Let an ambiguous rule be defined to be a rule experienced, reasonable, well-informed opponents

  • are unsure how to interpret in a circumstance (however unlikely the circumstance may be) that can legally arise during play, or
  • are sure how to interpret, except that each opponent interprets the rule oppositely.

Answers to earlier questions (here and here) have found the FIDE's 50-move rule to remain ambiguous according to the foregoing definition. I do not say that the rule remains ambiguous according to alternate definitions of ambiguity, but according to the foregoing definition I ask: is the 50-move rule the sole remaining such plausible ambiguity in the FIDE's laws of chess, or are there others?

If there are others, what are they, please?


To clarify, I do not seek opinions, but seek rather insights and facts. Players indeed have opinions or my question would not exist, but I do not ask what those opinions are. Rather, I ask the fact-based question of which rules have found (or can find) themselves to be objects of opposite interpretation.

I do not ask which interpretation is right.

  • All rules regarding the conduct of players/disturbing the opponent are (necessarily) rather vague and might be interpreted differently by different people. – user1583209 Jan 28 '18 at 23:16
  • Your definition of ambiguous is nonsense as the recent illegal move claim by Inarkiev versus Carlsen demonstrates. The illegal move rules are unambiguous as are the 50 move rule (your example) and 75 move rule. – Brian Towers Jan 29 '18 at 10:53
  • @BrianTowers: not so and not so. Regarding the latter, have I not documented the assertion? – thb Jan 29 '18 at 17:52
  • @user1583209: you are right, of course. I was thinking of game mechanics, for example a protocol by which two AIs could play without possibility of rules-related deadlock; but your point is well taken. – thb Jan 29 '18 at 17:57
  • The question is attracting quality answers but is disliked by some readers. Can you think of a way to reword it? – thb Jan 30 '18 at 22:35

Good question!

As of Jan 2018, the FIDE rules are a lot clearer than they used to be. I am particularly happy with 3.10 (which arrived in 2014 I think), which defines legal moves, illegal moves and illegal positions. (Curiously, legal positions are not defined, but I can we can safely induce what they must be from the other three definitions.)

I would like a term for what a legal move becomes when it is unplayable because the game has been terminated (through dead position, repetition, whatever... basically any cause of termination apart from mate & pat, when by definition no legal moves remain). But that's not really an ambiguity, just a lack of standard terminology.

One area of historical ambiguity has been largely cleared up: and that's the impact of castling and en passant rights on draw by repetition. Residual confusion is perhaps because the basis is different between castling and en passant. Castling rights depend purely on whether a king or rook has been moved or captured. There is no investigation of the possibility of actually castling. On the other hand, en passant status depends on whether the capture can actually be executed, or whether it is prevented by some direct or indirect pin or check. This distinction (which biologists might term "genotypic" versus "phenotypic") is completely rational, but I think that some people expect castling and en passant to be handled in the same way.

A final corner-case cobweb in this area remains: suppose king and rook have not moved, and then the rook is captured. Does the relevant castling right remain? says "The castling rights are lost only after the king or rook is moved." This could actually have an impact.

There is the famous issue of checkmate on the 50.0th non-capturing, non-pawn move. To me it is clear that the player has the choice over the board to draw if they want to, just as a player is allowed to resign in a position which is clearly not lost, or offer a draw in a clearly won position. I don't see why an arbiter should feel the need to interfere in the first case, but not in the other two.

Two other lacunae, other posters have competently detailed, but I will list for completeness:

(1) Both players resign (race condition).

(2) No-one has the move at game start.

In summary, as a problemist, the main change I would still like to see in the FIDE Laws is the completion of the partition into (1) rules of play, which are defined mathematically, requiring no arbiter (2) over the board conduct, about embedding the mathematical object that is a game of chess into the real world, and where the arbiter's judgement rightly holds sway.


This is purely academical, but there is an ambiguity whether the position after 1.Nf3 Nf6 2.Ng1 Ng8 is the same as the position at the beginning of the game.

Indeed, article 9.2.2 of the laws of chess states that:

Positions are considered the same if and only if the same player has the move, pieces of the same kind and colour occupy the same squares and the possible moves of all the pieces of both players are the same.

Indeed, the second and third requirements are met. The problem is with the first one. After 1.Nf3 Nf6 2.Ng1 Ng8 there is no doubt that white "has the move" since her opponent's move has just been made, and law 1.3 defines "having the move" as:

A player is said to ‘have the move’ when his opponent’s move has been ‘made’.

But who "has the move" in the starting position ? If we follow textually the rules of chess and the definition above, noone "has the move" in the starting position, since noone's opponent's move has been made.

