Royal Ladies must have legs together when sitting.
In case the term the “Duchess slant” is a new thing for you, here is an interesting fact that all the female members of the royal family are only allowed to sit in a single position. That is holding them together while this sit, or at most can have their legs crossed at their angles with their legs together or at a slant.
This particular rule is actually Royal etiquette that symbolizes modesty and makes sense. It is realistic to sit this way when these ladies have to wear dresses as skirts and to avoid any wardrobe-related miss hap they better keep their legs closed. This rule has been mandated to mitigate any potential malfunctioning with the royal dresses.