D20 Modern Rules
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Add your own D20M variant rule to D&D Wiki by clicking the link and following the instructions.
Supplemental Variant Rules
A supplemental variant rule can be implemented into a campaign with little or no change in the core dynamics. These rules cover an area that the core rules do not cover or do not cover well.
Transformational Variant Rules
A transformational variant rule, when implemented into a campaign, will change the core dynamics. These rules change the core rules to make them more playable or more realistic while staying within the d20 framework.
Radical Variant Rules
This new, self-contained rule system goes beyond the d20 framework, but bears much in common with d20 and D&D. The system is still in the early stages of development.
These are radical rule variants: