De jure is a term meaning 'in law', as opposed to de facto, meaning 'in fact'. By way of example, the Duchy of Normandy might in law (de jure) be part of theKingdom of France, but after the Norman Conquest is in fact (de facto) a part of the Kingdom of England.
Each county is part of a de jure duchy, each duchy is part of a de jure kingdom, and each kingdom is part of a de jure empire. There are, however, several exceptions to this rule. These are known as titular titles and have no land associated with them.
Crown laws apply to a kingdom's de jure territory, even if part of it is de facto in another kingdom.
De Jure Claims[]
One of the main consequences of de jure realms is the notion of de jure claims. If a ruler has a de jure title but does not own all of the counties which are de jure a part of that realm then he or she can war for them using the De Jure Claim casus belli. Note that this casus belli only allows one county to be contested at a time.
For example, the King of Denmark owns all of his de jure kingdom except for the Duchy of Skåne and its associated counties (Skåne, Halland, Blekinge and Bornholm), which are all held by the King of Sweden. Thus, the King of Denmark can use the De Jure Claim casus belli to declare war against the King of Sweden to claim one of Skåne, Halland, Blekinge or Bornholm.
De Jure Drift[]
If a duchy is part of a kingdom for 100 years that is not its de jure kingdom, its de jure kingdom changes to that which it has been a part of for 100 years. If this process is interrupted part way through the count of years that have passed will progress backwards to zero (where it can then be begun again), or until the kingdom into which it was drifting regains it and the process continues.
This process also works for kingdoms drifting in and out of empires, but not for counties drifting in and out of duchies. It is also possible for duchies and kingdoms to drift into titular kingdoms and empires, switching them from titular to de jure. If a kingdom or empire loses all of its de jure territory it becomes titular.