When the Palace doors close, their fancy titles go out the window.

In public Kate Middleton is referred to as the Princess of Wales, and her father-in-law, His Majesty King Charles III, but behind closed doors, their fancy titles reportedly no longer apply.

According to royal author Robert Jobson’s new book Catherine, the Princess of Wales, Middleton and Charles call each other by the cutest nicknames.

In an excerpt from the upcoming biography published by The Daily Mail, Jobson writes that the princess privately addresses Charles as “Grandpa,” while he likes to refer to her as his “beloved daughter-in-law” or “darling daughter-in-law.” Jobson also says that the monarch usually greets Kate with a “loving kiss” at family functions, and that they’re often seen “laughing and joking together.”

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“I think the [King] understands Catherine is a good influence on the entire family. He loves and truly appreciates everything she does,” a Palace courtier shared with Jobson in the book, adding that Middleton has a “calming” effect on Prince William. Calling Kate an “instinctive peace-maker,” the author revealed that she has “smoothed the way for William and his father to have a better relationship than they had in the past.”

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“[She] is somebody who always tries to see both sides of any dispute,” a source told Jobson, explaining that the “emotionally mature” princess “is a stabilizing influence.”
Charles and Middleton’s bond has only strengthened since they were both diagnosed with cancer earlier this year. “They are in this together, Kate and the King,” royal biographer Sally Bedell Smith previously told People. The two shared a private lunch together a day before Kate announced her cancer news with a video message in March, which Buckingham Palace noted Charles was “so proud” of her for doing so.