Two problem conventions re castling and capturing en passant
14 May 2011 | by Peter Wong
Two special moves in chess, castling and the en passant capture, differ from other moves in that their legality depends on not only the current position, but the prior play as well. An issue arises in composed problems when these special moves are an option in the diagram, since we’re not given the play that lead to it. Two problem conventions deal with such situations. In the case of castling, if the king and a rook of one side are on their original squares, castling is deemed legal in subsequent play, unless it can be proved that the king or the rook must have moved previously in a hypothetical game.
Chess Amateur 1913
Mate in 2, (b) Add BPg2
The two-mover above is a neat illustration of this rule. Since White is to play in the diagram, Black must have made the last move, with a piece that is still on the board. Neither of the pawns could have made this move, since they are still on their initial squares, so it must have been made by the king or the rook. Thus we’ve shown that Black has disturbed at least one of the two pieces previously, a fact that renders the castling move illegal. The key here is 1.Ra8! (threat: 2.B~) 1…Kf8 2.Be5, 1…Rg8+ 2.Bg3, and Black cannot play 1…0-0, which otherwise would be a refutation. In part (b) of this twin, with a black pawn added on g2, Black’s last move could have been made by this pawn. That means castling is now considered legal, and it would defeat the try 1.Ra8? The new key is 1.Be5! (2.Ra8) 1…0-0 2.Rg3.
The convention for en passant captures applies to problem positions where a pawn is on its fifth rank while an enemy pawn is adjacent to it on the same rank. In such cases, capturing the enemy pawn en passant is deemed illegal, unless it can be proved that the only possible last move was a double-step by that pawn.
Düna Zeitung 1897
Mate in 2
The second problem exemplifies this kind of proof. Black’s last move wasn’t made by the king, since if …Kg6-h6 was just played that would imply the two kings were standing next to each other, while …Kg7-h6 as the last move would mean that the f6-pawn had just given check, but that’s impossible because the squares where that pawn could have come from (e5, f5, and g5) are all occupied. Hence Black’s last move was made by the g5-pawn. This move wasn’t …g6-g5, because that would mean White was in check while Black has the move – an illegal situation. And since f6 and h6 are occupied, that rules out the alternatives …fxg5 and …hxg5, leaving …g7-g5 as the pawn’s only possible last move. Therefore 1.hxg6 e.p.! is legalised as the problem’s key, which forces 1…Kh5 2.Rxh7. For more examples of problems involving such retro-analysis, see Dennis Hale’s article.