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It is possible in a chess game to have a dead position in which both players have all eight pawns, one bishop, and a king--20 pieces total. If the two players' pawns are interlocked in "zig-zag" fashion and each player's bishop is the same color as his pawns, each player's army will be forever stuck behind his own wall of pawns, with no way to ever reach anything on the other side.

[FEN "4kb2/8/1p1p5/pPpPp1p1/P1P1PpPp/5P1P/8/4KB2 w - - 0 1"]

What is the largest number of pieces that can be on the board in a legally-reachable position such that either:

  1. The side on move has at least one legal move, but no sequence of legal moves would produce checkmate.

  2. It would be possible to play an arbitrary number of legal moves, but no sequence of legal moves would produce checkmate.

  3. No legal sequence of legal moves would produce checkmate or stalemate.

I would guess that the first of those might be possible with as many as 28 pieces on the board, but most escape-proof positions rely upon immobilized kings to block opposing pawns, making it hard to allow anything other than pawns to move safely. What are the actual limits?

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  • What exactly are you looking for? Are positions where the pieces are arranged such that each side can only move one piece back and forth good enough? Or do the pieces need more "freedom"? If so, how much more freedom? (Such almost-stalemates are pretty easy to construct, although maybe there would not be proof games if both sides have 16 pieces.)
    – TMM
    Sep 25, 2017 at 1:20
  • @TMM: Positions where pieces are limited to moving back and forth would be fine, if they are reachable via sequence of legal moves.
    – supercat
    Sep 25, 2017 at 1:38
  • You are asking three different questions, you have to delete it and post three separate questions. The questions are all so totally unrelated to each other that they must be posted on different sites in different languages at different times. The whole 1, 2, 3 thing makes me so very confused that we have to chat about it here for a long while now. I know that you know that feeling!
    – LocalFluff
    Oct 14, 2017 at 13:29
  • @LocalFluff: Before asking the question, I didn't know whether the answers to #1 and #3 would be different. While it seems unlikely that #3 would be possible with all 32 pieces on the board, I still don't know if the best position for #2 would guarantee that no stalemate would ever be possible. If it wouldn't, then any answer for #3 would also hold for #2, making #2 redundant.
    – supercat
    Oct 14, 2017 at 15:16

3 Answers 3

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11

Funny task. I think I'm still pretty far from the maximum, but here is a suggestion with

23 men:

[Event "?"]
[title "Challenge 3, 23 units"]
[FEN "NRN1k1bn/QRKpPp2/PPpP1Pp1/2P3Pp/7P/8/7B/8 w - - 0 1"]

With black to move, an almost-dishonest trick to reach 25:

[title "Challenge 3, BTM, 25 units"]
[FEN "BRN1Nkbn/QRKpPp2/PPpP1Pp1/2P3Pp/7P/8/1r6/B7 b - - 0 1"]

27 with promoted units:

[title "Challenge 3, Check, 27 unit, including promoted"]
[FEN "QNBk1bnr/RBpPpKbr/PpP1Pp1p/1P3P1q/6P1/8/1R6/b7 w - - 0 1"]

explanation: This is a dead draw because after the forced moves 1.gxh5 Bxb2, White could only play his king back and forth on f7 and g6 while Black's bishop explores the bottom half of the board. wRb2 could be replaced by a wB or a wN (or a wQ with obvious adjustment of the nature of the upper-left-corner stranded pieces), or even by a fourth bB while preserving legality. This is so many "degrees of freedom" that I strongly feel that 27 is not the maximum, and someone will soon come up with (at least) 28...

edit: indeed, Laska just scored a pretty 28 (or even 30 with "the trick").

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  • 3
    I like the trick for this 24!
    – RemcoGerlich
    Sep 25, 2017 at 10:21
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    Wow, it seems the 26 is even legal, black a-pawn captures twice to promote on a1 or c1, black d-pawn captures once, accounts for all white's missing pieces. The g-pawn may have promoted on g1 and got there because white's g-pawn captured twice on its way to g8, which happens to be white. Nice :-)
    – RemcoGerlich
    Sep 25, 2017 at 10:29
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    The trick for 24 is fine.
    – supercat
    Sep 25, 2017 at 13:42
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    I think some of these violate the terms of the question: "It would be possible to play an arbitrary number of legal moves" and "No legal sequence of legal moves would produce checkmate or stalemate".
    – D M
    Sep 25, 2017 at 16:30
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    @DM: There are three questions in one, which might or might not all have the same best answer. A situation where the player on move would have no legal move that didn't force stalemate would be a valid answer for the first. A situation where two players repeat the same sequence of four positions endlessly would qualify for any of them, since in the absence of a rule about draws by repetition, that sequence could be played arbitrarily many times, and in the presence of such a rule even the starting position would eventually lead to a draw by repetition if the game didn't by other means first.
    – supercat
    Sep 25, 2017 at 17:10
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I've come up with 23, in the following position, with promotions: 

