Answers.Body: <p>White's second to last move must be <strong>f5+</strong>.</p>
  
  <p>So, this is what the position looked like before White's second to last move - </p>
  
  <pre><code>   [FEN "b7/6pP/7k/7P/4PP2/2PP4/3B2KR/2Q1N2R w - - 0 1"]
  
     1. f5+ g5 2. hxg6+ 
  </code></pre>
  
  <p><strong>How to arrive at the solution -</strong> </p>
  
  <p>In this position, it is Black to play and the Black king is checked by a <strong>double-battery</strong> of the two rooks and the bishop and queen. What was White's last move? </p>
  
  <pre><code>        [FEN "b7/7P/6Pk/5P2/4P3/2PP4/3B2KR/2Q1N2R w - - 0 1"]
  </code></pre>
  
  <p>Since it is a double check, it must be some form of <strong>discovered check</strong>. However, since we have a double-battery, it cannot be possible that the discovered check was caused by a move of one of the pieces involved in the battery, because the other piece in the same battery would have already been checking the Black king if such were the case. Thus, it has to be some other "piece". But no such piece can uncover two attacks. Thus, it must be a very specific case of a pawn capture, an en passant, where two lines are opened at the same time. </p>
  
  <pre><code>     [FEN "b7/6pP/7k/5P1P/4P3/2PP4/3B2KR/2Q1N2R b - - 0 1"]
  
       1... g5 2. hxg6+
  </code></pre>
  
  <p>In the above, position, we see that the Black king is checked by a battery of queen and bishop. The only way that could have occurred in this position is if White played <strong>f4-f5</strong>. </p>
  
  <p>Thus, White's second to last move is <strong>f4-f5</strong>. </p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2014-04-18T17:50:33.440Z
Answers.Text: Nicely reasoned.  I think the position shown in the first version of the puzzle was a bit overcomplicated, though given the details of the position, you might want to mention the bishop on a8.  The simplified position eliminates the need for the battery as well as the black bishop, since there's no way white could have checked black by moving the bishop onto the a1-h8 diagonal.
Answers.UserDisplayName: supercat
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2014-04-17T23:45:44.033Z
Answers.LastActivityDate: 2019-09-21T17:38:55.493Z
Answers.LastEditDate: 2019-09-21T17:38:55.493Z
Answers.UserDisplayName: NM Wesley Falcao
Answers.Body: <p>The simplest version of the problem is 'cooked' - there are three distinct solutions:</p>
  
  <p>a) <em>setup - White king on B2, White pawn on F5, Black pawn on E7</em>
     --> <strong><em>1)</em> B2-B1+ E7-E5  <em>2)</em> F5xE5ep</strong></p>
  
  <p>b) <em>setup - White king on B2, White pawn on D5, Black pawn on E7</em>
     --> <strong><em>1)</em> B2-B1+ E7-E5  <em>2)</em> D5xE5ep</strong></p>
  
  <p>c) <em>setup - White pawn on D4, Black pawn on E7</em>
     --> <strong><em>1)</em> D4-D5+ E7-E5  <em>2)</em> D5xE5ep</strong></p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2016-04-29T20:23:19.853Z
Answers.Text: Looking back at the last comments on the puzzle, I see that perhaps c was the intended solution, implying that the White king should indeed be elsewhere.  Otherwise there are a lot more solutions, e.g. Black king at g5, g6, or g7, white king at c3; others as shown.  1. Kb2 Ke6, Kb1++.  If the White king is elsewhere, though, I don't know uncertain white's last move could be.
Answers.UserDisplayName: supercat
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2016-04-29T19:26:17.610Z
Answers.LastActivityDate: 2016-04-29T19:26:17.610Z
Answers.UserDisplayName: PMar
Body: <p>For the following position to occur in legal play, with Black on move, what would White's second-to-last move need to have been?  Note that Black is checked by two pieces on the h1-h8 file and two more on the c1-h6 diagonal, and both the d2 bishop nor the h2 must have been at their present locations when check was given.</p>
  <pre><code>b/7P/6Pk/5P/4P/2PP/3B2KR/2Q1N2R w K - 0 1
  </code></pre>
  <p>This is my first effort at a retrograde analysis problem: I think the position shown is reachable via legal moves, but that there's only one possible way the last three half-moves could have transpired.  Did I formulate the puzzle &quot;correctly&quot;?  Could/should it be improved?  Would it be better with fewer pieces?</p>
  <pre><code>8/8/4Pk/3P/8/8/5QK/B w K - 0 1
  </code></pre>
  <p>In the latter formulation, I like the way the White queen is placed so that it could have moved from b2 or d4, unblocking the bishop, but for the fact that it would have been giving check if it had been in either of those places.</p>
  <p>EDIT</p>
  <p>Here's yet another formulation, perhaps a bit more subtle, but I think the solution is still unique.  Note that in this revision, White's last move is not certain, but the second-to-last move is.</p>
  <pre><code>[FEN &quot;8/8/4Pk/8/8/8/8/B6K w - - 0 1&quot;]
  </code></pre>
  
