user-1: In this position, the black move 1. ... Qc3 results in two consecutive forced moves, Kxc3 and Ke7:
2rkr3/2p5/2q5/8/8/8/3KP3/3R4 b - - 0 1

1... Qc3 2. Kxc3 Ke7

What is the longest sequence of mutually forced moves that can be constructed on a standard chessboard?

user15543: There are two definitions of forced in play here:

Forced as in it the only legal move.

Forced as in you are bound by the rules of chess to make a move, i.e. your king is in check or it is your turn to move.


While there is some overlap between the two definitions, they are still separate categories in general. Here are records for each one, with and without promoted pieces.
Each record shall be recorded in length by plies, or half-moves, Since a player has a choice of what piece to promote to, promotion is not considered a fully forced move under definition #1.
One Legal Move, No Promoted Pieces-15 Ply
[Title "Bernd Schwarzkopf, Feenschach 1980 On Page 13"]
[FEN "8/2p5/2p5/1pP5/nP3p2/PP2B3/1pp1p3/brkbK3 b - - 0 1"]
[startflipped ""]

1... fxe3 2. bxa4 bxa4 3. b5 cxb5 4. c6 b4 5. axb4 a3 6. b5 a2 7. b6 cxb6 8. c7 b5

Source: The Die Schwalbe Chess Problem Database
Additional Source: A PDF of the original source
One Legal Move, Promoted Pieces-18 Moves
[Title "Karl Scherer, Feenshach 1980 On Page 13"]
[FEN "BQ4R1/2Q5/3Q4/4Q1pp/5B1P/6QK/Rrrrrrrq/R4nk1 w - - 0 1"]

1. Qxh2+ Rxh2+ 2. Bxh2+ Rxh2+ 3. Qxh2+ Rxh2+ 4. Qxh2+ Rxh2+ 5. Qxh2+ Rxh2+ 6. Qxh2+ Rxh2+ 7. Rxh2 g4+ 8. Rxg4+ hxg4+ 9. Kxg4

Source: The Die Schwalbe Chess Problem Database
Additional Source: A PDF of the original source
Multiple Legal Moves, No Promoted Pieces-12 Ply
[Title "Noam D. Elkies, Tim Krabbe’s Website Diary Entry #267 2004 (Version By Me)"]
[FEN "KBk5/P1P1p2p/2PN3P/P6p/2p3rP/2P3pB/6P1/8 b - - 0 1"]
[startflipped ""]

1... exd6 2. a6 d5 3. Bxg4+ hxg4 4. h5 d4 5. cxd4 c3 6. d5 c2 7. d6

Source: Diary Entry #267
Multiple Legal Moves, Promoted Pieces-22 Ply
[Title "Alexey Khanyan, Tim Krabbe's Website Diary Entry #267 2008, Mate In 11"]
[FEN "4Q2Q/4r3/4n1n1/1bbK1krn/RR1RR1RR/2qn1R1n/4n1nN/Q3Q3 b - - 0 1"]
[startflipped ""]

1... Ng2f4+ 2. Rfxf4+ N2xf4+ 3. Rgxf4+ Nh3xf4+ 4. Rhxf4+ Ndxf4+ 5. Rxf4+ Nhxf4+ 6. Rxf4+ Ngxf4+ 7. Rxf4+ Nxf4+ 8. Rxf4+ Kxf4+ 9. Qee5+ Qxe5+ 10. Qaxe5+ Rgxe5+ 11. Qxe5+ Rxe5+ 12. Qxe5#

Source: Diary Entry #267

Here are records if you want each one to end in checkmate.
One Legal Move, No Promoted Pieces-11 Ply
[Title "@Peter, Chess Stack Exchange 3/11/2015, Mate In 6"]
[FEN "7k/q5Q1/p4PPK/6PP/8/5P2/8/8 b - - 0 1"]
[startflipped ""]

1... Qxg7+ 2. fxg7+ Kg8 3. f4 a5 4. f5 a4 5. f6 a3 6. f7#

Source: This CSE question.
One Legal Move, Promoted Pieces-16 Ply
[Title "Bernd Schwarzkopf After Karl Scherer, Feenschach 1980 On Page 13, Mate In 8"]
[FEN "1KN4r/QRRRRRRr/kq6/p1q1R3/PP1q1b2/4q3/5q2/1r4q1 b KQkq - 0 1"]
[startflipped ""]

