The Duchess of Cambridge looked stunning in Princess Diana’s favourite tiara during a state banquet for U.S. President Donald Trump on Monday.

The banquet, held at Buckingham Palace, was part of the president’s three-day trip to the U.K.

During the lavish dinner, Kate Middleton wore the Lover’s Knot tiara, a family heirloom made with family-owned pearls and diamonds, Town and Country magazine reported.

The Duchess of Cambridge and United States Secretary of the Treasury Steven Mnuchin arrive through the East Gallery for a state banquet at Buckingham Palace. Photo: Getty Images.

First made in 1914 for Queen Mary, the tiara was passed down to her granddaughter, Queen Elizabeth II, after she passed in 1953.

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“In 1981, the Queen reportedly loaned the Lover’s Knot tiara to Princess Diana as a wedding gift,” the magazine continued.

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“Diana opted instead to wear the Spencer tiara, a family heirloom, on her big day, but the Lover’s Knot became one of her favourite pieces, despite being so heavy that it hurt her head.”

Princess Diana wearing the “Elvis dress” during a visit to the Culture Centre in Hong Kong in 1989. Photo: Getty Images.

On Monday, the Duchess paired the tiara with a white Alexander McQueen gown and sparkly diamond-and-sapphire earrings she borrowed from the Queen.

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But it’s not the first time the Duchess has worn her late mother-in-law’s favourite crown.

In 2018, the Duchess wore the headpiece during another state banquet, this time for King Willem-Alexander and Queen Máxima of the Netherlands.

Catherine, Duchess of Cambridge, during a state banquet at Buckingham Palace on Oct. 23, 2018. Photo: Getty Images

In 2015 and 2016, Kate wore the crown during a diplomatic reception at Buckingham Palace, and in 2017, she wore the tiara during a state visit of King Felipe and Queen Letizia of Spain.

Catherine, Duchess of Cambridge departs after Attending the annual diplomatic reception at Buckingham Palace on Dec. 8, 2016. Photo: Getty Images.

Town and Country magazine continued to report that when Princess Diana first divorced Prince Charles, the tiara was returned to Queen Elizabeth II and reportedly put into a safe.

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The tiara first came back into the spotlight in 2015 — after 20 years — when the Duchess of Cambridge wore it as a tribute during a diplomatic reception at Buckingham Palace.

But the timing of when the Duchess first wore this tiara may also come down to the Royal Family’s dress code, BBC reported.

“The old rule is that hats are never worn indoors after 6 p.m. because that is when the ladies changed into evening dress, and tiaras and the family jewels would come out,” said Diana Mather, a senior tutor for The English Manner etiquette consultancy, in 2017.

“Flashy diamonds and tiaras are not worn during the day, and only married ladies wear tiaras.”