But it might be difficult to find a county with Legalism 3 nearby, because due to the way technology spreads in the game, nearby counties will have similar levels of technology. When you switch to the Technology window and move your cursor around the map you can see the tech levels in different counties.
This removes the crown authority requirement and by extension the Legalism requirement. If you want to get Primogeniture but you lack the necessary Legalism level to raise your Crown Authority until it fulfills the requirement, then there are 3 options I am aware of:
