King Charles III, Princess Anne, and their siblings stood guard around the Queen's coffin at St Giles' Cathedral

• King Charles III and his siblings took part in the Vigil of the Princes on Monday.

• Charles III, Princess Anne, Prince Edward, and Prince Andrew surrounded Queen Elizabeth's coffin.

• This is only the third Vigil of Princes in recorded history.

King Charles III and his siblings paid tribute to their mother through the Vigil of the Princes.

On Monday, the new monarch, Princess Anne, Prince Edward, and Prince Andrew walked behind the hearse carrying the Queen's coffin as it processed on the Royal Mile in Edinburgh, Scotland, to St Giles' Cathedral for a Service of Thanksgiving in Queen Elizabeth's honor.

At the cathedral, the Queen's children stood guard around her coffin for the Vigil of the Princes.

They stood side by side with the Royal Company of Archers, who protect the Queen's coffin, for around 10 minutes, according to People. The vigil is a way for a monarch's children to mourn the loss of their parent, The Independent reported.

The Queen's four children looked somber in photos from the event, with each of them standing on one side of the coffin. Because the vigil took place in Scotland, King Charles III wore a kilt for the occasion.

Read more: A look inside St Giles' Cathedral in Edinburgh, where Queen Elizabeth II's coffin lies at rest

The Vigil of the Princes has only been held three times in recorded history: at the death of King George V in 1936; at the death of the Queen Mother in 2002; and on Monday, for Queen Elizabeth. Charles, Andrew, Edward, and Viscount Linley, the son of Princess Margaret, took part in the Queen Mother's vigil.

The Independent reported that Princess Anne, who accompanied her mother's coffin on its six-hour journey from Balmoral Castle to Edinburgh, is the first female royal to take part in the tradition.

Queen Elizabeth II died peacefully at Balmoral on Thursday. Her funeral will take place on Monday, September 19, and the United Kingdom — and the royal family — will be in a period of mourning until she is buried at St George's Chapel at Windsor Castle.

On Tuesday, the late monarch will arrive in London where she will lie in state at Westminster Abbey, allowing the public to pay their respects to the Queen.

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