In the letter, George V declared that the great-grandchildren of the Monarch would no longer be princes or princesses, except for the eldest son of the eldest son of the Prince of Wales.
That is why Prince William s children have the titles while Archie does not - although he will be entitled to one when Prince Charles becomes King.
It is for the same reason why Prince Andrew s daughters, Princess Beatrice and Eugenie, were given the titles from birth.
However, other royals have opted not to give their children the royal titles.
Princess Anne: The royal revealed the reason why she didn t give Zara and Peter titles (Image: GETTY)
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