As if there weren't enough things to remember about how to dress, how to drink tea, how to position cutlery and where to keep your tiara, there's also a correct way of holding your chin. At all times, or at least in so far as possible, Royals are told to keep their chin parallel to the ground.
This is all about public perception. Pointing your chin towards the air could be seen as arrogant and aloof. Tilting it towards your chest, and thereby bowing your head, might be interpreted as meek and lacking confidence. Therefore, the only remaining option is to hold it straight and exude a dignified middle ground at all times. We can't help but wonder if Catherine and Meghan were handed a book containing this information when they agreed to join the family, and if they were, how thick that book must have been.