Elizabeth became Queen in 1852 and was already a mum of two to Charles, born in 1948, the year after her wedding, and Anne two years later in 1950.
The late monarch and her eldest son shared a close bond as he was the first in the line of succession. However, the pair didn't always agree.
In Kate Nicholl's new book 'The New Royals', she delved into their relationship and revealed a time the late Queen overruled her son.
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The two would meet regularly as the Queen prepared Charles for his role, discussing matters of state and the future.
Former press secretary Charles Anson told the author how she prepared him: "Each monarch coaches and brings on the next generation.
"It's that continuity that appeals to so many people because it's not just an elected person who suddenly arrives in public life. It's this sense of generations, one after another, being trained and committing themselves to public service, to the nation, and to the Commonwealth in a constitutional monarchy above the fray of daily politics."
But that doesn't mean they always agreed, and it was the Queen who called the shots at the end of the day.
Writing about one such disagreement, Nicholl's said: "When Charles suggested that he and William should be the only two senior royals to lay a wreath at the Cenotaph in 2021, the Queen put her foot down.
"She insisted that other family members including Princess Anne, Prince Edward, and the Duke of Kent should be given the privilege."
Following his mother's death, King Charles promised to follow the late Queen's "selfless duty" in his address to both Houses of Parliament.
The King praised his mother's 'selfless' duty as he spoke in front of 900 MPs and peers, as he said: "While very young, Her Late Majesty pledged herself to serve her country and her people and to maintain the precious principles of constitutional government which lie at the heart of our nation. This vow she kept with unsurpassed devotion.
"She set an example of selfless duty which, with God's help and your counsels, I am resolved faithfully to follow.
"I cannot help but feel the weight of history which surrounds us and which reminds us of the vital parliamentary traditions to which members of both houses dedicate yourselves, with such personal commitment for the betterment of us all," the King continued.
"Parliament is the living and breathing instrument of our democracy. That your traditions are ancient we see in the construction of this great hall and the reminders of medieval predecessors of the office to which I have been called."
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