diff --git a/src/engine/game.test.ts b/src/engine/game.test.ts
new file mode 100644
index 0000000..d4fe312
--- /dev/null
+++ b/src/engine/game.test.ts
@@ -0,0 +1,73 @@
+import { describe, it, expect } from 'vitest';
+import { DuplicateGame } from './game';
+
+const START = 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR';
+
+function placement(fen: string) {
+  return fen.split(' ')[0];
+}
+
+describe('DuplicateGame', () => {
+  it('starts with four boards in the standard position, North to move', () => {
+    const g = new DuplicateGame();
+    expect(g.ply).toBe(0);
+    expect(g.currentPlayer).toBe('N');
+    for (const id of ['NW', 'NE', 'SW', 'SE'] as const) {
+      expect(placement(g.boards[id].fen())).toBe(START);
+    }
+  });
+
+  it("applies a synchronized move to the current player's two boards only", () => {
+    const g = new DuplicateGame();
+    g.applyMove({ from: 'e2', to: 'e4' }); // North
+    expect(g.ply).toBe(1);
+    expect(g.currentPlayer).toBe('S');
+    expect(placement(g.boards.NW.fen())).toContain('4P3'); // pawn advanced
+    expect(placement(g.boards.NE.fen())).toContain('4P3');
+    expect(placement(g.boards.SW.fen())).toBe(START); // untouched
+    expect(placement(g.boards.SE.fen())).toBe(START);
+  });
+
+  it('cycles the current player N -> S -> E -> W -> N', () => {
+    const g = new DuplicateGame();
+    g.applyMove({ from: 'e2', to: 'e4' }); // N
+    g.applyMove({ from: 'e2', to: 'e4' }); // S
+    g.applyMove({ from: 'e7', to: 'e5' }); // E
+    g.applyMove({ from: 'e7', to: 'e5' }); // W
+    expect(g.currentPlayer).toBe('N');
+    expect(g.ply).toBe(4);
+  });
+
+  it("throws on a move not legal on both of the player's boards", () => {
+    const g = new DuplicateGame();
+    expect(() => g.applyMove({ from: 'e2', to: 'e5' })).toThrow();
+  });
+
+  it('undo removes the last move and restores the position', () => {
+    const g = new DuplicateGame();
+    g.applyMove({ from: 'e2', to: 'e4' });
+    g.undo();
+    expect(g.ply).toBe(0);
+    expect(g.currentPlayer).toBe('N');
+    expect(placement(g.boards.NW.fen())).toBe(START);
+  });
+
+  it('rebuilds from a history array passed to the constructor', () => {
+    const g = new DuplicateGame([
+      { player: 'N', from: 'e2', to: 'e4' },
+      { player: 'S', from: 'e2', to: 'e4' },
+    ]);
+    expect(g.ply).toBe(2);
+    expect(g.currentPlayer).toBe('E');
+  });
+
+  it('resets the progress clock on a pawn move or capture', () => {
+    const g = new DuplicateGame();
+    g.applyMove({ from: 'e2', to: 'e4' }); // pawn move -> clock stays 0
+    expect(g.pliesSinceProgress).toBe(0);
+    g.applyMove({ from: 'e2', to: 'e4' }); // pawn move
+    expect(g.pliesSinceProgress).toBe(0);
+    g.applyMove({ from: 'g8', to: 'f6' }); // knight move -> clock increments
+    expect(g.pliesSinceProgress).toBe(1);
+  });
+});
diff --git a/src/engine/game.ts b/src/engine/game.ts
new file mode 100644
index 0000000..611e5b9
--- /dev/null
+++ b/src/engine/game.ts
@@ -0,0 +1,93 @@
+import { Chess } from 'chess.js';
+import type { BoardId, Player, SyncMove, HistoryEntry } from './types';
+import { BOARD_IDS, PLAYERS, PLAYER_BOARDS } from './boards';
+
+/** A chess.js move result has these fields we rely on. */
+interface ChessMoveResult {
+  piece: string;
+  captured?: string;
+}
+
+export class DuplicateGame {
+  readonly boards: Record<BoardId, Chess>;
+  readonly history: HistoryEntry[] = [];
+
+  /** Plies since the last capture or pawn move on any board (for the 50-move rule). */
+  pliesSinceProgress = 0;
+  /** Repetition keys of the whole 4-board system, one per position incl. the start. */
+  readonly repetitionKeys: string[] = [];
+
+  constructor(history: HistoryEntry[] = []) {
