user5784: 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.

user15543: In the Matplus.net forums, Arpad Rusz shared the longest such sequence in a 6-piece position. The length of it is 19 plies.
You can check the uniqueness for yourself by using the Sygyzy tablebases..
[FEN "3k4/8/8/p4pP1/3K3P/8/8/8 w - - 0 1"]

1. h5 Ke7 2. h6 Kf7 3. Ke5 Kg6 4. Ke6 f4 5. h7 Kxh7 6. Kf7 f3 7. g6+ Kh6 8. g7 f2 9. g8=Q f1=Q+ 10. Ke7


user167: 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 *


5784: <p>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) ?</p>

<p>There should not occur any repitition of a position.</p>

<p>I experimented with the endgame rook vs two (three) pawns and got some
 sequences I unfortunately do not remember.</p>
 15543: <p>In the <a href="http://matplus.net/start.php?px=1584295518&amp;app=forum&amp;act=posts&amp;fid=xshowm&amp;tid=2324" rel="nofollow noreferrer">Matplus.net forums,</a> Arpad Rusz shared the longest such sequence in a 6-piece position. The length of it is 19 plies.</p>
<p>You can check the uniqueness for yourself by using the <a href="https://syzygy-tables.info/?fen=3k4/8/8/p4pP1/3K3P/8/8/8_w_-_-_0_1" rel="nofollow noreferrer">Sygyzy tablebases.</a>.</p>
<pre><code>[FEN &quot;3k4/8/8/p4pP1/3K3P/8/8/8 w - - 0 1&quot;]

1. h5 Ke7 2. h6 Kf7 3. Ke5 Kg6 4. Ke6 f4 5. h7 Kxh7 6. Kf7 f3 7. g6+ Kh6 8. g7 f2 9. g8=Q f1=Q+ 10. Ke7
</code></pre>
 167: <p>To get the ball rolling, from the following position the first 6 moves for each side are forced in order not to lose. (If <code>7.h8=B</code> weren't available and <code>7.h8=Q</code> needed to be played, then the forced follow-ups <code>7...h1=Q+ 8.Qxa1+ Kxa1</code> would extend it to 8, but no cigar.)</p>

<pre><code>[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 *
</code></pre>