1... Qxa7+ 2. Rxa7+ Qxa7+ 3. Rxa7+ Qxa7+ 4. Rxa7+ Qxa7+ 5. Rxa7+ Qxa7+ 6. Rxa7+ Qxa7+ 7. Rxa7+ Rxa7 8. b5+ Rxb5+ 9. axb5#

Source: The Die Schwalbe Chess Problem Database
Additional Source: A PDF of the original source
Multiple Legal Moves, No Promoted Pieces-14 Ply
[Title "Noam D. Elkies, Tim Krabbe’s Website Diary Entry #267 2004 (Version By Me)"]
[FEN "KBk5/P1P1p2p/2PN3P/P6p/2p3rP/2P3pB/6P1/8 b - - 0 1"]
[startflipped ""]

1... exd6 2. a6 d5 3. Bxg4+ hxg4 4. h5 d4 5. cxd4 c3 6. d5 c2 7. d6 c1=Q 8. d7# 

Multiple Legal Moves, Promoted Pieces-See the above Alexey Khanyan problem.

user2299: You mean like this?
[Title "Vilhelm Röpke, Skakbladet 1942, Mate In 6"]
[FEN "K1k5/P1Pp4/1p1P4/8/p7/P2P4/8/8 w - - 0 1"]

White mates in 6.
I guess that's 9 consecutive forced moves. It would be eleven except for black's choice of promotion piece on his fifth move. I don't know if it's a record, and I don't know who composed this classic chess problem.
P.S. Thanks to Rosie F. for sourcing the problem. Quoting Rosie F.'s comment:

This problem is by Vilhelm Röpke. PDB cites it as Skakbladet, 1942.


user8860: A simpler infinite-loop setting:
[FEN "1kb5/1p1p4/1P1P4/8/8/4p1p1/4P1P1/5BK1 w - - 0 1"]


user2299: Inspired by Ed Dean's now deleted answer, here is another "infinite loop.”
[FEN "8/6p1/1p3pPk/1P3Pp1/1Pp3p1/KpP3P1/1P6/8 - - - 0 0 "]


user11144: Assuming you allow promoted material (since you didn't say anything :-),
this (on Page 13 of the PDF) is the (unfortunately, extremely unknown) finite record since ages. It shows the record for the longest sequence of only 1 legal move for each side, with use of promoted material.
[Title "Karl Scherer, Feenshach 1980, Page 13"]
[FEN "BQ4R1/2Q5/3Q4/4Q1pp/5B1P/6QK/Rrrrrrrq/R4nk1 w - - 0 1"]

1. Qxh2+ Rxh2+ 2. Bxh2+ Rxh2+ 3. Qxh2+ Rxh2+ 4. Qxh2+ Rxh2+ 5. Qxh2+ Rxh2+ 6. Qxh2+ Rxh2+ 7. Rxh2 g4+ 8. Rxg4+ hxg4+ 9. Kxg4 Kxh2


-1: <p>In this position, the black move 1. ... Qc3 results in <strong>two consecutive forced moves</strong>, Kxc3 and Ke7:</p>

<pre><code>2rkr3/2p5/2q5/8/8/8/3KP3/3R4 b - - 0 1

1... Qc3 2. Kxc3 Ke7
</code></pre>

<p>What is the longest sequence of mutually forced moves that can be constructed on a standard chessboard?</p>
 15543: <p>There are two definitions of forced in play here:</p>
<ol>
<li><p>Forced as in it the only legal move.</p>
</li>
<li><p>Forced as in you are bound by the rules of chess to make a move, i.e. your king is in check or it is your turn to move.</p>
</li>
</ol>
<p>While there is some overlap between the two definitions, they are still separate categories in general. Here are records for each one, with and without promoted pieces.</p>
<p>Each record shall be recorded in length by plies, or half-moves, Since a player has a choice of what piece to promote to, promotion is not considered a fully forced move under definition #1.</p>
<p>One Legal Move, No Promoted Pieces-15 Ply</p>
<pre><code>[Title &quot;Bernd Schwarzkopf, Feenschach 1980 On Page 13&quot;]
[FEN &quot;8/2p5/2p5/1pP5/nP3p2/PP2B3/1pp1p3/brkbK3 b - - 0 1&quot;]
[startflipped &quot;&quot;]

