It took 34 minutes, and every minute – every second – must have been excruciating.
On September 6, 1997 as 2.5 billion people globally watched, Prince William, 15, and Prince Harry, 12, bravely walked behind their mother's coffin for the journey to Westminster Abbey.
With their father Prince Charles, uncle Earl Spencer and grandfather Prince Philip by their sides, and as the world looked on, the young royals, heads bowed, silently endured an experience which today would be unthinkable.
In the days since Prince Philip, Duke of Edinburgh's death last Friday there has been an avalanche of tributes for a man who gave up any notion of his own career to dedicate himself to his wife, her crown and the nation.
Yes, he was at the forefront of conservation efforts decades before it became a mainstream issue. Yes, he deserves to be remembered as the Queen's greatest supporter and a man who worked up to the age of 95.
But we need to talk about another legacy of the so-called Iron Duke, which is the role he played in the momentous decision for William and Harry to walk behind their mum's coffin.
In those days in 1997, as the world reeled after the death of Diana in Paris and as London was gripped by a collective outpouring of public grief and anger the likes of which had never been seen before, the then 76-year-old Duke was resolutely focused on his grandsons.
During a funeral planning conference call, "We were all talking about how William and Harry should be involved and suddenly, came Prince Philip's voice," Anji Hunter, then-Prime Minister Tony Blair's head of government relations, told The Telegraph.
"We hadn't heard from him before, but he was really anguished. 'It's about the boys,' he cried. 'They've lost their mother.'"
The same week, when Philip and the Queen returned to the capital they undertook a walkabout outside the gates of Buckingham Palace to take in the floral memorial and to speak to the vast crowds who had gathered.
At one stage, a woman said to the Queen, "Take care of the boys, Ma'am!"
According to biographer Sally Bedell Smith, Philip responded with, "That's what we've been doing. That's what we've been doing!'"
While there had long been detailed blueprints in place for the Queen, the Queen Mother and Philip's funerals, when Diana was killed in Paris, Buckingham Palace was left scrambling. (Ultimately, the event was modelled after Queen Mother's blueprint, named Operation Tay Bridge.)
A key question that had to be answered: would Diana's sons walk behind the coffin?
According to biographers, it was Philip who helped decide the issue.
"Both boys, especially William, had been reluctant all week to commit to something so public," according to Smith. Nor were there plans at that stage for Philip to join the procession.
Then, their devoted grandfather stepped in.
"If you don't walk, you may regret it later," Philip reportedly said to William and Harry. "I think you should do it. If I walk, will you walk with me?"
Both boys agreed.
In the late-summer sunshine, as her funeral cortege passed St James's Palace, the four princes and the earl fell in behind the horse-drawn gun carriage bearing Diana's body.
Her brother would later reveal that they could hear people in the crowds "sobbing, wailing and shouting messages of love to Diana", per the BBC.
However, with the benefit of hindsight, was the choice for the boys to take such a public role the right choice? Did Philip's grandfatherly concern instead end up having unintended consequences?
Speaking in 2017, Harry told Newsweek, "My mother had just died, and I had to walk a long way behind her coffin, surrounded by thousands of people watching me while millions more did on television."
He added, "I don't think any child should be asked to do that, under any circumstances. I don't think it would happen today."
But later the same year, while being interviewed for the documentary Diana, 7 Days which marked 20 years since the Princess' death, he said, "Generally, I don't have an opinion on whether that was right or wrong. I am glad I was part of it. Looking back on it now, I am very glad I was part of it."
Prince William said during the same documentary that the procession was "one of the hardest things I've ever done".
"I felt if I looked at the floor and my hair came down over my face, no-one could see me," he said. "It wasn't an easy decision and it was a sort of collective family decision to do that."
He went on to say he had to find a balance "between me being Prince William and having to do my bit, versus the private William who just wanted to go into a room and cry, who'd lost his mother".
Also in 2017, their uncle and Diana's brother Charles, Earl Spencer said he had been "lied to" over the boys' involvement in the funeral.
He told the BBC's Today that in the lead-up to her funeral he had been a "passionate advocate" for the teenagers to not take part in the walk and that he had told courtiers that "she [Diana] just would not want them to do this".
"Then eventually I was lied to and told that they wanted to do it, which of course they didn't, but I didn't realise that," he said.
"It was the worst part of the day by a considerable margin, walking behind my sister's body with two boys who were obviously massively grieving their mother."
Earl Spencer went on to say the experience "was really harrowing" and that "I still have nightmares about it now".
If it was "harrowing" and nightmare-inducing for a grown man, what must the impact have been on two teenagers?
While Philip might have wanted to save his grandsons from future remorse over that day, while his actions were those of a loving grandpa, watching that footage of the princes today is a heart-rending experience.
Harry especially looks like a tiny, tragic figure buckling under the weight of unthinkable grief and the 30-odd minute procession looks like an exercise in the most exquisite, nearly unbearable suffering.
Should the boys have been allowed to grieve much more privately? Should they have, say, been driven from their then home at St James's Palace to the Abbey rather than being watched, silently, by the hundreds of thousands of people who lined the capital's streets?
I am not, for a second, criticising Philip.
He was a man very much of his generation and time, for whom the greater good and doing one's duty trumped all else. While intellectual and philosophical, nor was he reportedly a man given to letting emotion factor into things.
But consider this – William's son Prince George will turn 8 in a few months. Imagine him only four years from now being asked to walk behind his mother Kate, Duchess of Cambridge's casket. It is simply inconceivable.
Legacies are a tricky thing. When someone high-profile dies, it can be so easy to lose sight of missteps or miscalculations, to wallpaper over the tricky moments.
Philip was clearly driven by love for William and Harry and an unswerving dedication to looking out for their best interests, but it is impossible to gauge the full impact his offer might have had on the boys.
Next year will mark 25 years since that late summer day and those agonising 34 minutes.
Let's hope never again any member of the royal family, especially the younger members, be asked to make such a tortuous decision. Never again.