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What is the longest known sequence of moves with upto 6 pieces where each move of both sides is the only move drawing the game (any other move would lose the game) ?

There should not occur any repitition of a position.

I experimented with the endgame rook vs two (three) pawns and got some sequences I unfortunately do not remember.

3

To get the ball rolling, from the following position the first 6 moves for each side are forced in order not to lose. (If 7.h8=B weren't available and 7.h8=Q needed to be played, then the forced follow-ups 7...h1=Q+ 8.Qxa1+ Kxa1 would extend it to 8, but no cigar.)

[FEN "K5R1/8/p7/8/8/7P/8/1q1k4 w - - 0 1"]

1.Rg1+ $1 Kc2 $1 2.Rxb1 $1 Kxb1 $1 3.h4 $1 a5 $1 4.h5 $1 a4 $1 5.h6 $1 a3 
$1 6.h7 $1 a2 $1 7.h8=Q 
    ( 7.h8=B )
7...a1=Q+ $1 8.Qxa1+ $1 Kxa1 $1 *
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    already pretty good! – Peter Mar 11 '15 at 23:17
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do you mean this position?

2Q5/8/8/8/3n4/8/1b6/k2K4 b - - 0 1
  • No, BOTH sides must have only one move to draw. – Peter Mar 11 '15 at 19:46
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Here is my offering from what I found here on Matplus. This position has been kindly given by Arpad Rusz. This is a 5-men game with an 8-move sequence. So more surely exists with 6-men.

This extends ETD's answer by by two moves. While their are promotions before the end of the game, I think that as long as only one type of promotion works, that it counts as the only drawing move.

Now here is the game. It stops where it does because repetition will occur afterwards, and there are no more only drawing moves for White.

[FEN "8/1K4pk/5p2/8/8/8/P7/8 w - - 0 1"]

1. Kc6 g5 2. Kd5 Kg6 3. a4 Kf5 4. a5 g4 5. a6 g3 6. a7 g2 7. a8=Q g1=Q 8. Qc8+ Kf4

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