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This puzzle is not mine, although I don't have any reference as to where it originated from. I discovered it when I was a student, some fifteen years ago. It has been among my favourite chess puzzles since then. Could be very well known or a duplicate, but I couldn't find it in the database.

The following position is legal, except that the White King (and the White King only) is not displayed. Find it.

                  

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The white king must be on square

c3

Explanation:

First, we determine which side is to move. Unless the white king blocks the a4-d1 diagonal, the black king is in check. The king can't be on c2 since it would attack the black king; on b3, White would be to move but there's no way this double-check could have happened in a real game; there's no way to clear both the b3-d5 diagonal and the b3-b5 file with one move from the black rook or the black bishop. Black simply couldn't have played Bb4-d5 or Rc4-b5. So the black king is in check and White made the last move.

So, what was the last move?

The bishop can't have moved along the d1-a4 diagonal, because it would give check on c2 and d3 as well. So it must be a discovered check, caused by a move from the white king, from b3 to another square.

But ...

doesn't this lead to the same problem as before? On b3, the White king is still in double check. No, because there is a move which simultaneously clears the b3-d5 diagonal and the b3-b5 file, but it requires extra pieces: an en-passant capture from b4 to c3, capturing a pawn on c4. Since we have an extra (half-)move compared to the situation above, we can use that move to get rid of the black pawn.

So,

if White played 1. c2-c4, Black can play 1... b4xc3 e.p., White responds with 2. Kxc3 and we're in the diagram in the question with the White king on c3.

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  • $\begingroup$ ... but it doesn't include an explanation why this is the only possible solution (which took me a while to write down...) $\endgroup$ – Glorfindel Mar 26 at 15:59
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    $\begingroup$ @ArnaudMortier Glorfindel's answer eliminates other possibilities. $\endgroup$ – noedne Mar 26 at 15:59
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The last move in this position was Bd5+. White blocked the check with c4, black gave double check by capturing en passant bxc, and the king took the pawn on c3.
chessboard

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  • $\begingroup$ Um... to me, it looks like rot13(gur cnja vf ba p2)... :P $\endgroup$ – EKons Mar 26 at 15:49
  • $\begingroup$ I've ROT13-ed part of the comment so that it doesn't appear as an immediate spoiler. Check the image you've posted. $\endgroup$ – EKons Mar 26 at 15:51
  • $\begingroup$ That was super quick. Did you know the problem before? Do you know where it is from? $\endgroup$ – Arnaud Mortier Mar 26 at 15:54
  • $\begingroup$ Your answer says that "rot13(gur xvat gbbx gur cnja ba p3)", however that square looks empty, unless there's actually more behind the image. $\endgroup$ – EKons Mar 26 at 15:54
  • $\begingroup$ @EKons After the black pawn captures en passant, it is on c3 for the white king to capture. $\endgroup$ – noedne Mar 26 at 15:56

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