Standing on the right hand side of the escalators on the London Underground is as traditional for Brits as fish and chips or joining a long queue without knowing why.
So naturally it has caused a stir today as the Princess of Wales was spotted making a major faux pas as she travelled on the London Underground today.
William and Kate, dressed in a recycled Eponine London red coat, travelled on the Elizabeth Line for the first time as they took public transport to a pub to hear how London workers are preparing for the Coronation.
But as she took an escalator to exit the station, Kate was seen going against etiquette as she stood on the left-hand side of the moving staircase.
While it is not an official rule, the London Underground is peppered with signs advising travellers to "stand on the right".
The method allows for smooth sailing on the escalators, as the left side is clear for anyone walking to whizz up at their speed, while those stationary can stand on the right.
The royal couple travelled three stops and got off at Tottenham Court Road, in the shadow of Centrepoint tower block.
They boarded the Tube at Acton Main Line in west London and chatted to Transport for London (TfL) workers about their plans for the busy bank holiday weekend, as tens of thousands of people are expected in the capital.
Asking the group which of them would be working this weekend, Kate said: "Do you think there’s going to be extra pressure on the service during the coronation weekend? How do you manage that?"
One of the group then asked how the royal family’s preparations were going, to which Kate replied: "Yes, it’s going to be a very busy time. We’re getting there. I still feel like we’re trying to get ducks in a row."
Speaking to maintenance engineer Maria Serco about the opening of the line last year, William said: "I can’t quite believe it has been nearly a year. It’s quite nice to get the chance to see it all, after my grandmother opened it."
After the short journey from Acton, via Paddington to Tottenham Court Road, the couple were presented with gifts by Andy Lord, TFL’s commissioner.
They were given a plaque tile of a TFL Roundel, or “Crowndle” as Mr Lord described it, inscribed with “King Charles III,” and a TFL roundel lightbox for Prince George, Princess Charlotte and Prince Louis.
Dozens of stunned onlookers then rushed towards the escalator with their phones, to film the couple as they made their way up the escalator.
Kate then used an Oyster card as she left the station, walking a short distance with William to the Dog and Duck pub in Soho.
Speaking after the couple had left the station, Mr Lord said: “It was amazing to welcome them on their first visit on the Elizabeth Line. It’s coming up to the anniversary of the Queen opening it, and they were very impressed with it.”
Miss Serco said of William: “He was very interested in the technology, how we put the concrete in, the apprenticeships and training we offer.
"And I told him how much interest we’ve had from other countries around the world. It was really nice to thank them for all the royal family’s endorsement of the Elizabeth Line. We always knew we had their support. We feel the link to the royal family.”
Kensington Palace also invited tube and train enthusiast, Francis Bourgeois, to ride on the train with William and Kate as an observer.
He has more than a million followers on Instagram, and royal aides said it was hoped he might bring the experience to a “younger audience”.
The hospitality industry is hoping for a bumper weekend thanks to the Coronation, and the couple met representatives from the industry based in the Soho area.
The Dog and Duck is part of the Nicholson's group, which has several pubs across the UK and is celebrating its 150th anniversary this year.
William and Kate's trip to the pub comes ahead of a busy few days for the royals with the Coronation of King Charles now just two days away.
Yesterday - three days before the occasion - they attended a rehearsal at Westminster Abbey with their three children.
The royal family was greeted by the Dean of Westminster and shown inside the gothic church, where Charles and Camilla will be anointed and crowned on Saturday in front of 2,300 guests and a global audience of millions.
Future King George, nine, has the duty of being one of his grandfather's pages of honour and is expected to be tasked with holding the train of Charles's regal robes.
William will kneel before his father and vow to be his "liege man of life and limb" in the only homage of royal blood of the ceremony.
He will also help clothe the King in his golden robe royal, also known as the mantle, ahead of his crowning.
Louis, five, was spotted sat next to William in the back of a car as they left the rehearsal.
Also there were Camilla's grandsons, who will act as her pages of honour.
The King's sister, the Princess Royal, also attended after visiting soldiers from The Household Division at Wellington Barracks in the morning as they prepared for their roles in the military processions.
The rehearsal in central London came after a man was arrested when suspected shotgun cartridges were thrown into the grounds of Buckingham Palace on Tuesday evening.
Security minister Tom Tugendhat said police and intelligence agencies are mounting a "very complex" security operation around the ceremony - with the eyes of the world set to be on the UK.
"The police are, to put it mildly, all over it and our intelligence and other security forces are extremely aware of the challenges that we face and ready to deal with them - as the police did quite brilliantly yesterday," he told Times Radio on Wednesday morning.
Mr Tugendhat said the coronation is "an opportunity to showcase the United Kingdom to the world".
With last-minute preparations ramping up, early morning rehearsals were staged in the capital, revealing the first glimpses of the grandeur of the coronation processions.
Hundreds of soldiers, many on horseback, marched from Buckingham Palace past Trafalgar Square and Downing Street to Westminster Abbey shortly after midnight on Wednesday.
The diamond jubilee state coach and gold state coach travelled down The Mall as part of the dry-run for the full event.
Royal fans waited for more than three hours for the parade to make a return trip.
Crowds ran through St James' Park to find the best spots near the palace, with many trying to spot relatives in their military outfits.
