Monday, June 21, 2010

En Passant Chess Puzzle.

What an interesting little puzzle.

White to move and moves his bishop (lower right) up and to the right for checkmate. But, black has a response of double pushing his pawn still in its starting position up two to interpose, allowing a discovered checkmate by his bishop. At which point white takes blacks double pushed pawn "en passant" or 'in passing'. Now, interestingly enough, that would mean that black never actually got out of checkmate. The reason why you can double push your pawns is to speed up the game. The en passant move is so that people cannot abuse it to avoid being attacked by the other players pawns. So really the double pressed move is suppose to be two moves with only the opportunity by the other player to capture on the very next move or rather in the middle of the move that was just made. But, it's treated like a single move. So who really wins? Can black interpose if black can't reach the needed square? Because the whites response sets back the clock so to speak? Or does black win because the interposing allows for a discovered mate? Should you be allowed to capture the king in passing?



Jeffrey Rosenspan said...

I have the definitive answer, and I am happy to engage in a spirited debate if you don't agree.

Black wins. d5 is checkmate, no question. Two independent theories support this conclusion:

En passant is an option. If we ignore the checks, after Black's d4, White can certainly chose to play cxd3e.p. White is not compelled to, however, if neither king is in check. (En passant is obligatory only when there are no other legal moves.) Imagine a game: 1. d4 e5 2. d5 c5 3. option for dxc6e.p. or simply leave the d5 pawn where it is. Perhaps White will not take the black pawn En passant and choose instead to play c4 to defend the forward position of d5. The point is, En passant is not obligatory. This means Black's turn is over when Black plays d5, and in that moment White's turn begins. En passant does not involve two players moving pieces at the same time, there is still a turn for Black and a turn for White. As such, in your En passant puzzle the game ends with Black's d5++

En passant does not save the white king. If we allow the En passant, that removes the d5 pawn block and reveals a bishop attack on the Black king (for checkmate), but the white king remains in check. If a king is in check, that must be dealt with first. That's Chess 101. I was taught that checkmate means "the king will be taken next turn, no matter what, so the game is over". If we subscribe to that theory of checkmate, Black's d5 is checkmate, then White's cxd4e.p. is checkmate, and then Black's bishop takes the White king. White still loses first, so the game ends with Blacks d5++

If I'm wrong about any of the above, please correct me.

- Jeffrey L. Rosenspan

Tatarize said...

At issue though is not being an option, but the letter of the law. The reason it's En passant is that the piece was captured in passing. If the white pawn never arrived on that square then no move was permitted and it was mate. Blacks attempt to make a move that was prevented in passing means that that move wasn't legal to prevent mate. And white's move is mate. The question is letter of the law vs. spirit of the law. The spirit would be that double pushing that pawn would not be allowed because it doesn't prevent mate, because the pawn was captured halfway through that journey.