Princess Diana and Prince Philip’s respective personalities differed greatly—but the Duke of Edinburgh was a surprising support system to the Princess of Wales “when things were difficult.”

Diana’s unexpected ally came in the form of her father-in-law, particularly when Diana’s marriage to Prince Charles—Philip’s eldest son—became tumultuous. Philip stepped in when Diana “was at her most vulnerable and volatile,” royal biographer Gyles Brandreth wrote in his 2021 book Philip: The Final Portrait (and via Marie Claire). Brandreth was not only a royal biographer but also a personal friend of the royal family’s; following Diana’s shocking death on August 31, 1997, Brandreth apparently told Philip that “The public view of you, for what it’s worth, is of a grouchy old man, unsympathetic to his daughter-in-law.”

But that wasn’t the case: “But I happen to know…that when things were difficult, you wrote to Diana—kind letters, concerned, fatherly, loving letters from Pa, explaining how you knew, firsthand, the difficulties involved in marrying into the royal family,” Brandreth added. When Brandreth told Queen Elizabeth’s husband that “The impression the public has got is unfair,” Philip told him, “I’ve just got to live with it. It happens to a lot of people.”

In his letters to Diana, Philip confronted her with some truths and “he invited her to face the facts,” Brandreth wrote. “Essentially he wanted to make Diana think about her marriage, long and hard. And he did.”

“Philip’s letters to Diana were typical of his correspondence overall,” he continued. “They were sympathetic, but unsentimental, direct, but to a purpose.”

Diana was grateful for the support of her father-in-law and his letters, and “As soon as one arrived, she opened it, scanned it, usually burst immediately into tears and then shared it, as soon as possible, with her closest friends,” Brandreth added.