A
mapping technique which relies on changing a property of a map component in order to link it with an otherwise distinct map component. For example, the rule 'South becomes North' would lead to a direct equivalence between South Ealing and North Ealing (amongst others). Carefully crafted equivalences can be very precise in their effects, although overuse of this mapping can lead to playing something not unlike
Boardman's Combined Stations.
The effect of unsatisfied equivalences varies by ruleset. Chalk Farm '84 established the baseline behaviour of renaming the transformed but unlinked station, so in the previous example any play to South Wimbledon would instead be made to North Wimbledon. Holland Park 2000 added an option of leaving unsatisfied equivalences unchanged.
Note that transformation mappings can occasionally be disrupted by such otherwise harmless rules as the North Rule.
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