Pawn moves: the en passant capture

  A white and black pawn advancing on adjacent files will eventually meet diagonally, so that each attacks the other.  In order to avoid situations where one pawn could evade capture by an adjacent enemy pawn by using a double jump, a special rule called en passant (French for "in passing") exists. An example is shown in the diagram below. After the Black pawn has reached the fourth rank, the White pawn moves two squares by doing a double jump. To prevent the White pawn from "passing" in this way, the Black pawn is permitted to move to the square jumped over by the Black pawn, capturing just as if the White pawn had moved one only one square. The conditions under which this en passant capture is legal are:

1) a pawn may capture en passant an enemy pawn that has just made a double jump and that it could have captured had that pawn advanced only one square.

2) the en passant capture is allowed only as an immediate reply to the double jump. It may not be made at any later move.

A tale of en passant, told in three parts: 

In A, the White pawn at b2 hopes to make a run for it and break free from the grips of the Black a4 pawn, which is keeping b3 under control.

In B (after 1.b2-b4), the deed has been done - White has jumped over the Black pawn's capture square.  If Black wants to punish the frolicking White pawn for his impudence, he must do so now!

In C, (after 1. ... a4xb3) the end results are shown.  Black's a-pawn has transferred one square diagonally to the b-file, while White's pawn, alas, is no more.

Next: Pawn promotion