19

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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15

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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  • 1
    There we go! (And I deleted my answer, since it was flawed, as its infinite loop wasn't fully forced.) – ETD Mar 9 '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 9 '14 at 20:01
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    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 '14 at 20:08
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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 9 '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 5:09
24

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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  • 1
    Wow, that's quite a "machine". – supercat Sep 25 '17 at 0:17
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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 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 '19 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 '19 at 13:38
9

You mean like this?

[FEN "K1k5/P1Pp4/p2P4/Pp6/P1p5/2P5/8/8 - - - 0 0 "]

1. axb5 axb5 a6 b4 cxb4 c3 b5 c2 b6 c1=Q b7# 1-0

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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  • 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 5:37
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    @RosieF Thank you. I edited your comment into my answer. – bof May 26 '16 at 5:57
7

A simpler infinite-loop setting:

[FEN "1kb5/1p1p4/1P1P4/8/8/4p1p1/4P1P1/5BK1 w - - 0 1"]
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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) ​ ​ ​ ​ – user2668 Feb 22 '16 at 4:39
  • True, and then wBf1 and the e2/e3 pawns are not needed. – Noam D. Elkies Feb 22 '16 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 '16 at 7:46
  • Sorry, you're right. – Noam D. Elkies Feb 22 '16 at 14:30
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 choice of what piece to promote to, promotion is not considered a fully forced move for the definition #1.

One Legal Move, No Promoted Pieces-15 Ply

[Title "Bernd Schwarzkopf, Feenschach 1980"]
[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 Schwalbe Database

One Legal Move, Promoted Pieces-18 Moves

[Title "Karl Scherer, Feenshach 1980"]
[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 Schwalbe Database

Multiple Legal Moves, No Promoted Pieces-12 Ply

[Title "Noam D. Elkies, Tim Krabbe’s Website 2004 (Version)"]
[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: Journal Entry #267

Multiple Legal Moves, Promoted Piecess-22 Ply

[Title "Alexey Khanyan, Tim Krabbe Website 2008"]
[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: Journal 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, Mate In 8"]
[FEN "1KN4r/QRRRRRRr/kq6/p1q1R3/PP1q1b2/4q3/5q2/1r4q1 b KQkq - 0 1"]

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 Schwalbe Database

Multiple Legal Moves, No Promoted Pieces-14 Ply

[Title "Noam D. Elkies, Tim Krabbe’s Website 2004 (Version)"]
[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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  • 2
    I do feel obligated to point out that there is a choice of queens on some of the moves, – D M Apr 8 '19 at 21:53
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    My understanding of a forced move is that the player doesn't have a choice. – Evargalo May 6 '19 at 14:19
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    Can you explicit what would be "the second type, where the move is forced, but there are choices." It seems self-contradictory to me. – Evargalo May 7 '19 at 12:35
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    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 '19 at 12:14

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