Classic chess rules

En passant

If A's pawn moves forward two squares and B has a pawn on his fifth rank and on an adjacent file, B's pawn can capture A's pawn as if A's pawn had only moved one square. This capture can only be made on the immediately subsequent move. In this example, an en passant capture would remove the black pawn and the white pawn would move to the f6 square, marked by an "x".