EN PASSANT 

is a special pawn capture that can only occur immediately after a pawn makes a move of two squares from its starting square, and when it could have been captured by an enemy pawn had it advanced only one square. The opponent captures the just-moved pawn "as it passes" through the first square. The result is the same as if the pawn had advanced only one square and the enemy pawn had captured it normally.

The en passant capture must be made on the very next turn or the right to do so is lost.

 

Like any other move, if an en passant capture is the only legal move available, it must be played. 

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The en passant capture rule was added in the 15th century when the rule that gave pawns an initial double-step move was introduced. It prevents a pawn from using the two-square advance to pass an adjacent enemy pawn without the risk of being captured.