Official rule text
39.2 A system tile is on the edge of the game board if any of its sides are not touching another system tile. a The Ghosts of Creuss home system and the wormhole nexus are on the edge of the game board. RELATED TOPICS: System Tiles
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Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 39.2