They are often spotted warmly chatting or exchanging jokes when they get together for big royal family events thanks to their seemingly close bond.
But it appears their similar middle-class backgrounds and introduction into public life is not all the Princess of Wales and Duchess of Edinburgh have in common - it appears they almost could have had a title in common. It all stems back to 1999, ahead of Sophie marrying the late Queen's youngest son Prince Edward.
Eyebrows were raised with the title she was awarded before the wedding, and it's said this is because a plan to give her a title, which is now held by Kate, was axed in favour of a more unusual choice. When Edward married Sophie in 1999, he was not awarded a dukedom like his older brother Prince Andrew and instead became Earl of Wessex. This meant Sophie became the Countess of Wessex.
However, it is thought that she did almost end up with another title - and it was the same one the now Princess of Wales was awarded on her wedding day. According to one palace courtier, Edward was originally going to be offered the title Duke of Cambridge, which was eventually given to Prince William. If this original plan had gone ahead, Sophie would have been the Duchess of Cambridge - the title that eventually went to Kate.
However, the courtier explained to The Telegraph in 2010: "Prince Edward was going to be the Duke of Cambridge, but he watched the film Shakespeare in Love, which had a character called the Earl of Wessex. He liked the sound of it and asked the Queen if he could have that instead."
Meghan Markle 'to unleash her own memoirs' as Prince Harry's drops next weekHowever, last year, Edward and Sophie did see their titles change once more when they became the Duke and Duchess of Edinburgh. Edward was created Duke of Edinburgh on his 59th birthday back in March in honour of his late father Prince Philip - meaning Sophie became Duchess of Edinburgh.
Edward is the fifth Duke of Edinburgh. The title was first created for Prince Frederick, eldest son of King George II back in 1726. In 1764, it was given to Prince William, brother of King George III, as part of the joint title The Duke of Gloucester and Edinburgh. Up next was Prince Alfred, second son of Queen Victoria, who took it in 1866. And of course most recently it was handed to Prince Philip when he married the late Queen Elizabeth II.
Edward will be the Duke of Edinburgh for the rest of his life, however it will no longer be a hereditary role. This means it won't be passed down to James when he passes away. This could mean the title could be passed to one of the Prince and Princess of Wales’s children in the future, with Prince Louis the most likely candidate.