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Full-size image

Black has just delivered checkmate in the above position - but is the position legal?

If no, explain why.

If yes, give Black's mating move.

FEN: 3r4/2r4b/8/8/8/3Kp3/5k2/8 w - - 0 1

URL: lichess.org


Also hello, fellow puzzlers! Long time, no see!

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  • 1
    $\begingroup$ Welcome back, and very nice puzzle! $\endgroup$ – Deusovi Aug 5 at 2:34
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    $\begingroup$ Just out of curiosity - to the two downvoters, why the downvotes? $\endgroup$ – Brandon_J Aug 6 at 1:38
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I claim that

yes, the position is legal.

The reason:

The last move cannot have been by Black's rooks or bishop, because those would leave a piece attacking White. So those pieces must have been fixed, and there must have been pieces blocking those lines of attack.
But how is it possible for both of those pieces to disappear in a single move? Only one could have been taken in a capture, so the other piece is Black's. But then which piece could capture from a square and then not land on that square? There's only one way that could happen: an en passant capture, d4xe3.

enter image description here

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  • $\begingroup$ I suppose this was a tad simpler than I thought, lol - looks like you'll get the check, Deus. Nice answer as well, @El-Guest $\endgroup$ – Brandon_J Aug 5 at 2:36
  • $\begingroup$ I only just found out that there were other answers-I only refreshed the mobile page after I answered! $\endgroup$ – Rewan Demontay Aug 5 at 2:37
  • $\begingroup$ @Brandon_J Nah, I don't think it was as easy as our three answers would imply! You just happened to post it when two chess fans were online (and also I was here too). $\endgroup$ – Deusovi Aug 5 at 2:39
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    $\begingroup$ Seems a bit of a trend, but why is it every time I see an "impossible" chess puzzle, rot13(gurer'f nyjnlf na ra cnffnag vaibyirq)? $\endgroup$ – Darrel Hoffman Aug 5 at 13:21
  • $\begingroup$ This is probably the most famous retrograde analysis puzzle in existence, so it's no surprise that it was answered so quickly by three people... $\endgroup$ – BlueRaja - Danny Pflughoeft Aug 5 at 17:54
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I think that:

Yes it is legal.

The last sequence is

Bh7+ e4
dxe4 e.p. # !!

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It's easily legal. Black's last move was:

dex3# en passant!

Here's a possible sequence of the last few moves:

1. Rh7 Bxh7+ 2. e4 dxe3# *

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