+    this.boards = {
+      NW: new Chess(), NE: new Chess(), SW: new Chess(), SE: new Chess(),
+    };
+    this.repetitionKeys.push(this.systemKey());
+    for (const entry of history) this.applyMove(entry, entry.player);
+  }
+
+  get ply(): number {
+    return this.history.length;
+  }
+
+  get currentPlayer(): Player {
+    return PLAYERS[this.history.length % 4];
+  }
+
+  /** Whether `move` is a legal chess move on `board` in the current position. */
+  isLegalOnBoard(board: BoardId, move: SyncMove): boolean {
+    return this.boards[board].moves({ verbose: true }).some(
+      (m) =>
+        m.from === move.from &&
+        m.to === move.to &&
+        (m.promotion ?? undefined) === (move.promotion ?? undefined),
+    );
+  }
+
+  /** Apply one synchronized move to a player's two boards. Throws if illegal on either. */
+  applyMove(move: SyncMove, player: Player = this.currentPlayer): void {
+    const [a, b] = PLAYER_BOARDS[player];
+    if (!this.isLegalOnBoard(a, move) || !this.isLegalOnBoard(b, move)) {
+      throw new Error(
+        `Illegal synchronized move ${move.from}${move.to} for ${player}`,
+      );
+    }
+    const ra = this.boards[a].move(move) as unknown as ChessMoveResult;
+    const rb = this.boards[b].move(move) as unknown as ChessMoveResult;
+    this.history.push({ ...move, player });
+    const progress = isProgress(ra) || isProgress(rb);
+    this.pliesSinceProgress = progress ? 0 : this.pliesSinceProgress + 1;
+    this.repetitionKeys.push(this.systemKey());
+  }
+
+  /** Remove the last move by replaying the truncated history. */
+  undo(): void {
+    if (this.history.length === 0) return;
+    const replay = this.history.slice(0, -1);
+    for (const id of BOARD_IDS) this.boards[id].reset();
+    this.history.length = 0;
+    this.pliesSinceProgress = 0;
+    this.repetitionKeys.length = 0;
+    this.repetitionKeys.push(this.systemKey());
+    for (const e of replay) this.applyMove(e, e.player);
+  }
+
+  /** A repetition key for the whole system: each board's placement+castling+ep, plus the side to move. */
+  systemKey(): string {
+    return (
+      BOARD_IDS.map((id) => {
+        const parts = this.boards[id].fen().split(' ');
+        return `${parts[0]}${parts[2]}${parts[3]}`;
+      }).join('|') + ':' + this.currentPlayer
+    );
+  }
+
+  /** How many times the current system position has occurred. */
+  repetitionCount(): number {
+    const current = this.repetitionKeys[this.repetitionKeys.length - 1];
+    return this.repetitionKeys.filter((k) => k === current).length;
+  }
+}
+
+function isProgress(result: ChessMoveResult): boolean {
+  return result.piece === 'p' || result.captured != null;
+}
diff --git a/src/engine/test-helpers.ts b/src/engine/test-helpers.ts
new file mode 100644
index 0000000..7434792
--- /dev/null
+++ b/src/engine/test-helpers.ts
@@ -0,0 +1,23 @@
+import type { DuplicateGame } from './game';
+import type { SyncMove } from './types';
+
+/**
+ * Apply a list of [whiteMove, blackMove?] underlying ply-pairs symmetrically:
+ * North then South play the white move, East then West play the black move.
+ * While every move is symmetric all four boards stay identical, so each board
+ * behaves as an ordinary chess game — useful for reaching ordinary checkmate /
+ * stalemate / repetition positions. Omit blackMove for a final unanswered white move.
+ */
+export function playSymmetric(
+  game: DuplicateGame,
+  pairs: Array<[SyncMove, SyncMove?]>,
+): void {
+  for (const [white, black] of pairs) {
+    game.applyMove(white); // N
+    game.applyMove(white); // S
+    if (black) {
+      game.applyMove(black); // E
+      game.applyMove(black); // W
+    }
+  }
+}