user1656: This question demonstrates a game in which all 16 pawns promote for a total of 18 queens on the board. It takes 80 moves...

What is the shortest sequence of moves that achieves this goal?


user15543: As it appears, 48.0 moves may indeed be optimal. Here are more 48.0 moves proofs, sourced from rec.games.chess. The PGNs were sourced elsewhere.
[Title "Andreas Rein, Die Welt 1989"]
[FEN ""]
[startply "96"]

1. h4 g5 2. h5 g4 3. Rh2 g3 4. h6 gxh2 5. g4 e5 6. g5 Ne7 7. g6 a5 8. g7 a4 9. g8=Q a3 10. Qg2 Bg7 11. f4 c5 12. f5 c4 13. f6 c3 14. fxe7 f5 15. b4 Kf7 16. hxg7 h5 17. b5 h4 18. b6 h3 19. Bb2 f4 20. e4 f3 21. d4 d5 22. Nd2 Bf5 23. Kf2 cxd2 24. Kg3 Kg6 25. c4 Kg5 26. exf5 e4 27. c5 axb2 28. Bc4 dxc4 29. a4 f2 30. Qdf3 Ra7 31. bxa7 b5 32. a5 b4 33. a6 b3 34. c6 c3 35. c7 c2 36. d5 e3 37. d6 e2 38. d7 hxg1=Q 39. gxh8=Q bxa1=Q 40. axb8=Q h2 41. f6 b2 42. f7 h1=Q 43. a7 b1=Q 44. e8=Q Qdf6 45. c8=Q c1=Q 46. d8=Q d1=Q 47. a8=Q e1=Q 48. f8=Q f1=Q+


[TItle "Theodor Burian, Rochade Europa 11/1997"]
[FEN ""]
[startply "96"]

1. b4 a5 2. b5 a4 3. b6 a3 4. Bb2 axb2 5. a4 Ra7 6. bxa7 b5 7. h4 c5 8. h5 g5 9. h6 Bg7 10. hxg7 h5 11. g4 h4 12. a5 Rh5 13. gxh5 b4 14. f4 e5 15. f5 d5 16. f6 Ne7 17. fxe7 f5 18. h6 f4 19. e4 Bf5 20. exf5 Kf7 21. f6 f3 22. h7 g4 23. a6 g3 24. h8=Q g2 25. Kf2 h3 26. Kg3 c4 27. d4 c3 28. Nd2 cxd2 29. c4 b3 30. c5 e4 31. Bc4 dxc4 32. d5 h2 33. d6 f2 34. Qdh5+ Ke6 35. a8=Q gxh1=Q 36. e8=Q+ Qe7 37. a7 e3 38. d7 hxg1=Q+ 39. Qg2 c3 40. a8=Q f1=Q 41. c6 e2 42. c7 bxa1=Q 43. cxb8=Q b2 44. d8=Q c2 45. f7 b1=Q 46. g8=Q c1=Q 47. Qgh7 d1=Q 48. f8=Q e1=Q+

In a comment on the answer to the PSE version of this question, linked in the other answer, user @Dennis Jaheruddin came up with a provable lower bound for this challenge

"It takes 80 moves to move pawns forward. Obviously they cannot capture the king and queen so there are only 6 promotion fields left on each side. Basically this means it takes at least 4 halfmoves to clear additional promotion fields. Then there are 8 pawn rows that block eachother, each conflict will need to be solved by capturing at least 1 piece that is not on it's starting position, hence we need another 8 half moves. Now we already have a lower bound of 92 and there are still some issues to solve. As such 96 is probably optimal."

Figuring out and proving that more four halfmoves are necessary would be a momumentable achievement, finally bringing the 18 queens challenge to its knees. However, it seems no one knows where to start. Neither do I. As such, we are stuck at 92/96 proven plies. Hopefully, one day, a genius shall shine their light in our favor.

user2001: How about this strategy:
[FEN ""]

1. a4 b5 2. Na3 b4 3. h4 bxa3 4. b4 a5 5. b5 Na6 6. bxa6 Rb8 7. a7 Rb5 8. axb5 c5 9. Rb1 c4 10. Rb3 cxb3 11. Nh3 g5 12. c4 g4 13. c5 gxh3 14. g4 h5 15. g5 Nh6  16. gxh6 Rg8 17. h7 Rg5 18. hxg5 f5 19. Rg1 h2 20. Rg4 fxg4 21. g6 h4 22. c6 Bb7 23. cxb7 g3 24. b6 b2 25. a8=Q a2 26. b8=Q a4 27. b7 a3 28. Qf4 b1=Q 29. b8=Q a1=Q 30. Qbb4 a2 31. Qfc4 e5 32. f4 e4 33. Bg2 h3 34. Bf3 exf3 35. f5 g2 36. d4 Qab2 37. Qdc2 a1=Q 38. Kd2 f2 39. d5 h1=Q 40. d6 f1=Q 41. f6 Be7 42. dxe7 d5 43. g7 Kd7 44. g8=Q d4 45. e4 d3 46. e5 g1=Q 47. f7 Qc6 48. h8=Q h2 49. f8=Q h1=Q 50. e6+ Kc7 51. e8=Q Qbg7 52. Qbb2 Q1g6 53. Qff6 Qhh7 54. Kc3 d2 55. Qgf8 dxc1=Q 56. e7 Q8d7 57. Qef7 Q6h6 58. e8=Q


user478: 
Edit: best known is 96 half-moves by Friedrich Burchard & Friedrich Hariuc (1976). (link to answer)

