Watch: President Biden, first lady meet Queen Elizabeth II
Joe Biden has become the 13th president to have a one on one with the Queen after the monarch invited him and his wife, Jill, for tea at Windsor Castle.
The Bidens visit follows the G7 summit in Cornwall, where the Queen first met them on Friday evening, during a drinks reception held by the Royal Family at the Eden Project.
He is the fourth US president in recent times to have met the Queen at Windsor Castle, instead of Buckingham Palace which is where heads of state are usually entertained by the monarch.
In 2018, Donald Trump was invited to Windsor where he had tea with the Queen.
Here's how the two visits differed.
Trump got more time with the Queen
Both visits overran, according to reports, with Trump's scheduled half an hour meeting running over by 20 minutes.
The Bidens were reportedly over schedule by just 10 minutes.
The Bidens total visit was about an hour, with the couple arriving just after 5pm and leaving at about 6.03pm.
On his way to his next stop, Brussels, Biden told reporters the Queen had been "generous" and "gracious" and even remarked that she reminded him of his mother.
Biden's solo Guard inspection
In 2018, Trump was invited to inspect the Guard of Honour, as was Biden in 2021.
However in 2018, the Queen inspected the Guard with Trump. In 2021, the Queen stayed back on the dais as she spoke to Dr Biden.
The inspection of the Guard of Honour is something that the visiting head of state used to do with Prince Philip.
He was retired by 2018, when Trump was invited, so the Queen stepped into the role.
But in 2021, with the recent death of Prince Philip not far from her mind, the Queen asked Officer Commanding the Guard of Honour, Major James Taylor, and Major General Christopher Ghika to carry out the inspection with Biden.
Biden appears to avoid any major gaffes
In 2018, Trump was widely criticised when he appeared to make a major gaffe by standing in front of the Queen as they set out to inspect the Guard of Honour.
But Biden appeared to avoid any major mishaps in front of the Queen.
He arrived wearing sunglasses, but took them off. The Queen herself was wearing sunglasses during an engagement last week, when she received a rose in honour of her late husband, the Duke of Edinburgh.
Biden also appeared to offer an elbow to the Queen at one point, to help her walk down the steps, but she seemed to decline.
The president revealed details of their meeting later on, which is unlikely to be anything the palace will comment on, considering it a private chat.
He also made up for arriving after her on Friday evening to her drinks reception, and was early for his meeting on Sunday afternoon.
However, royal experts told Yahoo UK the Queen is rarely overly concerned by apparent gaffes.
Joe Little, managing editor of Majesty magazine told Yahoo UK: "The Queen, steeped in tradition after almost 70 years on the throne, has a very keen eye and will immediately notice that something is not as it should be.
"However, she fully appreciates that things do occasionally go wrong, but would never admonish her guests and make them feel uncomfortable. That's not her style."
Watch: Queen's Quip Gets G7 Leaders Laughing