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2014-09-23T21:14:09.470Z
Comments.Text: These puzzles are interesting and difficult. I think I will look into them.
Comments.UserDisplayName: CognisMantis
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2014-09-23T22:29:15.733Z
Comments.Text: @CognisMantis: Looking at the last one, I think I messed it up while trying to simplify it.  Perhaps the white king needs to be somewhere away from the a1-h8 diagonal, and a pawn is needed at a5?
Comments.UserDisplayName: supercat
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2014-09-24T12:20:20.883Z
Comments.Text: @supercat: in the last one, the white king should be elsewhere (h1 or so?) but then I still only see a unique solution, don't see why the pawn on a5 is needed.
Comments.UserDisplayName: RemcoGerlich
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2014-09-24T15:08:27.460Z
Comments.Text: @RemcoGerlich: If the white king is elsewhere, what could White's last move have been other than an en-passant capture, implying that Black's last move was e5?  But if Black's move was e5, what could white's move before that have been?  Am I missing something--stumped by my own puzzle?
Comments.UserDisplayName: supercat
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2018-08-21T07:34:02.260Z
Comments.Text: In the last diagram, the second-to-last move is not unique: -2.Kb2-b1+ e7-e5 -1.fxe6ep must be intention, but eg -2.Kb3-b2 Ke7-f6 -1.Kb2-b1+ is also possible.
Comments.UserDisplayName: Evargalo
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2018-08-21T18:15:00.723Z
Comments.Text: @Evargalo: Did I fix it now?
Comments.UserDisplayName: supercat
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2018-08-22T07:40:05.850Z
Comments.Text: @supercat : indeed you fixed it, but not in the most economical way. If you move the wK to h1, you can remove all the black Pawns.
Comments.UserDisplayName: Evargalo
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2018-08-22T18:52:42.267Z
Comments.Text: @Evargalo: Actually, I don't think neither that nor the version with the extra pawns works, since e5 must occur just before the e.p. capture, so White needs to have revealed the bishop before the capture, in some way that couldn't be done with pieces as they are.
Comments.UserDisplayName: supercat
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2018-08-22T19:15:36.843Z
Comments.Text: White: Kh1, Ba1, Pd4 ; Black : Kf6, Pe7. 1.d4-d5+ e7-e5 2.de6 ep.
Comments.UserDisplayName: Evargalo
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2018-08-22T19:56:39.863Z
Comments.Text: @Evargalo: Thanks.  I'd probably figured that out once but forgotten it.  Do you like the puzzle in that form?
Comments.UserDisplayName: supercat
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2014-04-17T23:27:37.910Z
Comments.Text: In chess composition one should try to show the idea in its purest form, without any unnecessary pieces. I like your idea, but I think you should work more on it. By the way, it's obviously black to move.
Comments.UserDisplayName: Dag Oskar Madsen
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2014-04-17T23:34:17.793Z
Comments.Text: @DagOskarMadsen: Do you like the above edit better?  Do you have any further suggestions for improvement?
Comments.UserDisplayName: supercat
Tags: <problems><retrograde-analysis><composing>
Title: Problem: Find White's second-to-last move
ThreadText: # Problem: Find White's second-to-last move
  
  (Asked by: username_0 on 2014-04-17)
  
  For the following position to occur in legal play, with Black on move, what would White's second-to-last move need to have been?  Note that Black is checked by two pieces on the h1-h8 file and two more on the c1-h6 diagonal, and both the d2 bishop nor the h2 must have been at their present locations when check was given.
  b/7P/6Pk/5P/4P/2PP/3B2KR/2Q1N2R w K - 0 1
  
  This is my first effort at a retrograde analysis problem: I think the position shown is reachable via legal moves, but that there's only one possible way the last three half-moves could have transpired.  Did I formulate the puzzle "correctly"?  Could/should it be improved?  Would it be better with fewer pieces?
  8/8/4Pk/3P/8/8/5QK/B w K - 0 1
  
  In the latter formulation, I like the way the White queen is placed so that it could have moved from b2 or d4, unblocking the bishop, but for the fact that it would have been giving check if it had been in either of those places.
  EDIT
  Here's yet another formulation, perhaps a bit more subtle, but I think the solution is still unique.  Note that in this revision, White's last move is not certain, but the second-to-last move is.
  [FEN "8/8/4Pk/8/8/8/8/B6K w - - 0 1"]
  