[Event "?"]
[Site "?"]
[Date "????.??.??"]
[Round "?"]
[White "?"]
[Black "?"]
[Result "*"]
[FEN "nb2k3/2p5/1pP5/1Pp5/2P1p1p1/3pPpPp/3P1P1B/4KNBN w - - 0 1"]

*

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5
  • Good work. I have experimented a bit and I think you can add Qg1, Rg2 and Rh3 for White (last sequence of legal moves, backwards: e3, Nf1, Qg1, Rg2, Rg1, Bh2, Rh3, Rh2, Bg1, Rg2, Rg1).
    – Annatar
    Sep 25, 2017 at 6:59
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    @Annatar with Rg2 it's not a dead position as black has fxg2.
    – RemcoGerlich
    Sep 25, 2017 at 7:23
  • @RemcoGerlich Oh darn. I focused too much on White's move options, not Black's. Forget about it, then.
    – Annatar
    Sep 25, 2017 at 7:24
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    This is not a dead position, white can play Bg1, Nh2, Nxg4.
    – RemcoGerlich
    Sep 25, 2017 at 7:24
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    Fixed the cook, with a promotion.
    – Herb
    Sep 25, 2017 at 12:33
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The three dead position challenges are (1) general (2) no finite mandatory stalemate (3) definitely no stalemate possible ever. (3)=>(2)=>(1).

Retro enthusiasts often sub-divide Task Records into 3 types:
- Type A = no additional information in the stipulation
- Type B = you are told who is on move (e.g. White here), but there is no check.
- Type C = one player is in check.

The posts so far have focused on OP's challenge (3). Here are a couple for challenge (1): 

[title "Challenge 1, Type B, 31 units, Last move?"]
[fen "brnk1N1B/qnb1pBR1/rbKpP3/p1pP2NQ/P1P2p2/1p3P1p/1P5P/5R2 w - - 0 1"]

White to move. Last move?

Black's last move must have been b4-b3. If the prior position was dead, the game would have already terminated. So White's move before that was a2-a4 or c2-c4, and Black chose not to make the en passant capture, which would have kept the game alive. No en passant convention is required here: White's double pawn move is the only way the game could have reached the current position without dying already.

This position was built many years ago, to force a unique last move. Without that additional goal, it's easy to make a 32 unit dead position. 

[title "Challenge 1, Type B, 32 units"]  
[fen "bqn1KN2/rrk1pB2/nb1pPp1p/p1pP1PpP/PpP3P1/1P2N1R1/4Q3/1R4B1 w - - 0 1"]

White to move.

Moving on to challenge 3, Olivier Pucher had 27 pieces, Type C. I offer:

[title "Challenge 3, Type A, 29 units"]  
[fen "qrn1KRRB/brk1pPN1/1p1pPp1p/1P1P1P1P/1p6/1Pp5/2P5/N5bB w - - 0 1"]

and:

[title "Challenge 3, Type C, 30 units"]
[fen "brn1KRRB/brN1pPN1/1bkpPp1p/p1pP1P1P/q1P5/1P6/1Q6/b4B2 w - - 0 1"]
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  • 1
    The difference between #2 and #3 is that a game of #3 could not end except by draw-by-repetition, draw-by-N-moves, or other such rule that is not based purely upon the position of pieces on the board, side on move, etc.. By contrast, #2 would allow for the possibility that players could end the game with a stalemated position.
    – supercat
    Oct 9, 2017 at 20:13
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    Thanks very much! Actually the laws of chess do say that a dead position game ends immediately. It's as sudden as checkmate or stalemate. Francois Labelle on his website wismuth.com has a list of games where players carried on going for a couple of turns before they realized! (Nit: he incorrectly terms such impossible moves "illegal".) But for formal chess problems, a recent 2015 convention says dead position, like 50 moves, only applies by default to retros (to protect e.g. self-stalemates). Most dead position problems are retro, so it's no issue. In others, as here, the condition is implied.
    – Laska
    Oct 10, 2017 at 6:37
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    If a position is dead, then by definition pretty much nothing the players could do would affect the outcome whether or not the game was recognized as "dead". The only situation I can think of where there could be an issue would be if someone resigned a dead position and later realize it was dead, and could plausibly claim the resignation was not deliberate bad sportsmanship (since bad sportsmanship might justify a forfeit even if the game would otherwise have been drawn). Otherwise, I would think that the official scores for games would include all the moves that were played.
    – supercat
    Oct 10, 2017 at 14:35
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    +1 for th beautiful 28-men dead draw. That will even be 29 if you force the move for black and add a wQb2...
    – Evargalo
    Oct 11, 2017 at 9:31
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    I wanted to see if I could get at least 2 more units by moving to Type C. I have just found such a position with 30 units, where 3 forced captures take place at the beginning. For the first time, I am not confident that this can be improved.
    – Laska
    Oct 11, 2017 at 23:37

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