My best score 104 half-moves:
[FEN ""]

1.a4 b5 2.a5 b4 3.a6 Bb7 4.axb7 h5 5.bxa8=Q h4 6.g4 h3 7.Bg2 hxg2 8.Ra3 bxa3 9.b4 gxh1=Q 10.e4 a2 11.b5 a1=Q 12.b6 a5 13.b7 d5 14.c4 Nd7 15.c5 Nb6 16.cxb6 Rh5 17.gxh5 g5 18.h6 g4 19.h7 g3 20.h4 a4 21.h5 a3 22.h8=Q a2 23.h6 axb1=Q 24.h7 g2 25.Bb2 d4 26.Bc3 dxc3 27.d4 c2 28.b8=Q c1=Q 29.b7 e5 30.Ke2 c5 31.Nf3 c4 32.Qba7 Bc5 33.dxc5 c3 34.Nd4 exd4 35.f4 Kd7 36.e5 Ne7 37.f5 Ng6 38.fxg6 f5 39.g7 f4 40.g8=Q Qca3 41.c6+ Ke7 42.e6 Kd6 43.c7 c2 44.Qhg7 c1=Q 45.Qc2 d3+ 46.Kf3 d2+ 47.Ke4 d1=Q 48.c8=Q f3 49.e7 f2 50.e8=Q f1=Q 51.h8=Q Qc7 52.b8=Q g1=Q+

Conclusion: it is impossible to make it in 80 half moves as it is required to bring the king to safety and in order to make some pawns swap files, you need to sacrifice minor pieces (at least some of them must not remain in their original squares).
I think it can be improved, but not that much.
Also, yes, the type of promotion affects makes the output differ because queens cover much space and you need to protect the king.

user478: Here is a solution in 96 plies. Found by Friedrich Burchard & Friedrich Hariu, it was published in the German magazine feenschach issue #33 in 1976 on page 22. It can be viewed and downloaded as a PDF (the challenge was issued in #31.
[Title "Friedrich Burchard & Friedrich Hariuc, feenschach #33 1976, Page 22"]
[FEN ""]
[startply "96"]

1. e4 f5 2. e5 Nf6 3. exf6 e5 4. g4 e4 5. Ne2 e3 6. Ng3 e2 7. h4 f4 8. h5 fxg3 9. h6 g5 10. Rh4 gxh4 11. g5 g2 12. g6 Bg7 13. hxg7 g1=Q 14. f4 h3 15. f5 h2 16. b4 a5 17. b5 a4 18. b6 a3 19. Bb2 Ra7 20. bxa7 axb2 21. a4 b5 22. a5 b4 23. a6 b3 24. c4 h1=Q 25. c5 h5 26. c6 Bb7 27. cxb7 c5 28. d4 c4 29. d5 Nc6 30. dxc6 c3 31. c7 c2 32. c8=Q c1=Q 33. b8=Q Qc7 34. a8=Q d5 35. a7 d4 36. Nc3 dxc3 37. Qa6 c2 38. Qa8b7 c1=Q 39. a8=Q Qd5 40. gxh8=Q+ Kd7 41. g7 bxa1=Q 42. g8=Q b2 43. f7 b1=Q 44. f8=Q h4 45. f6 h3 46. f7 h2 47. Qfa3 h1=Q 48. f8=Q exf1=Q+

Credit goes to d'alar'cop for his PSE answer.

1656: <p><a href="https://chess.stackexchange.com/q/4123/1656">This question</a> demonstrates a game in which all 16 pawns promote for a total of 18 queens on the board. It takes 80 moves...</p>
<blockquote>
<p>What is the shortest sequence of moves that achieves this goal?</p>
</blockquote>
 15543: <p>As it appears, 48.0 moves may indeed be optimal. Here are more 48.0 moves proofs, sourced from <a href="https://groups.google.com/g/rec.games.chess.misc/c/rgrG9aBWP1M/m/4rU7xIW4h2oJ" rel="nofollow noreferrer">rec.games.chess.</a> The PGNs were sourced <a href="https://www.edcollins.com/chess/queens.htm" rel="nofollow noreferrer">elsewhere.</a></p>
<pre><code>[Title &quot;Andreas Rein, Die Welt 1989&quot;]
[FEN &quot;&quot;]
[startply &quot;96&quot;]

