"Been playing some chess with my husband recently", my friend Megan began, as we sat with our drinks in Coffee Monarchy.

"How'd you get on?" I asked.

"Didn't get off to a good start, I'm afraid. In our first game, he managed to beat me in only seven moves. I've still got the moves here, for what it's worth."

Megan put her big shoulder-bag on her lap and brought out a scrap of paper from it, but just as she put it out on the table for me, she knocked her coffee over. Several tissues later we'd cleaned up the spill as well as we could. But that scrap of paper was in a sorry state, and the ink had run. Now the only bit I could make out was the very last move:

K x b7#

Now, given that this was the winner's seventh move, what were the moves that led up to this victory?


This problem was invented by Alex Fishbein and published in The Problemist, March 2016. It is reproduced at PDB. The bottom of that page contains links to the PDB site's publication details and privacy policy (in German).

This is an entry in Fortnightly Topic Challenge #40: Retrograde Analysis.

  • 1
    Are you certain you're not violating any copyright by reproducing this question that is published elsewhere? If so, please edit the question to include the license under which you're copying it (it needs to be compatible with SE's CC license) or how you got permission or what have you. – jpmc26 Oct 21 at 23:46
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    @jpmc26 I've added a link to this problem's entry in PDB. The bottom of that page contains links to the PDB site's publication details and privacy policy (unfortunately, only in German). Is this enough? – Rosie F Oct 22 at 5:41
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    Maybe I'm unclear. Did you just mean the chess problem in general? I had assumed the entire scenario came from someone else. But if you haven't copied anything and are just referencing a uniqueness proof (which is what I gather from the link), then there probably isn't an issue. The link should be more than enough in that case. Sorry for the confusion. – jpmc26 Oct 22 at 5:45
up vote 19 down vote accepted

Oooh, that's a nice one. First of all,

a mating move by a king is always discovered check.

Further more, it's a mating move by

Black, because White's king can get to b7 in 7 moves but there's no move left to set up a 'battery', i.e. a queen on a4 which would give mate after Kc6xb7.

I first tried (in vain) to

create a discovered check over the b8-h2 diagonal; it's easy to get the white king to e5, and the white bishop via a6 to b7 where it can be captured, but there's not enough time to weave a mating net.

So it must be

a major piece on c8 giving the discovered check.

It turns out that

we can just clear up enough pawns to make this work

with the following moves:

1. d4 c5 2. dxc5 Na6 3. Qxd7+ Kxd7 4. Kd2 Kc7+ 5. Kc3 Be6 6. c6 Rc8 7. cxb7 Kxb7#

Final position:

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    Gosh, that didn't take long! Correct! – Rosie F Oct 21 at 10:55
  • I had the same idea, and my best result looked suspiciously like your board. :) My attempts were 12 moves, then 9 moves, though I couldn't get down after that. lichess.org/study/iR2Wbtup Black has to make at least 6 required moves for this mating net (3 king, 1 knight, 1 bishop, 1 knight). The challenging bit is spending that last "free" move wisely. – Mateen Ulhaq Oct 21 at 21:18
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    Man, white really has to cooperate for this one to work. – Weckar E. Oct 22 at 2:49

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