i was playing ck2 in the scenario of charlemagne and after read about him in Wikipedia i saw that his empire be was divided by his sons after he dies because in the empire of france doesnt have primogeniture
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So have Charles the Great circumvent the legal traditions of the Franks going back generations? Not an especially good idea, that. If Charles himself doesn't develop an acute case of stabbed to death, his heir certain will and then well; civil war that the Franks tried to avoid with the whole even splitting thing.
Perhaps the easiest way would be to have a few generations of kings who happen to only have one son. If the empire's been passed on undivided for the last three or four generations, continuing to do so would seem more like a continuation of the status quo than a circumvention of legal tradition.