Illegal moves by grandmasters
On this page, I explain the draw by "impossibility of checkmate", and show examples where it is violated even by grandmasters at major tournaments.
Draw by impossibility of checkmate
The draw is stated three times in the FIDE Laws of Chess:
| Article 1.3 |
If the position is such that neither player can possibly checkmate, the game is drawn. |
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| Article 5.2b |
The game is drawn when a position has arisen in which neither player can checkmate the opponent's king with any series of legal moves. The game is said to end in a 'dead position'. This immediately ends the game, provided that the move producing the position was legal. (See Article 9.6) |
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| Article 9.6 |
The game is drawn when a position is reached from which a checkmate cannot occur by any possible series of legal moves. This immediately ends the game, provided that the move producing this position was legal. |
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Example
Let's look at the game Shakhriyar Mamedyarov (2757) vs Viswanathan Anand (2786), played in Dortmund 2007, round 5.
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Position after 68.b6. This is a live position because there still exists a sequence of legal moves that would lead to a checkmate. For example, 68...Kc8 69.bxc7 Kb7 70.c8=Q+ Ka7 71.Bd4#. |
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Position after 68...Kxb6. This is a dead position because neither player can possibly checkmate (note how the two bishops are on squares of the same color). This immediately ends the game, as explained in articles 5.2b and 9.6. |
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Mamedyarov inexplicably plays 69.Kd4. This move is illegal because the game already ended, according to the rules of chess. |
At this point you might expect Anand to refuse the illegal move, or to see the arbiter rush to the stage to correct the mistake. No such thing happened. What about a subsequent media uproar? None. Apparently no one noticed, except Noam Elkies who posted about it on a chess problem mailing list.
Classification of dead positions
Although the draw by impossibility of checkmate rule is easy to state ("neither player can possibly checkmate"), it may help to think of all the ways this can happen.
- Insufficient Material (IM):
The position is one of the following:
- KK
- KNK
- KBK, KBKB, KBBKB, ... (any number of bishops from any player, as long as they all are on squares of the same color)
- Forced Insufficient Material (FIM):
The players have no choice but to reach a position with insufficient material.
- Forced Stalemate (FS):
The players have no choice but to create a stalemate.
- Stalemate:
Stalemate itself is handled separately in the rules (Article 5.2a), but it could also be considered a special case of "neither player can possibly checkmate".
- Blocked position:
Players can play moves indefinitely, but a pawn wall makes checkmate impossible.
- Other:
One can imagine more exotic positions, for example a position where players are forced to produce either insufficient material or a stalemate, but can choose which one. These are rare.
Computer search
To find more examples of illegal moves, I used my chess engine to test games from the ICOfY Base (version 2011.1) which is a free database of 4.2M+ chess games, and from some other sources. The draw by impossibility of checkmate rule went into effect on July 1st, 1997 (FIDE Laws 1996/1997, articles 1.3, 9.6), so I'm limiting my examples to year 1998 and later. Before that, the rules were different (FIDE Laws 1992/1993, article 10.4) and only positions with insufficient material immediately ended the game.
Below are all the games that I found from 1998-2011 where an illegal move was played by a 2700+ rated player (in bold). (Here's a PGN file with all the games if you prefer.)
(*) Game obtained from PGN Mentor (Linares2009.pgn), somehow missing from the ICOfY database.
Such illegal moves are particularly common with 2700+ rated players because recent top tournaments often forbid draws by mutual agreement, which means that players are more likely to reach an unfamiliar draw by impossibility of checkmate.
Statistics and records from ICOfY Base 2011.1
- Number of games with at least one illegal move:
537 games out of 2356085 since 1998, so 0.023% of games.
- Number of illegal moves:
642 plies out of 180592923 since 1998, so 0.00036% of plies.
- Longest sequence of illegal moves in a game:
18 plies in Mikhail Glukhov (2205) vs Ilia Shkurikhin (2417), Izmailov's Memorial 2009 open (Tomsk), round 9.
- Most illegal moves by the same player since 1998:
9 plies, Mikhail Glukhov (2205), tied with Ilia Shkurikhin (2417).
- Most games with an illegal move by the same player since 1998:
5 games, Juan Manuel Bellon Lopez (~2420), tied with Sang Cao (~2500).
Thanks
Special thanks to Andrew Buchanan for teaching me this rule in 2004, and to Noam Elkies for pointing out its violation in professional chess.
Further Reading
- You may be interested in Andrew's page about chess problems based on the draw by impossibility of checkmate (see the "Dead Reckoning" section).
- The forum thread Master Games with illegal moves gives examples of more elementary illegal moves, like not noticing a check, or accidentally misplacing a piece. See the comments as you play the games (or click on "PGN" to know where to look). I didn't find any such examples in the ICOfY database, most likely because the database has been sanitized of such moves.
page last updated: May 15, 2011
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