While 'Primogeniture' is often the goal, 'Partition' laws can be exploited. If you have multiple children, grant them duchy titles before your death. This will ensure they inherit those titles and not split your primary kingdom/empire. You can also disinherit undesirable heirs using 'Renown' or force them into monastic vows. With 'House Seniority', you can ensure an older, more developed character inherits, even if they aren't your direct child. Master succession to prevent realm fragmentation.