1. h4 g5 2. h5 g4 3. Rh2 g3 4. h6 gxh2 5. g4 e5 6. g5 Ne7 7. g6 a5 8. g7 a4 9. g8=Q a3 10. Qg2 Bg7 11. f4 c5 12. f5 c4 13. f6 c3 14. fxe7 f5 15. b4 Kf7 16. hxg7 h5 17. b5 h4 18. b6 h3 19. Bb2 f4 20. e4 f3 21. d4 d5 22. Nd2 Bf5 23. Kf2 cxd2 24. Kg3 Kg6 25. c4 Kg5 26. exf5 e4 27. c5 axb2 28. Bc4 dxc4 29. a4 f2 30. Qdf3 Ra7 31. bxa7 b5 32. a5 b4 33. a6 b3 34. c6 c3 35. c7 c2 36. d5 e3 37. d6 e2 38. d7 hxg1=Q 39. gxh8=Q bxa1=Q 40. axb8=Q h2 41. f6 b2 42. f7 h1=Q 43. a7 b1=Q 44. e8=Q Qdf6 45. c8=Q c1=Q 46. d8=Q d1=Q 47. a8=Q e1=Q 48. f8=Q f1=Q+
</code></pre>
<hr />
<pre><code>[TItle &quot;Theodor Burian, Rochade Europa 11/1997&quot;]
[FEN &quot;&quot;]
[startply &quot;96&quot;]

1. b4 a5 2. b5 a4 3. b6 a3 4. Bb2 axb2 5. a4 Ra7 6. bxa7 b5 7. h4 c5 8. h5 g5 9. h6 Bg7 10. hxg7 h5 11. g4 h4 12. a5 Rh5 13. gxh5 b4 14. f4 e5 15. f5 d5 16. f6 Ne7 17. fxe7 f5 18. h6 f4 19. e4 Bf5 20. exf5 Kf7 21. f6 f3 22. h7 g4 23. a6 g3 24. h8=Q g2 25. Kf2 h3 26. Kg3 c4 27. d4 c3 28. Nd2 cxd2 29. c4 b3 30. c5 e4 31. Bc4 dxc4 32. d5 h2 33. d6 f2 34. Qdh5+ Ke6 35. a8=Q gxh1=Q 36. e8=Q+ Qe7 37. a7 e3 38. d7 hxg1=Q+ 39. Qg2 c3 40. a8=Q f1=Q 41. c6 e2 42. c7 bxa1=Q 43. cxb8=Q b2 44. d8=Q c2 45. f7 b1=Q 46. g8=Q c1=Q 47. Qgh7 d1=Q 48. f8=Q e1=Q+
</code></pre>
<p>In a comment on the answer to the PSE version of this question, linked in the other answer, user @Dennis Jaheruddin came up with a provable lower bound for this challenge</p>
<blockquote>
<p>&quot;It takes 80 moves to move pawns forward. Obviously they cannot capture the king and queen so there are only 6 promotion fields left on each side. Basically this means it takes at least 4 halfmoves to clear additional promotion fields. Then there are 8 pawn rows that block eachother, each conflict will need to be solved by capturing at least 1 piece that is not on it's starting position, hence we need another 8 half moves. Now we already have a lower bound of 92 and there are still some issues to solve. As such 96 is probably optimal.&quot;</p>
</blockquote>
<p>Figuring out and proving that more four halfmoves are necessary would be a momumentable achievement, finally bringing the 18 queens challenge to its knees. However, it seems no one knows where to start. Neither do I. As such, we are stuck at 92/96 proven plies. Hopefully, one day, a genius shall shine their light in our favor.</p>
 2001: <p>How about this strategy:</p>

<pre><code>[FEN ""]

1. a4 b5 2. Na3 b4 3. h4 bxa3 4. b4 a5 5. b5 Na6 6. bxa6 Rb8 7. a7 Rb5 8. axb5 c5 9. Rb1 c4 10. Rb3 cxb3 11. Nh3 g5 12. c4 g4 13. c5 gxh3 14. g4 h5 15. g5 Nh6  16. gxh6 Rg8 17. h7 Rg5 18. hxg5 f5 19. Rg1 h2 20. Rg4 fxg4 21. g6 h4 22. c6 Bb7 23. cxb7 g3 24. b6 b2 25. a8=Q a2 26. b8=Q a4 27. b7 a3 28. Qf4 b1=Q 29. b8=Q a1=Q 30. Qbb4 a2 31. Qfc4 e5 32. f4 e4 33. Bg2 h3 34. Bf3 exf3 35. f5 g2 36. d4 Qab2 37. Qdc2 a1=Q 38. Kd2 f2 39. d5 h1=Q 40. d6 f1=Q 41. f6 Be7 42. dxe7 d5 43. g7 Kd7 44. g8=Q d4 45. e4 d3 46. e5 g1=Q 47. f7 Qc6 48. h8=Q h2 49. f8=Q h1=Q 50. e6+ Kc7 51. e8=Q Qbg7 52. Qbb2 Q1g6 53. Qff6 Qhh7 54. Kc3 d2 55. Qgf8 dxc1=Q 56. e7 Q8d7 57. Qef7 Q6h6 58. e8=Q
</code></pre>
 478: <blockquote>
  <p><strong>Edit:</strong> best known is <strong>96</strong> half-moves by Friedrich Burchard &amp; Friedrich Hariuc (1976). <a href="https://chess.stackexchange.com/a/7743/478">(link to answer)</a></p>
</blockquote>

