En passant right

An en passant right may arise when an enemy pawn has just made a two-square double pawn push from its original square and finishes beside one of your pawns. On the immediate reply, your pawn may capture it as though the enemy pawn had advanced only one square.

The capture has unusual geometry: the capturing pawn moves one square diagonally onto the square the enemy pawn crossed, while the enemy pawn is removed from the adjacent square where it finished. The right exists only if the capture leaves the capturing side’s king safe, meaning it is not left under attack.

The right is temporary. If the player chooses any other move, the opportunity disappears and cannot be used later. Therefore, two boards with the same pieces on the same squares may differ for a repetition draw claim, which allows a player to claim a draw when the same position appears for the third time: a legal en passant capture may exist in one position but not the other.