The tunnels linking Route 5 to Route 6 and Route 7 to Route 8. In the Route 5-6 house an NPC trades you a Machoke for a Cubone - and because it counts as a trade, that Machoke evolves into Machamp the moment you receive it.
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Where something is met, and how. Characters are here as well as creatures: the guide records where Professor Oak stands and where each Gym Leader is fought.