The Rule
A pawn on its fifth rank may capture an enemy pawn on an adjacent file that has moved two squares in a single move, as if the pawn had moved only one square. The conditions are:
- the capturing pawn must be on its fifth rank
- the captured pawn must be on an adjacent file and move two squares in a single move
- the capture is optional, but if elected must be done immediately (cannot be done on a later turn)
Black to move
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White to move
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Black to move
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Such a move is the only occasion in chess in which a piece captures but does not move to the square of the captured piece (Burgess 2000:463).
Read more about this topic: En Passant