Law 1.2 just says:

The player with the light-coloured pieces (White) makes the first move

This confirms that the possibilities after 1.Nf3 Nf6 2.Ng1 Ng8 are exactly the same as in the starting position, but doesn't define "having the move" in the starting position. So, can we say that the two positions are identical in respect to article 9.2.2 ?

Well, that's where the ambiguity lies.

Obviously, this problem can only be relevant if someone claims for a threefold repetition when reaching a position 'identical' to the starting position. Which shouldn't happen too often. And such a claim might be denied anyway for lack of affrontment. So I suppose this should not prevent us to play chess with a free mind.


Both opponents have resigned in OTB game.

  • Yes this is a clear race condition. – Laska Apr 6 '18 at 8:58

I'm using the rules of january 2018.

There are many ambiguities left, and recently more have been introduced mostly by calling some things an "illegal move" that weren't considered illegal moves before, like using two hands to castle, seemingly without really considering all the implications of that.

It is in general legal to reply to an opponent's move before the opponent has pressed the clock, that's always been fine and allowed by the rules (mostly because of increments, it's important that the opponent still presses the clock, and then you press it).

So say player A castles using two hands, player B touches a piece to move it but then remembers what A does is wrong and he lets go of the piece. Then player A presses the clock.

Rule 4.1 says that you can't use two hands:

4.1 Each move must be played with one hand only.

Rule 4.8 says you can only complain about that right then, not later when you've started to move:

4.8 A player forfeits his right to claim against his opponent’s violation
of Articles 4.1 – 4.7 once the player touches a piece with the intention
of moving or capturing it.

Now the new 7.5.4 says that once the move is completed (clock pressed) then it counts as an illegal move and will be penalised as such:

7.5.4 If a player uses two hands to make a single move (for example in
case of castling, capturing or promotion) and pressed the clock, it shall
be considered and penalized as if an illegal move.

That penalty can be severe, like a game loss in case of a second illegal move.

So which is it, is it irrelevant that the castling happened with two hands because a piece was touched and the game has moved on, or is it really bad and potentially a game loss?

In that moment when B has touched his piece but A hasn't pressed his clock yet, nothing can be fixed (because it's B's move, and how do you fix moving with two hands anyway), nothing can be claimed (because of 4.8). But A's clock is running and once he presses it he's completed an illegal move.


Another guy mentioned "Both opponents have resigned in OTB game." as an answer.

I want to comment on that but I don't have enough points.

I want to say that this is not a matter of law, but a matter of fact.

2 opponents can not resign at the same time. In fact, 1 of them will have resigned first, and that first resignation will mean that the other player is the winner.

However, I can see how it could become a question of ambiguous fact. One possibility is that the black player may mark his scoresheet as 1-0 (therefore resigning in law), while the white player also marks on his scoresheet 0-1 (also resigning). The two players themselves may not even be aware of who resigned first, but there will be in fact a first resigner.

Since the only question here is the facts, this can not be "an ambiguity in the laws of chess". The laws are clear, but when the facts are unclear, it can be difficult to apply the laws for sure.

  • 2
    I think that this is a good answer. The reason your answer is significant may be nonobvious to players who have not implemented a computer chess interface or the like, but I appreciate the answer, at least. One only regrets that I could not think of a more popular way to ask the question (for, if the question were more popular, then your answer would likely attract more upvotes). – thb Feb 1 '18 at 22:57
  • 1
    Just because the rule doesn't say what to do if both resign at exactly the same second (exactly the same microsecond if you want, but the same second should be enough for the arbiter to be unable to decide who was first) doesn't mean that this event cannot occur. If I say "I resign", you can actually say the same thing at exactly the same time. (to say it mathematically, "anteriority/posteriority is a partial order but not a full order on the collection of all events") – Evargalo Feb 7 '18 at 9:10
  • 3
    (Special relativity makes the partial ordering of event sequencing much more profound. Your attempts to resign just need to be separated by a spacelike interval). – Ben Millwood Feb 13 '18 at 18:25
  • 1
    I've witnessed a case where both players thought they were resigning at the same time, shook hands, and each reported a loss to their respective team captains. In the end it turned out that only of them had actually said "I resign" so he lost. – RemcoGerlich Apr 6 '18 at 7:56
  • 1
    @thb: first division of the Dutch national club competition, about ~2150 level. One player (A) sacrificed a piece, combination looked winning but had a huge hole in it that he saw immediately after playing it. Player A disgusted with himself. Opponent (B) thought for a long time. In the end A can't handle it anymore and sticks out his hand to resign, B immediately shakes it and mumbles "yeah, I resign" but A doesn't realize. Both players report a loss to their team leader. Eventually A won, B lost. – RemcoGerlich Mar 10 '19 at 20:05

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