So I start as the king of France and give my younger sons in marriage to various duchesses, thinking, what the heck, I can use them in case the main line fails.
As it so happens... every one of those marriages became a matrilinear marriage. My oldest son was an imbecile and fathered no heir, and then the kingdom went to the second son who had 3 sons of different dynasty.
I had to introduce seniority, but that meant increasing authority = rebellions = game over in the next generation since the only Capet left was a legitimized bastard hated by all.
As it so happens... every one of those marriages became a matrilinear marriage. My oldest son was an imbecile and fathered no heir, and then the kingdom went to the second son who had 3 sons of different dynasty.
I had to introduce seniority, but that meant increasing authority = rebellions = game over in the next generation since the only Capet left was a legitimized bastard hated by all.