The Mirror Rule, Explained

Square boards have a hidden second-player advantage: whatever the first player plays, the second player can answer with the reflected or rotated twin of that move. If P2 copies faithfully for the whole game, the final board is perfectly symmetric, both players own identical groups, and the scores tie. Mirror play forces a draw.

Here cyan has mirrored green across the vertical axis three times in a row. If both players keep going, there is no move green can make that cyan can't echo — every green group is matched by an identical cyan group on the other side of the board. Draws are boring, so Jelmata bans the tactic outright.

Detection isn't limited to one axis. The engine checks every move pair against all seven symmetries of a square board. Any single one of them is enough — P2 doesn't get to dodge by picking the 270° rotation instead of a vertical flip.

• Vertical reflectionLeft ↔ right. Each cell's column is flipped; the row stays put.
• Horizontal reflectionTop ↔ bottom. The row is flipped; the column stays put.
• Main diagonal reflectionSwap column and row. A move at (0, 3) mirrors to (3, 0).
• Anti-diagonal reflectionFlip across the other diagonal — the top-right and bottom-left corners are the fixed points.
• 90°, 180°, 270° rotationsThree rotations around the board's center. A corner move rotates to each of the other three corners as you step through them.

This board shows a 180° rotation mirror: every cyan cell is the green cell rotated a half turn around the center square. The vertical-mirror example on card 1 and this rotational example are both flagged by the same rule.

Opening moves on a small board sometimes look mirrored by accident, so the engine holds fire until three complete move pairs all fit the same symmetry. One or two coincidental reflections cost you nothing.

Cyan has answered each green move with its main-diagonal twin — a cell at (0, 1) is mirrored to (1, 0), (1, 3) to (3, 1), and (2, 4) to (4, 2). After the third pair, the rule triggers and cyan forfeits.

• Pair 1Green (0, 1) → cyan (1, 0). Fits main-diagonal.
• Pair 2Green (1, 3) → cyan (3, 1). Still fits main-diagonal.
• Pair 3Green (2, 4) → cyan (4, 2). Third consecutive match — cyan forfeits, green wins.

The check re-runs over the entire move history every turn, so there's no escape hatch. Once the pattern breaks, mirror play can't be resumed later: the earlier matched pairs are still on the board, and any future deviation means no single symmetry fits the whole game.

Why the Rule Exists

Without this restriction, the optimal strategy on any square Jelmata board would be “P2 copies P1.” Every game between strong players would drift toward a tie, and Jelmata's real scoring mechanic — the product of your group sizes, which rewards splitting the board into multiple corners — would never get a chance to matter.

The rule is one-sided on purpose. P1 can happily play into symmetric shapes; only P2 is punished for echoing them. That asymmetry is the price P2 pays for moving second on a game where symmetric positions would otherwise be too strong a refuge.