Writing in his shocking memoir Spare, Prince Harry said that after then-Prince Charles gave birth to Princess Diana's youngest son, "It's wonderful! Now we have an heir and a spare.

Whether or not this exchange really took place is not known, but it illustrates one certainty in the British royal family: throughout history, there have often been "spares" who have been at odds with the heir to the throne, rebelling in order to establish their own identity.

Princess Margaret was one, and so was Prince Andrew. For some royal experts, however, Prince William and Princess Kate are doing their utmost to prevent this from happening with their children, Prince George, Princess Charlotte, and Prince Louis.

"William and Catherine are determined to break the cycle of heirs and spares," Kinsey Schofield told the Daily Star.

"I don't think the word 'spare' has ever been uttered in their home to describe one or more of their babies. It is not on their radar. They wanted a large, loving family."

Instead, the Crown Prince and Princess are preparing their children for royal life as best they can.

"William and Catherine are teaching their children about royal responsibilities.

"You will notice how calm and prepared the children are for big royal events. You will see that Princess Charlotte takes great pride in her family and her responsibilities. [It] reminds me of Princess Royal."

This preparedness was definitely visible when the children made their last public appearance at their grandfather's coronation on May 6.