Royal protocol dictates that the Queen should always be addressed as Your Majesty or Ma'am (to rhyme with jam). Prince George, however, has his own special name for the Head of State: Gan-Gan.
The Duchess of Cambridge revealed how her 2-year-old son addresses his great-grandmother and also disclosed the Queen leaves "little gifts" for George and his sister Princess Charlotte in their bedrooms when they stay with her.
Catherine joined nine other members of the royal family to paint an intimate portrait of the Queen for the ITV documentary Our Queen at Ninety, which will air in Britain on Easter Sunday.
The film-makers spent almost two years filming the two-hour programme, which shows the Queen as a family woman and a stateswoman and also picks up on the light-hearted side of royal life.
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Prince William describes seeing people become so nervous at meeting the Queen that they faint in front of her, and his uncle Prince Andrew speaks about her "intelligence network" that reports every piece of gossip to her.
The Queen, who will be 90 on April 21, allowed cameras to film everything, from her weekly meeting with Prime Minister David Cameron to feeding carrots to her Canadian horse Elizabeth at the Royal Mews in Windsor.
Catherine has clearly formed a close bond with the Queen, and describes how the monarch has given her valuable guidance and support as she learns the ropes of public life.
William credited the Queen for allowing him to plough his own furrow in life, something which has led to recent accusations that he is work-shy.
The Queen is never happier than when she is at Balmoral, described by Princess Eugenie as "the most beautiful place on Earth".
The Duke of York's younger daughter said: "I think she really, really loves the Highlands ... walks, picnics, dogs, a lot of dogs, there are always dogs and people coming in and out all the time. Family-wise we're all there, so it's a lovely base for Granny and Grandpa to be."