Different time periods. In what way?
As an example: Charlemagne and the Golden Bull of 1356 do not belong in the same game. Or perhaps the Viking Age, the Crusades, and the Hundred Years War do not belong in the same game. The Abbasid Caliphate and the Mongol Empire. There are plenty of different things we can point to as things that just Do Not Belong in the same game in the same way you declare the rise and fall of rome, and the conidtions before/after those states, belong in different titl;es, and yet these are loosely accepted in CK2
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