En passant

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En passant (from French: "while [the pawn is] passing") is a maneuver in the board game of chess. The en passant rule applies when a player moves a pawn two squares forward from its starting position, and an opposing pawn could have captured it if it had only moved one square forward. The rule states that the opposing pawn may then capture the pawn as if it had only moved one square forward. The resulting position is the same as if the pawn had only moved one square forward and then the opposing pawn had captured as normal. En passant must be done on the very next turn, or the right to do so is lost. The move is unusual in that it is the only occasion in chess in which a piece captures but does not move to the square of the captured piece. In chess notation, en passant captures are sometimes denoted by "e.p." or similar, but that isn't required.

Example of en passant capture

Historically, en passant is one of the last series of major rule changes in European chess that occurred in the 14th to 15th century, together with the introduction of the two-square first move for pawns, castling, and the unlimited range for queens and bishops. Because of their separation from European chess prior to that period, the Asian chess variants do not feature any of these moves.

The idea behind en passant was that when the two-square first move for pawns was introduced to speed up the opening phase, this should not allow pawns to sneak past opposing pawns. Although a novice introduced to en passant by an opponent in the course of a game will often react with incredulity at the apparent illogic of this rule, upon closer examination, it is quite logical. As its name implies, the concept is that a pawn, which ordinarily moves only one square at a time, cannot move immediately to a square two rows ahead. It is thus vulnerable to being captured "in passing" through the first square to get to the second. The same principle can be seen in the rule that one cannot castle through check. Since a king ordinarily moves only one square at a time, he cannot move two squares at once, and thus renders himself vulnerable to being captured in passing through the first square. Since by the conventions of chess, a king is not allowed to put himself into check, so castling through check is not allowed.

Other usages of the term en passant

The term en passant is sometimes used among medical personnel as a label for unexpected or accidental findings seen while performing diagnostics aimed at another ailment. For example; finding a tumor while performing a chest x-ray to depicture a suspected pneumonia would label the tumor an en passant-finding.

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