US president Joe Biden and his wife Jill have said they are excited for their sit down with the Queen during their trip to the UK for the G7 summit.
But the American leaders might have already breached apparent royal protocol before their Sunday tea.
The couple reportedly arrived at the Eden Project reception after the Queen on Friday evening as she and other members of the Royal Family hosted world leaders during the summit.
Protocol on this varies depending on the occasion.
For example, at a state event, like a banquet, guests are seated before the Queen enters the room.
A guide of Regal do's and don'ts published by the BBC suggests guests should arrive before the Queen.
However, on some occasions she is seen welcoming the guests, greeting them one by one, which of course means she would be there first.
Marlene Koenig, royal historian, told Bazaar.com in 2019: "When it is a formal event, a state dinner, attending the ballet (sitting in the royal box), a reception, a wedding, the royals are the last to arrive."
She added: "At state dinners, all of the guests are in their seats before the royals arrive.
"The Queen enters first followed by other royals. She will walk with the head of state."
Etiquette guide Debrett's states: "It is correct for everyone to arrive before the royal personage and protocol rules that no guest should leave an event before a member of the Royal Family."
However any apparent faux pas is likely to have been ignored by the Queen, who is not actually overly concerned by protocol.
Watch: Queen uses ceremonial sword to cut cake
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."
It's the Bidens first time meeting the Queen this week, although they have met both her son Prince Charles and her grandson Prince Harry.
Ahead of the meeting, Dr Biden said: "Joe and I are both looking forward to meeting the Queen. That’s an exciting part of the visit for us. We’ve looked forward to this for weeks and now it’s finally here. It’s a beautiful beginning."
She was asked after her meeting with the Duchess of Cambridge if she had sought advice for meeting the Queen from Kate.
But she replied: "No I didn't. We've been busy. Were you not in that room? We were talking education."
The couple will go to Windsor Castle on Sunday afternoon for tea with the monarch.
Biden will inspect a Guard of Honour before the trio goes for a private meeting.