It is contrasted with de facto, which means "of fact".
Each duchy, kingdom and empire has a lawfully right land area in the game. If another ruler has occupied land that rightfully belongs to you, then you will have a de jure Casus Belli on that land area. For counties, the count holding the county title is the de jure liege of all holdings within the province.
The de jure ruler of a Kingdom/Empire also decides the Crown Authority of the realm. De facto owners of parts of a de jure realm are granted voting rights for laws, if the rulers are of the same religion.
Titles where the Elective succession law applies, will have all its de jure vassal rulers of the same religion as nominators.
De jure drift
In general, a duchy will assimilate into a kingdom after being controlled by that kingdom for 100 years, and a kingdom will similarly assimilate into an empire in the same time frame. Every duchy in the game is considered to be part of a specific de jure kingdom. For example, in 1066, Flanders is part of the de jure kingdom of Frisia, but is entirely controlled by the King of France. If France controls Flanders for long enough, Flanders will eventually be considered part of the de jure territory of France. This will make the local rulers of Flanders regard the French king more favorably, and will also make it easier for France to reclaim the territory if it is conquered by another realm or declares independence.
A duchy will begin to drift into a kingdom when the following conditions are met:
- The entire duchy is within the realm of the king.
- The king does not hold the crown of the current de jure kingdom to which the duchy belongs.
- The ducal title either does not exist or is held by the king or by a vassal of the king.
- The duchy is not part of the kingdom of Jerusalem
- The kingdom is not a tribal title
- The kingdom is its owner's primary title
If this is the case the duchy will, after one year, appear shaded with the colors of the assimilating kingdom (blue over orange, in this case) on the de jure kingdoms screen. Hovering over the duchy will show a tooltip indicating the number of years until full assimilation.
Whenever the conditions for assimilation are not met - for example, if the local ruler of Flanders rebels against France, or if one of the counties in Flanders is conquered by the Holy Roman Empire - the counter will tick backwards instead of forwards. When the conditions are again met - for example, if the duke's rebellion is put down - the counter will resume ticking forwards.
Kingdoms assimilate into empires in an identical fashion - the emperor must hold all territory in the de jure kingdom.
Assimilation allows titular kingdoms and empires to become non-titular. It can also potentially completely eliminate a de jure kingdom (the de jure kingdom would, at that point, become a titular kingdom).
Assimilation can be both good and bad from a gameplay perspective:
- an assimilated Duke becomes more loyal, can vote in elections, and since patch 2.0 will provide more levies if assimilated in the capital realm,
- but the assimilated Duchy no longer counts towards the total territories needed to claim a crown. For example, suppose as the King of Scotland you control the Duchy of Ulster in northern Ireland. Before the duchy assimilates, you control 4/13 counties in Ireland - you'd only need 3 more to proclaim yourself the Irish King. But if Ulster assimilates into Scotland, you'll now have 0/9 counties in Ireland and need to conquer 5 more to become King of Ireland.
As of patch 1.08, kingdoms no longer assimilate out of de jure empires that have no holder.