With just under five years between them, it's not always easy to see the close resemblance between Princes George and Louis .
Add in to the mix the fact that the youngest Cambridge has been largely kept out of the public eye, and we don't get many opportunities to compare them.
Because we love to try and detect who babies take after, popular theory suggests that Little Louis takes after the Duchess of Cambridge's dad, Mike.
But when you see the two brothers up close and side-by-side, the similarities are uncanny.
Now, a royal family super fan has given us the opportunity to see how much they resemble each other.
The picture shows Prince Louis at a few months old, and Prince George at a year old and they have near-identical expressions and features.
It's not just their physical resemblance which bonds the Cambridge kids together , they're also said to be very close.
Ingrid Seward, editor of Majesty magazine, told People magazine : "The Cambridge kids are really lively. Clearly the kids are having fun.
A source told People magazine that George and Charlotte have grown especially close, because play dates with outsiders can sometimes be 'tricky.'
It's believed that playing with ther kids comes with security issues, and a natural suspicion of letting new people in.
