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Once more, here is a new retrograde chess puzzle! This one might have turned out a bit tricky, so if you like a challenge, please have a go! :)

In the game leading up to the position below, both players somehow managed to get their pieces clumped up in the northwest corner of the grid. Somehow, in the process of doing this, seven units were captured. How did this happen?

For each of the missing units, can you determine which piece they fell victim to?

Final position, FEN: RBqk3N/RprPp2p/bKppp1p1/N1p5/2P5/1P6/B1PP3P/8

(13+12), FEN: RBqk3N/RprPp2p/bKppp1p1/N1p5/2P5/1P6/B1PP3P/8

As usual, please provide your reasoning in your answer. Have fun solving this! :^)

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2 Answers 2

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All captures are almost unambiguously defined

White: a2xNb3, b3xNc4, e6xRd7, g3xBf4(g5xBf6), f2-f8=B, g2-f8=B.

Black: a7xBb6, b6xBc5, f7xQe6.

The main point of the puzzle is to return black's f7xe6 move. To do this, you need to turn two pieces into pawns on d8. The rook and queen are not suitable - there is nothing to make an overlap on e8, the knight cannot get to f8. Only the elephant remains. Given that we can't pull out bishop b8, both elephants will be advanced. And you can only get them from the a7-c5 pawn. Everything else is details and maneuvers.

A short solution plan.

  1. We consider the balance. We find out which pawns were captured. We come to the conclusion that white has advanced two pawns and turned them into pieces on f8 (this is described in sufficient detail in Alex Ravsky's answer).

  2. Find the key that opens the cage in the northwest corner. This is the return of black's move f7:e6. To return this move, you must first return the f2 and g2 pawns home, that is, turn some pieces into pawns on f8. We can only get two white rooks out of the square, but they cannot be turned into pawns, we do not have a free piece to overlap on e8. It is also impossible to get to f8 with a knight. Maybe only with an elephant, but our only elephant is sitting in a cage and can't get out. Therefore, the only option to get the two bishops we are looking for is if the c5 black pawn returns them to us. In order to return two moves with the c5 pawn, you must first remove two white rooks from the square and place the white king on a8.

  3. Next, we just play the game according to this scenario (we keep in mind that in order for the white king to exit from the last horizontal, it is necessary to leave a window on g7, along the way it turns out that on d7 the white pawn must return the black rook, since the pawn chain will be formed by that time and the entrance and exit of the black rooks from the house will be impossible) and make sure that everything is correct. I was acting backwards, as is customary in retroanalysis. I didn't have any problems. Everyone left with everyone and returned home.

An important caveat. The white pawn f2 can only return the black dark bishop, which is not suitable for overlapping on e8.

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  • $\begingroup$ Well, this is correct. But you could perhaps consider giving a few more details to make your answer more understandable for non-experts :) $\endgroup$ Commented Jun 15 at 14:21
  • $\begingroup$ It is difficult to guess what exactly will be incomprehensible to "non-specialists". I have neither the time nor the desire to describe the "from and to". But I would be happy to answer specific questions. $\endgroup$ Commented Jun 15 at 20:21
  • $\begingroup$ Looks much more thorough already! One more slight subtlety is that black can also not uncapture anything that we can use as a shield on e8 to unpromote the rooks (since there are already two knights on the board). And freeing the black queen from the cage for that job is also not possible, even though there is some freedom of movement. $\endgroup$ Commented Jun 16 at 21:13
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This answer is in progress. Also it has a gap, see OP's comment.

Currently we answered the question for all white pieces and the black dark-squared bishop.

The missed pieces balance. White: the queen and two pawns (maybe promoted). Black: a rook, the dark-squared bishop, and two knights.

White pawn b3 captured no pieces, because otherwise the white bishop could not reach a2. So white pawn c4 captured two pieces. None of them was the dark-squared bishop.

It is easy to see that all three white pieces were captured by pawns, via f7xg6 and a7xb6xc5. So black pawns c6 and d6 are originated from c7 and d7, respectively. Therefore the last move of white pawn d7 was a capture. That capture was e6xd7, because otherwise it was c6xd7, but then this pawn captured at least two pieces to reach vertical c, so White captured at least five pieces, a contradiction. Clearly, the captured piece was not the dark-squared bishop.

The white pawn from g2 could not reach vertical e or its left to be captured, because of the capture limit, so it was promoted. To reach the eight horizontal it captured a piece, because the vertical g is blocked by black pawn g6. The capture was at vertical f, because vertical h is blocked by black pawn h7. The captured piece was the dark-squared bishop, because it was not captured by three White's captures considered above.

Since we detected all four White's captures, white pawn d7 captured no more pieces, so it originates from e2. Therefore white pawn c4 originates from a2.

The white pawn from f2 could not reach vertical e or its left to be captured, because it captured no pieces, so it was promoted too. Both white pawn promotions were at f8, and so they were after the black pawn f7 freed vertical f by the capture f7xe6. So the captured piece was the white queen. Thus both promoted white pawns were captured by the black pawn from a7 via a7xb6xc5.

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    $\begingroup$ This is a good start! Note, however, that it is also plausible that some original pieces were captured by Pa7 and the promoted pawns just replaced them. This makes the white side of the balance a bit less certain. $\endgroup$ Commented Jun 14 at 18:44

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