1... fxe3 2. bxa4 bxa4 3. b5 cxb5 4. c6 b4 5. axb4 a3 6. b5 a2 7. b6 cxb6 8. c7 b5
</code></pre>
<p>Source: <a href="https://pdb.dieschwalbe.de/P1374771" rel="nofollow noreferrer">The Die Schwalbe Chess Problem Database</a></p>
<p>Additional Source: <a href="http://problem64.beda.cz/silo/feenschach51_1980.pdf" rel="nofollow noreferrer">A PDF of the original source</a></p>
<p>One Legal Move, Promoted Pieces-18 Moves</p>
<pre><code>[Title &quot;Karl Scherer, Feenshach 1980 On Page 13&quot;]
[FEN &quot;BQ4R1/2Q5/3Q4/4Q1pp/5B1P/6QK/Rrrrrrrq/R4nk1 w - - 0 1&quot;]

1. Qxh2+ Rxh2+ 2. Bxh2+ Rxh2+ 3. Qxh2+ Rxh2+ 4. Qxh2+ Rxh2+ 5. Qxh2+ Rxh2+ 6. Qxh2+ Rxh2+ 7. Rxh2 g4+ 8. Rxg4+ hxg4+ 9. Kxg4
</code></pre>
<p>Source: <a href="https://pdb.dieschwalbe.de/P1303797" rel="nofollow noreferrer">The Die Schwalbe Chess Problem Database</a></p>
<p>Additional Source: <a href="http://problem64.beda.cz/silo/feenschach51_1980.pdf" rel="nofollow noreferrer">A PDF of the original source</a></p>
<p>Multiple Legal Moves, No Promoted Pieces-12 Ply</p>
<pre><code>[Title &quot;Noam D. Elkies, Tim Krabbe’s Website Diary Entry #267 2004 (Version By Me)&quot;]
[FEN &quot;KBk5/P1P1p2p/2PN3P/P6p/2p3rP/2P3pB/6P1/8 b - - 0 1&quot;]
[startflipped &quot;&quot;]

1... exd6 2. a6 d5 3. Bxg4+ hxg4 4. h5 d4 5. cxd4 c3 6. d5 c2 7. d6
</code></pre>
<p>Source: <a href="https://timkr.home.xs4all.nl/chess2/diary_14.htm" rel="nofollow noreferrer">Diary Entry #267</a></p>
<p>Multiple Legal Moves, Promoted Pieces-22 Ply</p>
<pre><code>[Title &quot;Alexey Khanyan, Tim Krabbe's Website Diary Entry #267 2008, Mate In 11&quot;]
[FEN &quot;4Q2Q/4r3/4n1n1/1bbK1krn/RR1RR1RR/2qn1R1n/4n1nN/Q3Q3 b - - 0 1&quot;]
[startflipped &quot;&quot;]

1... Ng2f4+ 2. Rfxf4+ N2xf4+ 3. Rgxf4+ Nh3xf4+ 4. Rhxf4+ Ndxf4+ 5. Rxf4+ Nhxf4+ 6. Rxf4+ Ngxf4+ 7. Rxf4+ Nxf4+ 8. Rxf4+ Kxf4+ 9. Qee5+ Qxe5+ 10. Qaxe5+ Rgxe5+ 11. Qxe5+ Rxe5+ 12. Qxe5#
</code></pre>
<p>Source: <a href="https://timkr.home.xs4all.nl/chess2/diary_14.htm" rel="nofollow noreferrer">Diary Entry #267</a></p>
<hr />
<p>Here are records if you want each one to end in checkmate.</p>
<p>One Legal Move, No Promoted Pieces-11 Ply</p>
<pre><code>[Title &quot;@Peter, Chess Stack Exchange 3/11/2015, Mate In 6&quot;]
[FEN &quot;7k/q5Q1/p4PPK/6PP/8/5P2/8/8 b - - 0 1&quot;]
[startflipped &quot;&quot;]

1... Qxg7+ 2. fxg7+ Kg8 3. f4 a5 4. f5 a4 5. f6 a3 6. f7#
</code></pre>
<p>Source: <a href="https://chess.stackexchange.com/questions/8789/what-is-the-longest-known-sequence-of-forced-moves-leading-to-a-checkmate-for-on">This</a> CSE question.</p>
<p>One Legal Move, Promoted Pieces-16 Ply</p>
<pre><code>[Title &quot;Bernd Schwarzkopf After Karl Scherer, Feenschach 1980 On Page 13, Mate In 8&quot;]
[FEN &quot;1KN4r/QRRRRRRr/kq6/p1q1R3/PP1q1b2/4q3/5q2/1r4q1 b KQkq - 0 1&quot;]
[startflipped &quot;&quot;]

