Official rule text
39.1 The game board consists of all system tiles that were placed RELATED TOPICS: Anomalies, Movement during setup, even if the sides of those tiles do not touch any other system tiles, such as the Ghosts of Creuss’ home system.
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 39.1