What is the default Matrilineal Marriages game rule in CK3?

Default: AI characters will arrange Matrilineal marriages if they belong to the player's close family, or a reputable Dynasty, or an Equal realm, or are strong Rulers. Female Dominated realms default to Matrilineal. (patch 1.20.1.2)

Does changing the Matrilineal Marriages rule disable achievements?

No: none of its 5 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Historicity

Matrilineal Marriages

Default
Default (no Player exception)
Always
Female Dominated & Equal
Never

Matrilineal Marriages is a game rule in Crusader Kings III (Historicity) with 5 settings: Default (the default), Default (no Player exception), Always, Female Dominated & Equal and Never. None of its settings disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Matrilineal Marriages game rule in CK3?: Default: AI characters will arrange Matrilineal marriages if they belong to the player's close family, or a reputable Dynasty, or an Equal realm, or are strong Rulers. Female Dominated realms default to Matrilineal. (patch 1.20.1.2)
Does changing the Matrilineal Marriages rule disable achievements?: No: none of its 5 settings carries the flag blocks_achievements in patch 1.20.1.2.

5 settings

Default
Default
AI characters will arrange Matrilineal marriages if they belong to the player's close family, or a reputable Dynasty, or an Equal realm, or are strong Rulers. Female Dominated realms default to Matrilineal.
default_matrilineal_marriages
Default (no Player exception)
AI characters will arrange Matrilineal marriages if they belong to a reputable Dynasty, or an Equal realm, or are strong Rulers. Female Dominated realms default to Matrilineal.
The script checks it once, in 1 place
Scripted rules 1
- ai_wants_matrilineal_marriage
Every has_game_rule = matrilineal_marriages_no_player_exception in game/common and game/events, by the top-level object it sits in.
matrilineal_marriages_no_player_exception
Always
AI characters will want to arrange Matrilineal marriages for all female Dynasty members. Like with Default rules, Patrilineal marriages still happen if deemed favorable enough.
The script checks it once, in 1 place
Scripted rules 1
- ai_wants_matrilineal_marriage
Every has_game_rule = matrilineal_marriages_always in game/common and game/events, by the top-level object it sits in.
matrilineal_marriages_always
Female Dominated & Equal
AI characters will only arrange Matrilineal marriages if part of a Female Dominated or Equal realm. Like with Default rules, Patrilineal marriages still happen if deemed favorable enough.
The script checks it once, in 1 place
Scripted rules 1
- ai_wants_matrilineal_marriage
Every has_game_rule = matrilineal_marriages_female_and_equal in game/common and game/events, by the top-level object it sits in.
matrilineal_marriages_female_and_equal
Never
AI characters will never arrange Matrilineal marriages, unless part of a Female Dominated realm.
The script checks it once, in 1 place
Scripted rules 1
- ai_wants_matrilineal_marriage
Every has_game_rule = matrilineal_marriages_never in game/common and game/events, by the top-level object it sits in.
matrilineal_marriages_never

Source: game/common/game_rules/00_game_rules.txt line 377 (key matrilineal_marriages), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.