When two pieces move to the same empty square simultaneously, a “collision” occurs. The outcome depends on the hierarchy of the pieces:

• Unequal collision: The piece with the higher hierarchy wins. The order from highest to lowest is: Queen, Rook, Knight, Bishop, Pawn.

• Equal collision: The piece of the player who registered their move first wins; the other piece is removed.RULE 1. Simultaneity

In XYMYX, capturing one’s own piece (except the King) is allowed, encouraging proactivity and anticipation—essential elements in a game with simultaneous movements. There are two main scenarios:

• Simple Absorption: If a player moves one of their own pieces to a square already occupied by another of their pieces, the moving piece will remain, and the initially placed piece will be removed.

• Double Absorption (Three pieces on the same square): If, during a simultaneous move, a player moves one of their pieces to a square where they already have another piece, and the opponent also moves a piece to the same square intending to capture, the outcome will be:

• Only the piece belonging to the player with two pieces in the square will remain.

• Among those two, the piece that moved during that turn will stay on the board.

• The opponent’s piece and the original piece of the player will be removed.

This mechanism ensures that absorption can be strategically used to anticipate opponent moves and protect key piece

If a Queen or Rook delivers a check, it becomes “frozen” and cannot move in the next turn. This adds balance, limits the dominance of these pieces, and promotes the strategic use of other pieces on the board.

Check can only be countered by moving the King to a safe square. Traditional defenses, such as blocking or capturing the attacking piece, do not apply due to simultaneity.

If a piece attempts to capture another, but the target piece moves at the same time, the capture does not occur. This also applies to pawns, which can only capture in adjacent squares.

Since XYMYX does not use black and white pieces, both Queens start the game to the left of their respective Kings, symbolizing equal conditions for both players.

In XYMYX, castling is not possible if any of the involved pieces (including the King or the Rook) is under attack at the time of the move. This adjustment maintains consistency with the game’s simultaneous logic and reinforces the importance of careful defensive planning.

In XYMYX, en passant captures do not exist, simplifying the game mechanics.

This is a very rare scenario since pawns can only capture when moving diagonally, not when advancing forward. However, if in a simultaneous move two pawns advance forward and land on the same empty square, the pawn of the player who registered their move first prevails.

Another possible scenario is when a King moves to an empty square at the same time as an opponent’s pawn also moves there. It is important to understand that the King was able to move to that square because the pawn, unable to capture while moving forward, does not threaten it, leaving the square free for the King. The result in this case is that the King remains on the square, and the pawn is removed from the board.

When a pawn reaches the eighth rank, it is promoted immediately to any piece chosen by the player. This new rank takes effect instantly.

If, at that exact moment, an opponent’s piece attempts to capture the promoted pawn, a collision occurs. To resolve it, the hierarchy of the promoted piece is considered, not that of the original pawn. The collision rules apply as usual: the higher-ranked piece prevails, or if the pieces are equal, the move registered first takes priority.

In XYMYX, each simultaneous turn is structured into two distinct and consecutive moments, and it is essential to understand them correctly in order to apply the rules fairly:

First Moment: This is the state of the board as it stands before the players make their simultaneous moves. It is the moment to analyze possible plays, identify threats, and plan actions.

Second Moment: This is the state of the board after both players have recorded their moves simultaneously. This new board reflects the consequences of the plays, such as new checks or any changes in the situation caused by the moves. The rules must not be analyzed or reinterpreted based on this second reality. All evaluations regarding the validity or legality of the actions must be made exclusively by considering the first moment.

Practical Conclusion in XYMYX: A player plans their move by ensuring that, at the moment of recording it, the move is legal and correct according to the first reality.

The second moment is simply the physical consequence of the blind decisions made during the first moment, and it cannot retroactively alter the validity of the recorded moves.

The major difference with traditional chess is that, in XYMYX, a single turn represents two moves: both players’ moves, made simultaneously. This breaks the classical chess paradigm, where each turn represents only one move, and makes a turn in XYMYX a concatenated representation of two simultaneous moves. This dynamic opens new strategic possibilities and adds a deeper layer of complexity to the game.