However, a royal expert has now revealed the one rule that both of the ladies follow.
With their busy social calendars and constant photo opportunities, the pair are surely always keen to look their regal best.
We see them wearing the latest designer fashions and the most dazzling jewellery.
However, pay close to attention to their daytime attires, as they'll be dazzlingly different to what the duchesses wear after dark.
While Marilyn Monroe declared diamonds were a girl's best friend, etiquette expert Myka Meier disagrees.
She says the royal ladies only wear diamonds after 6pm so they don't appear "too flashy".
At night, you’ll see the diamonds come out, and that’s in order to not come across as flashy.
Speaking to News.com.au, she said: "They have a team who know what is appropriate for the event they’re attending, and Meghan especially would be given a lot of advice on what to choose."
Revealing that diamonds are a no-no, Myka said other jewels are worn "pre-6pm".
She said: "Before 6pm, you’ll see metallics, gemstones, pearls, sapphires. At night, you’ll see the diamonds come out, and that’s in order to not come across as flashy in your appearance."
Etiquette expert Grant Harrold of The Royal Butler backed the comments and said: "The more significant event, the more likely you are to see them wearing diamonds."
Conservative hemlines are also expected of the royal ladies, says Myka, while less is more when it comes to flashing the flesh.
She revealed: "At formal engagements, we typically always see the shoulders covered. Cleavage is something that is not practised, especially during the day."
Hopefully Meghan has sought advice on what to wear on her first trip to Balmoral.
This week, it's expected that the duchess will head to the Scottish Highlands to visit the Queen at her sprawling estate with husband Prince Harry and baby Archie.
Let us know what you think of our story on our Facebook page @EntertainmentDailyFix.