1... Qxa7+ 2. Rxa7+ Qxa7+ 3. Rxa7+ Qxa7+ 4. Rxa7+ Qxa7+ 5. Rxa7+ Qxa7+ 6. Rxa7+ Qxa7+ 7. Rxa7+ Rxa7 8. b5+ Rxb5+ 9. axb5#
</code></pre>
<p>Source: <a href="https://pdb.dieschwalbe.de/P1303798" rel="nofollow noreferrer">The Die Schwalbe Chess Problem Database</a></p>
<p>Additional Source: <a href="http://problem64.beda.cz/silo/feenschach51_1980.pdf" rel="nofollow noreferrer">A PDF of the original source</a></p>
<p>Multiple Legal Moves, No Promoted Pieces-14 Ply</p>
<pre><code>[Title &quot;Noam D. Elkies, Tim Krabbe’s Website Diary Entry #267 2004 (Version By Me)&quot;]
[FEN &quot;KBk5/P1P1p2p/2PN3P/P6p/2p3rP/2P3pB/6P1/8 b - - 0 1&quot;]
[startflipped &quot;&quot;]

1... exd6 2. a6 d5 3. Bxg4+ hxg4 4. h5 d4 5. cxd4 c3 6. d5 c2 7. d6 c1=Q 8. d7# 
</code></pre>
<p>Multiple Legal Moves, Promoted Pieces-See the above Alexey Khanyan problem.</p>
 2299: <p>You mean like this?</p>
<pre><code>[Title &quot;Vilhelm Röpke, Skakbladet 1942, Mate In 6&quot;]
[FEN &quot;K1k5/P1Pp4/1p1P4/8/p7/P2P4/8/8 w - - 0 1&quot;]
</code></pre>
<p>White mates in 6.</p>
<p>I guess that's 9 consecutive forced moves. It would be eleven except for black's choice of promotion piece on his fifth move. I don't know if it's a record, and I don't know who composed this classic chess problem.</p>
<p><strong>P.S.</strong> Thanks to <a href="https://chess.stackexchange.com/users/10380/rosie-f">Rosie F.</a> for sourcing the problem. Quoting Rosie F.'s comment:</p>
<blockquote>
<p>This problem is by Vilhelm Röpke. <a href="http://pdb.dieschwalbe.de/search.jsp?expression=probid=%27P1180283%27" rel="nofollow noreferrer">PDB</a> cites it as <em>Skakbladet</em>, 1942.</p>
</blockquote>
 8860: <p>A simpler infinite-loop setting:</p>

<pre><code>[FEN "1kb5/1p1p4/1P1P4/8/8/4p1p1/4P1P1/5BK1 w - - 0 1"]
</code></pre>
 2299: <p>Inspired by Ed Dean's now deleted answer, here is another "infinite loop.”</p>

<pre><code>[FEN "8/6p1/1p3pPk/1P3Pp1/1Pp3p1/KpP3P1/1P6/8 - - - 0 0 "]
</code></pre>
 11144: <p>Assuming you allow promoted material (since you didn't say anything :-),
<a href="http://problem64.beda.cz/silo/feenschach51_1980.pdf" rel="noreferrer">this (on Page 13 of the PDF)</a> is the (unfortunately, extremely unknown) finite record since ages. It shows the record for the longest sequence of only 1 legal move for each side, with use of promoted material.</p>

<pre><code>[Title "Karl Scherer, Feenshach 1980, Page 13"]
[FEN "BQ4R1/2Q5/3Q4/4Q1pp/5B1P/6QK/Rrrrrrrq/R4nk1 w - - 0 1"]

1. Qxh2+ Rxh2+ 2. Bxh2+ Rxh2+ 3. Qxh2+ Rxh2+ 4. Qxh2+ Rxh2+ 5. Qxh2+ Rxh2+ 6. Qxh2+ Rxh2+ 7. Rxh2 g4+ 8. Rxg4+ hxg4+ 9. Kxg4 Kxh2
</code></pre>
