In the wake of the 9/11 terrorist attacks the Queen ordered for the US national anthem to be played at the Changing of the Guard.
Almost 21 years ago the world was struck with shock as two planes were hijacked and crashed into the World Trade Centre in New York City.
Almost 3,000 people died in the terrorist attack that would go on to spark the 'War on Terror'.
Tributes from all around the world poured in as the world offered its condolences for the shocking attack, and the Queen showed her solidarity with the US in her own way.
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Two days after 9/11 at the Queen's request the Band of the Coldstream Guards played The Star Spangled Banner in a show of solidarity with the people of the United States at their time of hardship.
Many onlookers watched with silent reverence, while others waved the US flag in commemoration of the bonds between the US and UK after the terrorist attack.
It was an incredibly moving gesture, not least because in the centuries long history of the Changing of the Guard no other national anthem had ever been played during the ceremony.
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Breaking with that tradition at the Queen's order upended a status quo which had lasted almost 600 years and dated back to the time of King Henry VII
A month later the Band of the Coldstream Guards accompanied the Pipes and Drums of the Scots Guards in a goodwill tour of New York City.
20 years on from the 9/11 attacks the Band of the Welsh Guards played the US national anthem in a demonstration of the enduring solidarity between the UK and US.
Their performance of The Star Spangled Banner was then followed by a minute's silence in commemoration of the victims of the terrorist attack.
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As a tradition the Changing of the Guard dates back to the time of the Tudor monarchs when Henry VII created the royal bodyguards to protect himself from potential threats.
The first Guards regiments were raised in the 17th century to protect the future King Charles II while Oliver Cromwell ruled England.
The ceremony used to be conducted at the Palace of Whitehall before the monarch's official residence was switched to St James's Palace.
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These days the main ceremony is held at Buckingham Palace, and the songs played range from traditional tunes to more contemporary music.
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Joe Harker