26

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?

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5 Answers 5

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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 "]
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6
  • 15
    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 9, 2014 at 20:01
  • 3
    Yes. It's in Article 1 of the rules, even ( fide.com/fide/handbook.html?id=124&view=article ). Infinite loops aren't really compatible with the laws of chess.
    – RemcoGerlich
    Mar 9, 2014 at 20:08
  • 3
    @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 9, 2014 at 21:26
  • 16
    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 9, 2014 at 21:44
  • 11
    Since the longest sequence of legal moves from this position is 0, perhaps it should not be the accepted answer!
    – M.M
    Aug 31, 2016 at 5:09
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
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5
  • 1
    Wow, that's quite a "machine".
    – supercat
    Sep 25, 2017 at 0:17
  • 5
    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, 2018 at 13:10
  • 1
    Can we get attribution in the answer?
    – hkBst
    Jan 18, 2019 at 9:57
  • 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 9, 2019 at 8:41
  • @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, 2019 at 13:38
12

A simpler infinite-loop setting:

[FEN "1kb5/1p1p4/1P1P4/8/8/4p1p1/4P1P1/5BK1 w - - 0 1"]
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4
  • 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) ​ ​ ​ ​
    – user2668
    Feb 22, 2016 at 4:39
  • True, and then wBf1 and the e2/e3 pawns are not needed.
    – Noam D. Elkies
    Feb 22, 2016 at 6:29
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    It would be the Bc8 and the d pawns that aren't needed, rather than the Bf1 and e pawns. ​ ​
    – user2668
    Feb 22, 2016 at 7:46
  • 1
    Sorry, you're right.
    – Noam D. Elkies
    Feb 22, 2016 at 14:30
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.

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4
  • I'm not sure c1=Q can be considered forced, you had other promotions
    – Alan
    May 21, 2014 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, 2014 at 19:52
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    This problem is by Vilhelm Röpke. PDB cites it as Skakbladet, 1942.
    – Rosie F
    May 26, 2016 at 5:37
  • 1
    @RosieF Thank you. I edited your comment into my answer.
    – bof
    May 26, 2016 at 5:57
1

There are two definitions of forced in play here:

  1. Forced as in it the only legal move.

  2. 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.

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3
  • 7
    My understanding of a forced move is that the player doesn't have a choice.
    – Evargalo
    May 6, 2019 at 14:19
  • 4
    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 9, 2019 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, 2021 at 9:14

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