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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 types of moves are an apparent option in the diagram position. Even though problem positions are required to be legal or reachable from the initial array, their histories are not given – to indicate, for instance, whether a king has moved before. (FEN information is completely irrelevant in composed problems, which don’t come from actual games.) Two basic conventions are designed to handle such situations and determine if a castling move or an en passant capture is allowed. These important rules apply to all kinds of compositions, but in particular they are the basis of many retro-analytical problems, where the focus is on deducing the preceding play.
The first convention relates to castling right when a player’s king and rook are on their original squares. In such positions, 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. The twin problem below with two parts neatly illustrates this rule.
Since White is to play in the diagram, Black must have made the last move, with a unit 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 have determined 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. Part (b) adds a black pawn on g2. Now Black’s last move could have been made by this pawn, rather than the king or the rook. With no proof that the latter pieces have ever moved, castling is now considered legal, and it would defeat the try 1.Ra8? The new key is 1.Be5!, threatening 2.Ra8. Since 1…Rg8 no longer checks, Black’s only defence is 1…0-0, answered by 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.
Friedrich Amelung
Düna Zeitung 1897
Mate in 2
The second problem exemplifies this kind of proof. Suppose Black’s previous move was …Kg7-h6, which would indicate 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. And clearly …Kg6-h6 wasn’t the last move as that would mean the two kings were standing next to each other. Hence Black’s last move was made by the g5-pawn. This move wasn’t …g6-g5, because that would imply White was in check while it was Black’s turn – an illegal situation. The alternatives …fxg5 and …hxg5 can be ruled out since f6 and h6 are occupied. The sole possibility left is the double-step …g7-g5; therefore 1.hxg6 e.p.! is legal as the problem’s key, and it leads to 1…Kh5 2.Rxh7.
Consider the same position but without the e5-pawn. In that case, Black’s previous move could have been …Kg7-h6, preceded by exf6+. That means …g7-g5 wasn’t necessarily the last move and therefore 1.hxg6 e.p. is forbidden, i.e. there is no solution. For more examples of problems involving such retro-analysis, see Dennis Hale’s article.
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