So we know they call each other 'William' and 'Catherine' in public, but obviously they'd call each other something else in private. They are Royal after all, and the Royals love a nickname...
If we were to hazard a guess as to what that might be, 'darling' would be up there. As would 'poppet' – because, well, that's what Prince William called his wife on the steps of St. Mary's Hospital, as they presented the newborn Prince George to the press.
But a new nickname has come to light, one Kate accidentally let slip whilst looking at an award-winning garden at the RHS Chelsea Flower Show on Monday.
Speaking to Channel 9's Today Show, the garden's designer – Australian TV presenter and landscaper Charie Albone – revealed: 'I got to meet Prince Will and Prince Harry. Prince William was quite funny actually. He came up and said: "What [flower] is this one?", and I replied: "That's a Buxus". Princess Catherine just turned to him and said...'
Wait for it dear readers...
Wait for it...
'"Babe, we've got those. We've got loads of those."'
BABE! BAAAAAAAABE!
When asked to confirm if she said that exact word, Charlie replied: 'Yeah, she actually called him "babe", and said "we've got loads of those".'
Of course, that's not the first nickname Prince William has had in his life. Prince Harry revealed once that Princess Diana used to call William, 'Wombat', which came about following their visit to Australia when he was a boy. Close friends are said to call him 'Wills'.
Kate on the other hand was once known as 'Squeak'. The Middletons had two guinea pigs, one called Pip and the other Squeak, and because her sister's 'Pippa', the future Duchess had to take on 'Squeak'.
Other royal nicknames include Tillabet – which is what The Queen called herself as a young girl – and Lillibet, which is what her family and friends affectionately call her... all except her husband Prince Philip. He prefers the moniker 'Cabbage'. Charming!
Prince Charles and Camilla are said to call each other 'Fred' and 'Gladys', whilst Prince Harry used to be known as 'Spike'.
When Prince George was first born, reportedly he was called 'PC' – which stood for 'Prince Cambridge' – whereas Kate at that time was known as 'MC' ('Mother Cambridge'). As we reported in March, Prince George is pretty good at coming up with nicknames of his own. For instance, he likes to call The Queen 'Gan Gan'.
But back to babe... Might one day it progress to 'bae'? We truly hope so.
READ MORE: When Prince George embarrassed Prince Harry in front of the Obamas...!
READ MORE: All the best moments from The Queen's 90th Windsor birthday celebrations