A ruler's primary title is the top tier title which they most identify with.
A ruler with multiple top tier titles can switch the primary title at any time. This is true even for counts, who are not required to use their capital as their primary title.
The choice of primary title is important in a number of ways:
- The heir of the primary title is the character you will play as once your current character dies.
- Under gavelkind, the primary title goes to the eldest. Under elective gavelkind, the primary title goes to the elected character.
- The capital can be moved to the de jure capital of the primary title, even if you have already changed your capital. This is the only way for a merchant republic to change its capital.
- During a regency, a regent whose primary title is within the relevant kingdom or empire may decide to lower crown authority of the kingdom/empire. This can only happen once to each liege.
- De jure vassals under your primary title (one tier below) will not have the "desire <title>" opinion penalty. As such, when holding two or more titles of the same tier, making the title with the most de jure vassals the primary title can cut down significantly on discontent among your vassals. This is more significant if you have a titular title; making another title of the same tier (with de jure vassals) the primary title is advised.
- Newly created kingdoms and empires have their crown laws and succession laws copied from the primary title.
- Crown laws of the primary title are applied in some outlying areas.
- The "Fabricate Claim on Liege" plot requires the target title to be a direct de jure liege or vassal of your primary title.
- It is not possible to usurp the primary title from a merchant republic.
- It is not possible to usurp the sole top title from a ruler of a different religion, if the ruler personally holds any of its (direct or indirect) de jure vassal titles.