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If one player (Black) gives another player (White) odds of x moves, stipulating that White must not move any man beyond his fourth rank until Black has made one move, what's the smallest value for "x" that would force checkmate by the time it is Black's turn to move? And what would those x moves be?

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    I seem to remember reading this in a book, and it was somewhere around 16 moves, but I can't be sure, and I don't remember the moves.
    – Daniel
    Jun 14, 2012 at 17:01
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    I remember reading it in a book too. I guess it was something like the 16-mover or the 14-mover in the answer I just posted.
    – bof
    Oct 27, 2019 at 0:11
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    You asked for the least number of moves, so why don't you accept my 14 move solution instead of that 15 move solution?
    – bof
    Apr 8, 2020 at 10:49
  • I also remember it from a (German) book :-) Maybe the "shortest" solution still isn't optimal. Also, "only 4th" is a bit arbitrary. (The book story was a GM always gave this odds, until someone clever busted him with the below setup.)
    – Hauke Reddmann
    Apr 5, 2022 at 7:38

2 Answers 2

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Interesting puzzle.

I came up with this.

[FEN ""]

1. d4 null 2. Qd2 null 3. Qf4 null 4. a4 null 5. Ra3 null 6. Re3 null 7. Nf3 null 8. h4 null 9. Nh2 null 10. Ng4 null 11. Rhh3 null 12. Rhf3 null 13. Nc3 null 14. Ne4 null 15. g3

After Black makes any move, it is a forced checkmate according Houdini. The best move is ’15... f6.’ It allows for mate in 10 moves.

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    You wasted a tempo with your queen at the beginning. 1. d4 2. Qd2 3. Qf4 gets her there one move quicker, so really you have it in 15 moves.
    – ETD
    Jun 15, 2012 at 5:07
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I think something like this is the easiest solution. The set-up takes 16 moves, but then it's only two more moves to checkmate and you don't need a computer to verify it. This is almost the same as Akavall's set-up, except that a knight is posted on c4 instead of g4.

[FEN ""]

1. a4 null 2. d4 null 3. h4 null 4. Nf3 null 5. Na3 null 6. Nd2 null 7. Ne4 null 8. Nc4 null 9. Qd2 null 10. Qf4 null 11. Ra3 null 12. Re3 null 13. Rhh3 null 14. Rhf3 null 15. g3 null 16. Bh3

But you can stop after only 14 moves, and Stockfish announces a forced mate in 7. (Note that after 14. Rhf3 White is threatening both 15. Qxf7# and 15. Nd6+ followed by 16. Nxd6#; only by moving the d pawn can Black parry both threats.)

[FEN ""]

1. a4 null 2. d4 null 3. h4 null 4. Nf3 null 5. Na3 null 6. Nd2 null 7. Ne4 null 8. Nc4 null 9. Qd2 null 10. Qf4 null 11. Ra3 null 12. Re3 null 13. Rhh3 null 14. Rhf3 d6 (14... d5 15. Qxf7+ Kd7 16. Ne5#) 15. Qxf7+ Kd7 16. Nc5+ dxc5 (16... Kc6 17. Na5+ Kb6 18. Qb3+ Kxa5 19. Qb5#) 17. Ne5+ Kd6 18. dxc5+ Kxc5 19. Qc4+ Kd6 (19... Kb6 20. Qb5#) 20. Qd4+ Ke6 21. Nd3#
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  • Thanks for this - certainly worth +1. I would term these “ply” not “moves”, particularly since the entire point is that black’s moves are null!
    – Laska
    Nov 24, 2020 at 8:04
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    You should add the line 14... Nh6 as well I think. And also 14... f6 and 14... f5. Then you covered all moves that isn't mate in one by 15. Qxf7# As you already know I am sure ;). For all three follows 15. N5d6+ cxd6 16. Nxd6#. PS I don't think someone can change the accepted answer afterwards? But too bad the OP didn't even react to your comment :/.
    – Carlo Wood
    Apr 4, 2022 at 10:39
  • @CarloWood Thanks. You can unaccept an accepted answer and accept a different answer, but some people don't like to do this.
    – bof
    Apr 5, 2022 at 5:01

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