Seniority is a succession law where the eldest member of the dynasty inherits all titles.
The patricians of merchant republics always use this law, although they can override it with the honorary title of Designated Heir.
Feudal rulers (other than Muslims and Indians) can also choose succession by seniority. Without the 
Conclave DLC, for realms subject to crown authority, it requires Medium crown authority or higher.
Benefits
- Tends to unite the titles in the dynasty, in contrast to gavelkind.
- Avoids regencies due to child rulers.
- Can lead to greater score if successive characters have high prestige due to dynasty prestige or titles.
- Truces and Law change cooldowns reset more often - allowing you to start more wars and pass more laws.
- Many long-term ruler-based penalties become much less of an issue. Most notably tyranny, lack of a coronation, favours and kin-slaying.
- With the Conclave DLC activated, can be passed without any tech requirements.
Drawbacks
- Older rulers may suffer from the "Short Reign" opinion penalty for their entire reign.
- May be difficult to adopt another succession law, due to the "need to rule for 10 years" requirement.
- All the direct disadvantages of short reign, such as:
- Less time to make use of claims and favors.
- Inability to make adequate progress in desired societies.
- Inability to accumulate many WoL bonuses, or to make adequate use of the costly ones (tower, maze).
- Greater loss rate of artifacts stemming from frequent succession.
- Strategic complications caused by more frequent jumps in ruler's stats. (For example you can't build lasting plans that depend on high intrigue because your high intrigue is not likely to last.)
Opinion
A ruler using seniority succession will face the following opinion modifiers:
| Character | Opinion modifier | Reason |
|---|---|---|
| Oldest child (of preferred gender) | -15 | They would certainly prefer primogeniture. |
| Other children (of preferred gender) | -10 | They are unlikely to inherit. |
| All other dynasty members | +5 | They may get a chance to inherit at some point, as heir is moving between distant dynasty branches. |
| Non-dynastic vassals | -5 | Titles are likely to remain within ruler's dynasty for a long time. |