<p>My best score <strong>104</strong> half-moves:</p>

<pre><code>[FEN ""]

1.a4 b5 2.a5 b4 3.a6 Bb7 4.axb7 h5 5.bxa8=Q h4 6.g4 h3 7.Bg2 hxg2 8.Ra3 bxa3 9.b4 gxh1=Q 10.e4 a2 11.b5 a1=Q 12.b6 a5 13.b7 d5 14.c4 Nd7 15.c5 Nb6 16.cxb6 Rh5 17.gxh5 g5 18.h6 g4 19.h7 g3 20.h4 a4 21.h5 a3 22.h8=Q a2 23.h6 axb1=Q 24.h7 g2 25.Bb2 d4 26.Bc3 dxc3 27.d4 c2 28.b8=Q c1=Q 29.b7 e5 30.Ke2 c5 31.Nf3 c4 32.Qba7 Bc5 33.dxc5 c3 34.Nd4 exd4 35.f4 Kd7 36.e5 Ne7 37.f5 Ng6 38.fxg6 f5 39.g7 f4 40.g8=Q Qca3 41.c6+ Ke7 42.e6 Kd6 43.c7 c2 44.Qhg7 c1=Q 45.Qc2 d3+ 46.Kf3 d2+ 47.Ke4 d1=Q 48.c8=Q f3 49.e7 f2 50.e8=Q f1=Q 51.h8=Q Qc7 52.b8=Q g1=Q+
</code></pre>

<p><strong>Conclusion:</strong> it is impossible to make it in 80 half moves as it is required to bring the king to safety and in order to make some pawns swap files, you need to sacrifice minor pieces (at least some of them must not remain in their original squares).</p>

<p>I think it can be improved, but not that much.</p>

<p>Also, yes, the type of promotion affects makes the output differ because queens cover much space and you need to protect the king.</p>
 478: <p>Here is a solution in 96 plies. Found by Friedrich Burchard &amp; Friedrich Hariu, it was published in the German magazine feenschach issue #33 in 1976 on page 22. It can be viewed and downloaded as a <a href="http://problem64.beda.cz/silo/feenschach33_1976.pdf" rel="nofollow noreferrer">PDF</a> (the challenge was issued in <a href="http://problem64.beda.cz/silo/feenschach31_1975.pdf" rel="nofollow noreferrer">#31.</a></p>
<pre><code>[Title &quot;Friedrich Burchard &amp; Friedrich Hariuc, feenschach #33 1976, Page 22&quot;]
[FEN &quot;&quot;]
[startply &quot;96&quot;]

1. e4 f5 2. e5 Nf6 3. exf6 e5 4. g4 e4 5. Ne2 e3 6. Ng3 e2 7. h4 f4 8. h5 fxg3 9. h6 g5 10. Rh4 gxh4 11. g5 g2 12. g6 Bg7 13. hxg7 g1=Q 14. f4 h3 15. f5 h2 16. b4 a5 17. b5 a4 18. b6 a3 19. Bb2 Ra7 20. bxa7 axb2 21. a4 b5 22. a5 b4 23. a6 b3 24. c4 h1=Q 25. c5 h5 26. c6 Bb7 27. cxb7 c5 28. d4 c4 29. d5 Nc6 30. dxc6 c3 31. c7 c2 32. c8=Q c1=Q 33. b8=Q Qc7 34. a8=Q d5 35. a7 d4 36. Nc3 dxc3 37. Qa6 c2 38. Qa8b7 c1=Q 39. a8=Q Qd5 40. gxh8=Q+ Kd7 41. g7 bxa1=Q 42. g8=Q b2 43. f7 b1=Q 44. f8=Q h4 45. f6 h3 46. f7 h2 47. Qfa3 h1=Q 48. f8=Q exf1=Q+
</code></pre>
<p>Credit goes to <em>d'alar'cop</em> for his <a href="https://puzzling.stackexchange.com/a/3545/4376">PSE answer.</a></p>
