Longest sequence of mutually forced moves

Question

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?

score 33 · Answer 1 · edited Apr 06 '20 at 21:30

33

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

edited Apr 06 '20 at 21:30

Rewan Demontay

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answered Aug 28 '16 at 20:06

Hauke Reddmann

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Wow, that's quite a "machine". – supercat Sep 25 '17 at 00:17
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To make that position reachable, black must previously have had the queen on h1 and just captured a white pawn, knight or rook on h2, which then starts the forced sequence. – Silas S. Brown Dec 31 '18 at 13:10
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Can we get attribution in the answer? – hkBst Jan 18 '19 at 09:57
Well, Karl Scherer himself :-) See: "feenschach" 51, 07-09/1980, S. 424 for a dead wood source. (Props to "apf" from schachfeld.de!) – Hauke Reddmann May 09 '19 at 08:41
@Rewan: Not regularly, neither on MatPlus :P feenschach is a German periodical for fairy chess, anybody at MatPlus could provide you with a scan if needed ('xept me - I'm doing virtually only orthodox 2# and don't have it). – Hauke Reddmann May 16 '19 at 13:38

score 17 · Accepted Answer · edited Apr 06 '20 at 21:20

17

Inspired by Ed Dean's now deleted answer, here is another "infinite loop.”

[FEN "8/6p1/1p3pPk/1P3Pp1/1Pp3p1/KpP3P1/1P6/8 - - - 0 0 "]

edited Apr 06 '20 at 21:20

Rewan Demontay

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answered Mar 09 '14 at 19:14

bof

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There we go! (And I deleted my answer, since it was flawed, as its infinite loop wasn't fully forced.) – ETD Mar 09 '14 at 19:18
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Although technically, since neither side can possibly checkmate the other here, it's an immediate draw (just like stalemate). So there aren't any legal moves at all here, the game is over. – RemcoGerlich Mar 09 '14 at 20:01
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Yes. It's in Article 1 of the rules, even ( http://www.fide.com/fide/handbook.html?id=124&view=article ). Infinite loops aren't really compatible with the laws of chess. – RemcoGerlich Mar 09 '14 at 20:08
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@Remco To be fair, you can have infinite loops as long as both players play along, because the 50 moves rule only says may be drawn - technicality obviously. – Voo Mar 09 '14 at 21:26
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No, this has nothing to do with the 50 moves rule. See rules 1.3 or 5.2.b in the rules (my link). Or even rule 9.6 -- the rules describe this situation three times! It's an immediate draw, no claims. – RemcoGerlich Mar 09 '14 at 21:44
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Since the longest sequence of legal moves from this position is 0, perhaps it should not be the accepted answer! – M.M Aug 31 '16 at 05:09

score 12 · Answer 3 · answered Nov 29 '15 at 23:18

12

A simpler infinite-loop setting:

[FEN "1kb5/1p1p4/1P1P4/8/8/4p1p1/4P1P1/5BK1 w - - 0 1"]

answered Nov 29 '15 at 23:18

Noam D. Elkies

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One can [put white pawns on a7 and f3, put black pawns on c6 and h2, and move black's king to a8] to get 9 consecutive only-legal moves. (without an immediate draw by dead reckoning; see Remco's comments to the other answer) – Feb 22 '16 at 04:39
True, and then wBf1 and the e2/e3 pawns are not needed. – Noam D. Elkies Feb 22 '16 at 06:29
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It would be the Bc8 and the d pawns that aren't needed, rather than the Bf1 and e pawns. – Feb 22 '16 at 07:46
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Sorry, you're right. – Noam D. Elkies Feb 22 '16 at 14:30

score 11 · Answer 4 · edited Jan 28 '21 at 23:35

11

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.

edited Jan 28 '21 at 23:35

Rewan Demontay

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answered Mar 09 '14 at 18:16

bof

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I'm not sure c1=Q can be considered forced, you had other promotions – Alan May 21 '14 at 18:33
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@Alan That's why I said "I guess that's 9 consecutive forced moves. It would be eleven except for black's choice of promotion piece on his fifth move." – bof May 21 '14 at 19:52
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This problem is by Vilhelm Röpke. PDB cites it as Skakbladet, 1942. – Rosie F May 26 '16 at 05:37
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@RosieF Thank you. I edited your comment into my answer. – bof May 26 '16 at 05:57

Rewan Demontay · Answer 5 · 2021-02-06T08:14:31.737

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"]

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.

My understanding of a forced move is that the player doesn't have a choice. — Evargalo, May 06 '19 at 14:19
I am still not sure what you mean, but I am certain this is not the usual definition of a forced move in chess. — Evargalo, May 09 '19 at 12:14
After your edit, I am happy to see many nice examples in your answer but I failed to understand the difference between your "one legal move" and "multiple legal moves" sections. In both cases, each player has only one legal move at each turn ? — Evargalo, Aug 23 '21 at 09:14

Longest sequence of mutually forced moves

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