What is the default Custom Duchies game rule in CK3?

On: The creation of custom non-historical Duchies, and the ability to integrate Counties into them is allowed. (patch 1.20.1.2)

Does changing the Custom Duchies rule disable achievements?

No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Historicity · Titles & Realm

Custom Duchies

On · default
Player Only
Off

Custom Duchies is a game rule in Crusader Kings III (Historicity and Titles & Realm) with 3 settings: On (the default), Player Only and Off. 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 Custom Duchies game rule in CK3?: On: The creation of custom non-historical Duchies, and the ability to integrate Counties into them is allowed. (patch 1.20.1.2)
Does changing the Custom Duchies rule disable achievements?: No: none of its 3 settings carries the flag blocks_achievements in patch 1.20.1.2.

3 settings

On
Default
The creation of custom non-historical Duchies, and the ability to integrate Counties into them is allowed.
The script checks it 2 times, in 2 places
Decisions 2
- Expand Duchy
- Found a New Duchy
Every has_game_rule = on_custom_duchies in game/common and game/events, by the top-level object it sits in.
on_custom_duchies
Player Only
The creation of custom non-historical Duchies, and the ability to integrate Counties into them is allowed for players only.
The script checks it 2 times, in 2 places
Decisions 2
- Expand Duchy
- Found a New Duchy
Every has_game_rule = on_custom_duchies_player in game/common and game/events, by the top-level object it sits in.
on_custom_duchies_player
Off
The creation of custom non-historical Duchies, and the ability to integrate Counties into them is disallowed.
off_custom_duchies

Source: game/common/game_rules/00_game_rules.txt line 981 (key custom_duchies), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.