In June 1987, Diana, Princess of Wales, and Sarah, Duchess of York, arrived at Royal Ascot together. The two were clearly in high spirits, caught naughtily poking a courtier in the bottom with an umbrella.

It was a cheeky moment that reflected their relationship: They were not only close friends but provided essential support to one another, two outsiders who had married into the most famous family in the world.

Later, their respective separations and divorces would follow an eerily similar trajectory. However, when Diana was tragically killed one summer's night in Paris in 1997, the two former mates hadn't spoken in about a year.

Dating future Dukes

Strangely enough, Diana and Fergie were distantly related (fourth cousins) and their mothers, Frances Shand Kydd and Susan Barrantes, respectively, had gone to school together. (Both of their mothers had also left their families for other men too.)

Come the early '80s, they were two young women who moved in the same upper class London circles and became friends, being photographed at the polo together. (Fergie's father was Prince Charles' longtime polo manager.)

When Diana and Charles tied the knot, Fergie was inside Westminster Abbey, invited by her good friend.

Later, the Princess of Wales would play cupid, inviting the fun-loving Fergie (then a PR for a small Swiss publishing house) to stay at their country mansion Highgrove when Prince Andrew was also in attendance. The rest is messy royal history.

Partners in crime

While the Queen and Prince Philip might have approved of their second son's choice of a wife, she was raising regal eyebrows long before the wedding, becoming the first Windsor bride to enjoy a bachelorette party. A bottle of prosecco and some private giggles? No way.

Instead, Fergie and Diana dressed up as policewomen and caused a ruckus when they tried to gatecrash Andrew's stag do. Later, still wearing the same uniforms, they headed to famed club Annabel's for a boogie. In 1986, Fergie officially joined The Firm, wedding Andrew and becoming the Duchess of York and giving Diana an official partner-in-royal-crime.

Buckingham Palace BFFs

From the outset, Diana and Fergie provided invaluable support to one another, as they both struggled with the rules and responsibilities they had been lumped with courtesy of their marriages. In the claustrophobic world behind palace gates, they were confidants, helpmates and allies who knew the other could uniquely understand the challenges inherent to royal life.

"I always felt safe in mimicking Diana — she was so far ahead of me on the learning curve, such a deft ambassadress for Britain and the royal family," Fergie would write years later in her autobiography. "I thought Diana had all the answers."

However, things started to sour between the pair. After her marriage, Fergie enjoyed a wave of public adoration. She was the vibrant, energetic new face on the Buckingham Palace balcony, a breath of fresh air blowing through the royal corridors who brought a cheeky delight to her official duties.

Diana, by contrast, was privately contending with mental health issues, bulimia and her disintegrating marriage to Charles.

Publicly, the more formal, reserved Princess of Wales had started to look stilted and stuffy in contrast to her bubbly, rambunctious sister-in-law.

Making things even more complicated was Her Majesty and the Duke of Edinburgh's genuine fondness of Fergie, while Diana's relationship with her in-laws only got frostier and frostier. Ditto Charles, who is alleged to have once told Diana, "I wish you would be like Fergie — all jolly."

A close friend of the family told Vanity Fairin 1987: "At first it was lovely for the Princess to have someone of her own age to giggle with. Now I think she can't help but make comparisons. The Duchess has all the perks and none of the real stresses and strains. As time goes by, that will become more and more apparent. I predict there will be competition."

"I got terribly jealous (of Fergie) and she got terribly jealous of me," Diana would later tell her biographer Andrew Morton. "She kept saying to me: 'You mustn't worry, Duch, everything is going to be fine, let me do this, let me do that'. I couldn't understand it, she was actually enjoying where she was, whereas I was fighting to survive."

Still, they enjoyed getting up to some merry hijinks, such as taking a tandem bike while staying at Balmoral in 1991 and careening across a golf course before taking the Queen Mother's expensive Daimler with Diana in the driver's seat and donning the chauffeur's cap.

One another instance, after a state dinner at Buckingham Palace, a journalist remembers catching the two women kicking off their shoes and running in their ball gowns down the lengthy, ornate corridor after they were allowed to leave.

All the single (royal) ladies

By the early '90s, both Diana and Fergie's marriages were struggling, and the couples both announced they were separating in 1992. (The same year Princess Anne revealed the end of her marriage to Mark Philips, resulting in the Queen famously opining that year had been her "annus horribilis".)

Despite outwardly appearing to be close, Diana and Fergie's relationship was becoming far more complicated.

With both women looking to extract themselves from the Windsors' clutches, Diana was said to have viewed Fergie as the "canary down the mineshaft", according to biographer Andrew Morton — that is, to see how the family would treat a royal deserter.

It has also been alleged Diana tipped off the paparazzi that Fergie was holidaying with her "financial adviser" John Bryan in the south of France when he was infamously caught sucking her toes, reportedly to distract from the Squidgygate tapes.

The final chapter

In 1996, the Yorks officially divorced, leaving Fergie with mounting debts and life as a single mother. She penned her autobiography My Story the same year, a move Diana gave her approval for. The only thing — she didn't want the Duchess to mention her.

While the two, now both former members of the royal family, had had a tumultuous friendship, one simple line in Fergie's bestseller put the final nail in the coffin of her relationship with Diana. She wrote: "Diana helped me by giving me all her shoes (and less happily, her plantar warts)."

No matter that it was only a brief mention of a very common, minor ailment, the Princess was infuriated.

Incensed, Diana cut Fergie out of her life immediately, according to reports. Biographer Tina Brown says the Princess never spoke to her sister-in-law again despite Fergie's profuse apologies.

When Diana hosted a birthday party for Hong Kong businessman David Tang in 1997, she reportedly told him he could invite anyone he wanted. When he suggested Fergie, the Princess retorted, "Absolutely not," according to Brown's The Diana Chronicles.

So, would that have been the final sad chapter in their friendship?

Royal editor Robert Jobson told a documentary this year that he thought the two women "probably would have repaired that relationship, but it didn't happen. It's a sad ending."

Decades on, Diana is still a presence in Fergie's life. Earlier this year, she launched an anti-bullying campaign and penned an open letter saying: "Women, in particular, are constantly pitted against and compared with each other in a way that reminds me of how people tried to portray Diana and me all the time as rivals, which is something neither of us ever really felt."

Of all the losses that Diana's death represented — as a mother, sister, aunt and friend — it is haunting to think of the ways the more media-savvy royal might have helped steer the scandal-prone Duchess.

In an interview, Fergie has been quoted saying of Diana: "I loved all her funny mad ways."

Us too. Us too.