Diana, Princess of Wales had 2,500 guests at her July 29, 1981 wedding to Prince Charles, but when the 20-year-old walked down the aisle of St. Paul's Cathedral with her father Earl Spencer, she looked for one specific person: Camilla Parker Bowles, her husband's former girlfriend — and later, his second wife.

"I knew she was in there, of course. I looked for her," the late royal told biographer Andrew Morton in 1991. "So walking down the aisle, I spotted Camilla, pale gray, veiled pillbox hat, saw it all, her son Tom standing on a chair. To this day you know — vivid memory."

Her candid interview with the English journalist, revised and re-released last summer as Diana: Her True Story — In Her Own Words, recently appeared in the National Geographic documentary Diana: In Her Own Words as well. But the factoid begs the question why Camilla was there at all.

The bride had developed some serious animosity for her fiancé's ex in the lead-up to her wedding, according to author Penny Juror. "She became jealous — obsessing about Camilla Parker Bowles — suspicious, turning against people she appeared to like, convinced they were out to get her," she wrote in The Duchess: Camilla Parker Bowles and the Love Affair That Rocked the Crown.

Given her distrust and dislike of Camilla, it seems unusual that she made the guest list. However, the invitation makes more sense considering that her husband Andrew Parker Bowles played a significant role in the wedding procession. As the Commanding Officer of the Household Cavalry Mounted Regiment, he would oversee the ceremonial escort that accompanied the couple's carriage ride through London.

Perhaps even more importantly, Prince Charles and Camilla had remained close friends even after their romance in the early '70s. The future king knew that the palace disapproved of his former girlfriend as a potential wife, but the two continued to spend time together even after Camilla married in 1974. Diana initially didn't know the extent of their relationship however.

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Prince Charles chats with Camilla Shand in a circa-1970s taken at a polo match.

"Instead of explaining to Diana at the outset that Camilla was an old girlfriend, he had presented her as nothing more than a friend," Juror wrote. "It didn't occur to him that she needed to know before someone else told her ... He came clean after the engagement, admitting that Camilla had been one of his most intimate friends, but reassured Diana that from now on there would be no other women."

That didn't ultimately soothe Diana's fears though. She discovered a number of gifts exchanged between the two, including gold chain bracelet given before the wedding with the letters G and F: "'Gladys' and 'Fred' — they were their nicknames," the princess told Morton.

It's no surprise then that she reportedly shot down further involvement of the Parker Bowles family in the bridal party. The group eventually included two page boys, but the couple had initially discussed a third candidate.

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Prince Charles and Lady Diana Spencer walking down the aisle after their wedding ceremony on July 29, 1981.

"When it came to choosing her attendants for the wedding, she vetoed Charles' suggestion that Camilla's son, Tom, his godchild, be in the wedding party," Juror wrote. "She agreed to several of his other godchildren, but there was a big scene about Tom Parker Bowles and Charles backed down."

The Parker Bowles family didn't attend the post-wedding breakfast at Buckingham Palace either, although the groom reportedly had a moment with his longtime friend during the ceremony.

"Charles glanced over at Camilla he walked down the aisle with a 'slightly plaintive, sad look' on his face," Juror says. "Their wonderful affair was over and this was reality kicking in."

The Prince of Wales later admitted to rekindling his romance with Camilla after his first marriage had "irretrievably broken down" in 1986. He divorced Diana a decade later and waited nearly another 10 years to wed his longtime love.

This time the event included way fewer guests. Only close family members attended the civil ceremony at Windsor Castle in 2005, but not everyone. His mother Queen Elizabeth II and father Prince Philip only came to the following service of prayer and reception.

preview for A Timeline of Prince Charles and Camilla Parker Bowles' Royal Romance