Game concept

Co-Monarchy

The game also links it as Co-Monarchies, Co-Monarch, Co-Monarchs.

From the game's encyclopedia

Co-Monarchies are a type of Power Sharing that provides Feudal rulers with a way to select a Designated Heir even in earlier Eras.

The cost for this is entering into a permanent Power Sharing agreement with them, where they will have easier access to many of the same powers as an Entrenched Regent.

Co-Monarchs are only available to Kings and Emperors.

The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.

Co-Monarchy is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Co-Monarchies are a type of Power Sharing that provides Feudal rulers with a way to select a Designated Heir even in earlier Eras." It is part of Co-Rulership. The game also links it as Co-Monarchies, Co-Monarch and Co-Monarchs. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.

for a guide, post or blog

Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.