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You are required to find position such that:

1) Any move made by White or Black leads to stalemate for both sides immediately.

2) The position can be reached from the starting position of a chess game.

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  • Is this a puzzle from a magazine or something?
    – Herb
    Commented Oct 26, 2017 at 3:13
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    To be precise, 'mutual stalemate' is impossible - once one side gets stalemated, the game is over, and there is no need to assess whether the other side is 'stalemated' or not. To elaborate, a stalemate is said to occur when there is no legal move for the side to move. Once the game's up, there is no question of the other side having to move, hence there is no point in debating whether their move is legal or not.
    – Myungjin Hyun
    Commented Oct 31, 2017 at 8:54
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    "Mutual stalemate" is a problem concept (and easier to define than "mutual checkmate" which is also a thing!) Basically one side gets stalemated in a situation where, if the other side were on move, the position would still be stalemate.
    – Laska
    Commented Oct 31, 2017 at 17:44

3 Answers 3

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We are looking for a diagram which can be part of a legal position with either White to move or Black to move. The further constraint is that any legal move by either side must result in diagram which is stalemate, whoever is on move. I am trying to maximize the number of pieces.

At least 30 pieces is possible:

[title "Move by either player would be immediate mutual stalemate"]
[fen "brnbKRRN/qnk1pBN1/rb1pPpPp/p1pP1P1P/PpP5/8/1P6/8 w - - 0 1"]

Black g pawn captured the two missing (White) pieces to promote to dark-squared bishop, and also unblocked the g-file for the White pawn to advance. The only legal moves (whoever has the move) would be b3=, but the positions are dead, as would any which satisfy this challenge.

One tricky point is that if Black is on move, then White has many possible last moves, so by the en passant convention, Black is not permitted to capture bxa3ep or bxc3ep.

EDIT: As few as 8 pieces is possible.

[title "Move by either player would be immediate mutual stalemate"]
[fen "5k1K/p3pP1P/4P3/P7/8/8/8/8 w - - 0 1"]

If Black moved last it must have been Ke8-f8, Ke8xNf8 or Ke8xBf8. White has many more choices for last move. But in either case the position is legal.

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    Very interesting position. However, you should revise whether this position can be reached from the starting position of a chess game: you will probably need waiting moves by black to make white final's move (which looks like it should be g6), but you have none!
    – Pablo S. Ocal
    Commented Oct 31, 2017 at 17:10
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    @Pablo S. Ocal: thanks for your thoughtful response. I am pretty sure the position is legal though. White could have made any pawn move last. The last move for Black might have been perhaps c6-c5, which locked in bNb7 & bBb6 which were free to e.g. make waiting moves before that.
    – Laska
    Commented Nov 1, 2017 at 3:39
  • I completely overlooked that. You could probably add a white queen on h7 then. However, you have two dark squared black bishops! (which I noticed just now)
    – Pablo S. Ocal
    Commented Nov 1, 2017 at 21:16
  • @Pablo S. Ocal: yes the promotion is accounted for, and is not considered a defect for this type of construction. The black g P captured twice (wQ & wQB) to promote to a dark square B.
    – Laska
    Commented Nov 2, 2017 at 2:26
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7k/6pP/6P1/4p3/8/1p2P3/pP6/K7 w - - 0 1

This is a proposed solution, I wonder if we can find more patterns like this with more pieces on the board

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    Nice solution! Didn't think this sort of a configuration was possible!
    – Keshav
    Commented Oct 26, 2017 at 7:06
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    One way to add a piece would be an entombed bishop on c1 with a white pawn on d2 and a black pawn on d3.
    – itub
    Commented Oct 28, 2017 at 11:14
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If you want a solution that doesn't rely on the en passant convention, here is a solution with 29 men based on @Laska's answer. The missing White knights ensure that one of the black pawns could have promoted. The mutual stalemate occurs when either sides moves a pawn to f3, the only legal move.

[Title "me after @Laska, chess.stackexchange.com 4/2/2019, Mutual Stalemate In 1"]

[FEN "bqnbKRQB/rnk1pBRP/rb1pP1P1/p1pP4/PpP2p2/1P6/5P2/8 w - - 0 1"]

A fun fact is that the White dark-squared bishop can be replaced by a knight and the position would still yield the same.

Here’s an alternative 8-piece solution. This shows how two pieces is the least that one side can have.

[Title "me, chess.stackexchange.com 4/2/2019, Mutual Stalemate In 1"]
[FEN "KBk5/P1P1p3/2P5/4P3/8/8/8/8 w - - 0 1"]
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