If you look closely at pictures of Kate Middleton, especially those taken in 2014, you’ll notice she wears the exact same thing in most of them: the Cartier Ballon Bleu watch.
Princess Diana use to own a Cartier Tank Francaise watch, which was a present from her father, John Spencer, and she was frequently spotted wearing it in the months leading up to her death (perhaps to replace the one she was wearing in that famous two watches picture).
It was one of Prince William’s favourite pieces from his mother’s collection, and he kept it as a memento. He has another special connection to the brand, as Princess Diana apparently bought him a Cartier watch of his own, which she had engraved for his birthday.
With Cartier so sentimental to him, it was an extra thoughtful gift to his wife. He also went the extra mile by adding a sapphire to the frame of the watch, so it would match Catherine’s engagement ring, which of course used to belong to Princess Diana.