They may have shared the common bond of marrying into the most famous family in the world.
But their seemingly vastly different personalities and approaches to life meant the Princess of Wales and the Duchess of Sussex never really hit it off.
In his memoir Spare, Prince Harry recounts several times that his wife Meghan seemed at loggerheads with his sister-in-law Kate in the early days of their romance.
He says the two women had a row over bridesmaid dresses in the lead-up to his wedding that left Meghan in tears and that Kate "grimaced" when the duchess borrowed her lipgloss.
Meanwhile, he writes how Kate asked for an apology from Meghan when she accused her of having "baby brain" following the birth of Prince Louis.
However, early in the memoir, Harry pinpoints the exact moment he realised Meghan and Kate might not have been on the same page - and it happened during the women's very first encounter with each other.
The first time they met was in 2016 when Prince William and Kate were invited over to have dinner at Nottingham Cottage by Harry and Meghan.
In Spare, Harry notes how Meghan cooked and that all seemed to be going well as they chatted about Wimbledon and the legal drama Suits.
However, there was one big difference between the women - and Harry said it was all down to what they were wearing.
He writes: "The only possibly discordant note I could think of was the marked difference in how the two women dressed, which both of them seemed to notice.
"Meg: ripped jeans, barefoot. Kate: done up to the nines. No big deal, I thought."
Harry's observation in Spare comes after Meghan also took aim at Kate during her and Harry's Netflix docuseries, which was released last week.
As she recalled their first meeting she said she was "surprised" by the behind-the-scenes formality of members of the royal family.
Speaking in episode two of the six-part Harry & Meghan Netflix, she said: "When Will and Kate came over, and I met her for the first time, they came over for dinner, I remember I was in ripped jeans and I was barefoot.
"I was a hugger. I've always been a hugger, I didn't realise that that is really jarring for a lot of Brits.
"I guess I started to understand very quickly that the formality on the outside carried through on the inside.
"There is a forward-facing way of being, and then you close the door and go 'You can relax now', but that formality carries over on both sides. And that was surprising to me."
Spare by Prince Harry and published by Penguin Random House is out now.
