US President Donald Trump will finally meet Britain's Queen Elizabeth II on Friday, throwing one of the world's most unscripted leaders into a situation full of convention and tradition.
But the Queen has met 11 of the 12 serving US presidents during her reign -- she never officially met Lyndon Johnson -- and with decades of experience on her side, she is unlikely to be ruffled by Trump's unpredictable nature. She's hosted far more controversial figures, from Syria's Bashar al-Assad back to Romanian dictator Nicolae Ceausescu.
Here's what to look out for when the Queen meets her next US President and first lady, Melania Trump.
The last US leader to drop in on Windsor Castle was President Barack Obama in 2016, and he was picked up from Marine One by Prince Philip personally in his Range Rover, a move that raised a few eyebrows among the Secret Service.
Prince Philip will not play chauffeur this time, but the Queen will welcome Trump and the first lady at the quadrangle of Windsor Castle in royal fashion. A guard of honor will give a royal salute and the US national anthem will be played. The Queen and the President will then inspect the guard of honor before a military march-past.
The Queen doesn't expect people to bow to her, least of all foreign heads of state, but many choose to anyway. Observers will be interested to see how reverential the Trumps will be to the world's longest-serving head of state. This is the guidance on the royal website should they wish to follow it: "For men this is a neck bow (from the head only) whilst women do a small curtsy. Other people prefer simply to shake hands in the usual way. On presentation to The Queen, the correct formal address is 'Your Majesty' and subsequently 'Ma'am,' pronounced with a short 'a,' as in 'jam.'" Obama went for the "Your Majesty" but skipped the bow at Windsor.
Tradition dictates that the Queen speak first and initiate any body contact, which at most extends to a handshake. Michelle Obama famously broke this protocol as first lady when she put her arm around the Queen in 2009, not that the Queen appeared to mind. President Trump is partial to a bit touchy-feely too, and is famous for his awkward handshakes. In February 2017, Trump shook Japanese Prime Minister Sinzo Abe's hand for a whole 19 seconds, repeatedly patting his hand. He is also prone to gruffly pulling people in toward him once a handshake has begun.
This isn't a formal state visit, so the Trumps shouldn't expect a banquet, rather a high tea, which is a minefield of etiquette. British high society bible, Debretts, offers the following advice: "If a waiter places a teapot on the table without pouring the tea the person nearest the pot should pour for everyone. If the teapot contains loose tea, pour through a tea strainer. Add the milk or lemon and sugar. After stirring, remove the spoon from the cup and place it on the saucer. Hold the handle of the teacup between your thumb and forefinger; don't hold your little finger in the air. Don't dunk biscuits in your tea unless in an informal setting, and don't slurp -- even if it is piping hot."
The other Mrs. Trump
How long will it take for Trump to mention his mother, Mary Anne MacLeod Trump, who hailed from Scotland? He told The Times of London last year: "My mother was very ceremonial, I think that's where I got this aspect because my father was very brick-and-mortar, he was like, and my mother sort of had a flair, she loved the Queen, she loved anything -- she was so proud of the Queen. She loved the ceremonial and the beauty, cause nobody does that like the English. And she had great respect for the Queen, liked her. Anytime the Queen was on television, an event, my mother would be watching. Crazy, right?"
Where's Prince Philip?
Prince Philip is famous for his culturally insensitive gaffes, and a meeting with Trump would no doubt provide for a risqué cup of tea. But alas, Buckingham Palace has confirmed the Prince, who is now retired, will not attend this time. In China in 1986, Prince Philip described Beijing as "ghastly" and infamously told British students there: "If you stay here much longer you'll all be slitty-eyed." In 2003, when greeted by Nigeria's President Olusegun Obasanjo, who was wearing traditional robes, he quipped: "You look like you're ready for bed."
A picture marking the meeting will go down in anglo-American history, and here it is all about the optics. How will this one compare with the others she has stood for? What can we read in to the body language?
Trump isn't the only head of state in the room with a Twitter account. How will Buckingham Palace sum-up the visit, as opposed to the White House and of course the President's personal account?