The strange chess rule that players often mistake for a software bug
In short: In chess, most captures involve landing on the square occupied by an enemy piece. But 'en passant' breaks this logic. It is a special case where a pawn captures an opponent by moving to an empty square, effectively catching a pawn that tried to skip past it.
The rule, meaning 'in passing' in French, was likely introduced between the 13th and 16th centuries alongside the two-square pawn advance to accelerate gameplay. While the two-square move was a later addition to early chess, the en passant rule may have arrived at the same time or slightly later. Evidence for the rule's existence in the 16th century can be found in the writings of Spanish master Ruy López de Segura.
To execute the move, an enemy pawn must have just advanced two squares on its previous turn. To execute the capture, the pawn travels diagonally ahead into the space vacated by the opponent's piece, simulating a single-step advance. This right is fleeting; if the capture is not exercised immediately on the very next turn, the opportunity is lost forever.
The rule has historically caused significant debate. In the 19th century, players argued whether the capture was a 'privilege' that could be declined. However, after discussions involving figures like Howard Staunton, who noted the move is mandatory when it is the only legal move available, the rules were amended to clarify its status. Today, it remains a source of confusion for casual players, who frequently report it as a bug or hacking on internet chess platforms.
The complexity of the rule is such that it can even define whether two positions are considered different in terms of threefold repetition. If a position allows for an en passant capture and another does not, they are treated as distinct states.
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Sources
- Summary by Intellect · source: En passant
- Also: Wikidata — en passant · CC0
- Also: Wikidata — En passant · CC0
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