Dead Reckoning

This problem was moved here from Thread 148235 at greenpawn's request.

A. Buchanan
StrateGems 2002

White to move. What was the last move?

This relies on a certain FIDE rule: If a position is reached where there is no possible checkmate, even with the most unskilled play, the game is immediately drawn. No further moves are permitted.

Step 1 for solving is: Is the diagrammed position 'dead' or 'alive'? Is a checkmate possible if both sides cooperate?

OK SG I'll bite.

Just to give you the pleasure of proving me wrong and
perhap pull some others into this problem before it gets buried.


The last move was Ke8-d8

I want a full explanation as to why it's wrong. (clues boys, suck clues out of him.)
(has it something to do with Black's b6 pawn?)

Originally posted by tomtom232

Are we sure the position is legal? How did that bishop get on a8?


Edit: The obvious answer is promotion but where did and how was the pawn promoted?

This was posted by tomtom232 in the other thread.

I don't see why this is a problem. Why does this bishop have to be a promoted pawn?

Hi SG

Having fun. I can get this.


Not figured out the mate yet but the g-pawn is obviously a tempo waster.

Hmmm.....you did not say who mates who?

Originally posted by WanderingKing
This was posted by tomtom232 in the other thread.

I don't see why this is a problem. Why does this bishop has to be a promoted pawn?

It doesn't. Tomtom232 is just talking nonsense... don't listen to him.

Some observations:

1. There's no tricky business with en passant or castling here. Everything moves like it should.

2. If we disregard the legality of the position and try to find a cooperative mate, then white's next move must be Ke6. g5 is the only other possiblity, but this inevitably leads to a draw.

3. Thanks to the the white g4 pawn, the pieces can be manipulated so that the resulting position is almost the same, but with black to move and with the white pawn on g5.

I haven't been able to find a mate. Since I had no ideas here, I just tried to analyze it by brute force. All lines I tried ended in a draw.

However, I think the position must be "alive" for two reasons. First, the puzzle says it's "white to move". If the position were dead, the game would have already ended, and white wouldn't be "to move". Second, these puzzles are supposed to surprise, and this position being dead is no surprise.

Originally posted by greenpawn34
Hi SG

Having fun. I can get this.

[fen]Kb1k1b2/b1p1p1pB/1pP1P1P1/1P6/p5P1/P7/8/8 w - - 0 11[/fen]
Not figured out the mate yet but the g-pawn is obviously a tempo waster.

Hmmm.....you did not say who mates who?

There doesn't seem to be a mate.... the black bishops can't move so we can discount them. The black king can't get off the back rank so he's accounted for. The white king can't get past the sixth rank so he can't capture that a4 pawn and there is also no way for the white bishop to take that pawn either.

White to move. What was the last move?

So we have to figure out Black's last move (perhaps retract it and then mate.)

There will be a logical explanation (and solution...but who mates who?)

How about this.
(I won't do the SG pm thing because I'm sure it's wrong - we are trading ideas.
and SG's answer will give us clues.)

This was the position with White to play.


It has to be a Knight on d8. No other piece can legally be there.

White played 1.Kb7 Kxd8.

So we take back 1.Kb7

Originally posted by greenpawn34
How about this.
(I won't do the SG pm thing because I'm sure it's wrong - we are trading ideas.
and SG's answer will give us clues.)

This was the position with White to play.

[fen]Bb1Nkb2/b1p1p1p1/KpP1P1P1/1P6/p5P1/P7/8/8 w - - 0 1[/fen]
It has to be a Knight on d8. No other piece can legally be there.

White played 1.Kb7 Kxd8.

So we take b ...[text shortened]... } 1... Kd8 2. Ka6 Ke8 3. g5 {See the first note.} 3... Kd8 4. Nf7+ Kc8 5. Bb7 {mate}[/pgn]

Wait - how did you get the Knight to f7?

"Wait - how did you get the Knight to f7?"

That's a clue.

I'm wrong.

Sorry -- I really don't get this. So the first clue is that we have to check if the position on the diagram is dead or alive. If it's dead though then it's the end of the game. But it's "white to move", so it's not the end of the game. So the position is alive.

Should I be worrying about this or is it not important?

I think the mate looks like this



Which looks possible from the position GP posted earlier.

Edit : ...although the Black king might struggle to get tucked up in there, now that I think on!

Originally posted by WanderingKing
Sorry -- I really don't get this. So the first clue is that we have to check if the position on the diagram is dead or alive. If it's dead though then it's the end of the game. But it's "white to move", so it's not the end of the game. So the position is alive.

Should I be worrying about this or is it not important?

I think the logic is -

if the diagrammed position is dead then for a legal move to have just been played the position a turn earlier mustn't have been dead
this would suggest a capture or some crucial pawn move has just been made,
also it means black had to have more than one legal move available because otherwise the diagrammed position is forced

On the other hand, if a mate is possible (I think one is) then we have more options for what the last move could have been.

Right. Never mind having Knight on f7. This looks better.

Blacks last move was a5-a4 we take that back and rather clumsily do this.