Okay so I'm not sure how to word this eloquently, I'm sorry, but please bear with me. Let's say that a lord, eg. a Duke had two sons. The older one got married, had a male heir, and then died while his father, the duke, was still alive and ruling the lands that were meant to be passed down to him.
Would they then still follow the rule of primogeniture and, after the duke's death, pass the lands down to the older son's son, as his father was meant to get them and pass them down to him, or would those lands have be given to the younger brother, as the Duke's oldest living heir? I'm interested in France, sometime in the High Middle Ages, but all answers are welcome.