry and Meghan's children were present for the day's events.
Both couples waved at the mass of mourners before departing in the same vehicle driven by Prince William.