  **Tags:** problems, retrograde-analysis, composing
  
  > **Comments:**
  > **username_4**: In chess composition one should try to show the idea in its purest form, without any unnecessary pieces. I like your idea, but I think you should work more on it. By the way, it's obviously black to move.  
  > **username_0**: @DagOskarMadsen: Do you like the above edit better?  Do you have any further suggestions for improvement?  
  > **username_1**: These puzzles are interesting and difficult. I think I will look into them.  
  > **username_0**: @username_1: Looking at the last one, I think I messed it up while trying to simplify it.  Perhaps the white king needs to be somewhere away from the a1-h8 diagonal, and a pawn is needed at a5?  
  > **username_2**: @username_0: in the last one, the white king should be elsewhere (h1 or so?) but then I still only see a unique solution, don't see why the pawn on a5 is needed.  
  > **username_0**: @username_2: If the white king is elsewhere, what could White's last move have been other than an en-passant capture, implying that Black's last move was e5?  But if Black's move was e5, what could white's move before that have been?  Am I missing something--stumped by my own puzzle?  
  > **username_3**: In the last diagram, the second-to-last move is not unique: -2.Kb2-b1+ e7-e5 -1.fxe6ep must be intention, but eg -2.Kb3-b2 Ke7-f6 -1.Kb2-b1+ is also possible.  
  > **username_0**: @username_3: Did I fix it now?  
  > **username_3**: @username_0 : indeed you fixed it, but not in the most economical way. If you move the wK to h1, you can remove all the black Pawns.  
  > **username_0**: @username_3: Actually, I don't think neither that nor the version with the extra pawns works, since e5 must occur just before the e.p. capture, so White needs to have revealed the bishop before the capture, in some way that couldn't be done with pieces as they are.  
  > **username_3**: White: Kh1, Ba1, Pd4 ; Black : Kf6, Pe7. 1.d4-d5+ e7-e5 2.de6 ep.  
  > **username_0**: @username_3: Thanks.  I'd probably figured that out once but forgotten it.  Do you like the puzzle in that form?  
  
  ---
  
  ## 2 answers
  
  ### Answer by username_5 on 2014-04-17. Score: 7
  
  White's second to last move must be f5+.
  So, this is what the position looked like before White's second to last move - 
     [FEN "b7/6pP/7k/7P/4PP2/2PP4/3B2KR/2Q1N2R w - - 0 1"]
  
     1. f5+ g5 2. hxg6+ 
  
  How to arrive at the solution - 
  In this position, it is Black to play and the Black king is checked by a double-battery of the two rooks and the bishop and queen. What was White's last move? 
          [FEN "b7/7P/6Pk/5P2/4P3/2PP4/3B2KR/2Q1N2R w - - 0 1"]
  
  Since it is a double check, it must be some form of discovered check. However, since we have a double-battery, it cannot be possible that the discovered check was caused by a move of one of the pieces involved in the battery, because the other piece in the same battery would have already been checking the Black king if such were the case. Thus, it has to be some other "piece". But no such piece can uncover two attacks. Thus, it must be a very specific case of a pawn capture, an en passant, where two lines are opened at the same time. 
       [FEN "b7/6pP/7k/5P1P/4P3/2PP4/3B2KR/2Q1N2R b - - 0 1"]
  
       1... g5 2. hxg6+
  
  In the above, position, we see that the Black king is checked by a battery of queen and bishop. The only way that could have occurred in this position is if White played f4-f5. 
  Thus, White's second to last move is f4-f5.
  
  > **Comments:**
  > **username_0**: Nicely reasoned.  I think the position shown in the first version of the puzzle was a bit overcomplicated, though given the details of the position, you might want to mention the bishop on a8.  The simplified position eliminates the need for the battery as well as the black bishop, since there's no way white could have checked black by moving the bishop onto the a1-h8 diagonal.  
  
  ---
  
  ### Answer by username_6 on 2016-04-29. Score: 2
  
  The simplest version of the problem is 'cooked' - there are three distinct solutions:
  
  a) *setup - White king on B2, White pawn on F5, Black pawn on E7*
  --> ***1)* B2-B1+ E7-E5 *2)* F5xE5ep**
  
  b) *setup - White king on B2, White pawn on D5, Black pawn on E7*
  --> ***1)* B2-B1+ E7-E5 *2)* D5xE5ep**
  
  c) *setup - White pawn on D4, Black pawn on E7*
  --> ***1)* D4-D5+ E7-E5 *2)* D5xE5ep**
  
  > **Comments:**
  > **username_0**: Looking back at the last comments on the puzzle, I see that perhaps c was the intended solution, implying that the White king should indeed be elsewhere.  Otherwise there are a lot more solutions, e.g. Black king at g5, g6, or g7, white king at c3; others as shown.  1. Kb2 Ke6, Kb1++.  If the White king is elsewhere, though, I don't know uncertain white's last move could be.  
  
  ---
  
  