Check the pinned threads in https://www.chess.com/club/custom-variants
The most relevant ones I've linked below:
- https://www.chess.com/clubs/forum/view/requirements-for-submission
- https://www.chess.com/clubs/forum/view/how-to-submit-new-custom-positions
- https://www.chess.com/clubs/forum/view/how-to-use-variants-world
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