user315: 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?
FACTS VERSUS OPINION
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.

user28162: The longer you study the rules about game endings, the more ambiguities you find. As other answers have pointed out, three of the huge ones are:

The effect of writing down a checkmating move to claim a 50 move draw (Art. 9.2.1.1)
Whether the starting position counts for repetition draws (Art. 9.2.2)
What exactly "a pawn could have been captured en passant" means in the repetition draw rule (Art. 9.2.2.1)

Another question is whether resignation is legal in a situation where you cannot be checkmated by any series of legal moves. The glossary at the end of the FIDE Laws, which is considered an integral part of the Laws, defines "resigns" as follows:
"Where a player gives up, rather than play on until mated."
On its face, this definition seems to imply that you can only resign if you can legally be checkmated. (Interestingly, the USCF rules also define resignation in a way that implies at least the possibility of checkmate). Now, maybe we shouldn't read any significance into this definition. Maybe it's just sloppiness and carelessness on the part of whoever wrote the glossary.
Or maybe not. In several other places, the FIDE Laws noticeably go out of their way to make sure that someone won't lose without a theoretical checkmate. Art. 6.9 states that you cannot lose on time if your opponent cannot legally checkmate you. Art. 7.5.5 protects you from losing when checkmate is impossible even if you make two illegal moves yourself. And the Quickplay Finish Guidelines go so far as to shield you from losing in situations where checkmate is merely unlikely!
So, I would say there's certainly a reasonable case for taking the glossary definition of "resigns" at face value.
Next, there are many issues with the draws in Art. 9.6.1 for fivefold repetition, and Art. 9.6.2 for 75 moves. First, if it turns out that checkmate cannot legally occur for either side prior to hitting the 5th repetition, or the 75th move, does that mean it would be an immediate 'dead position' draw under Art. 5.2.2, on the grounds that checkmate is impossible by any series of legal moves? And by the same principle, if a player who could not legally be checkmated prior to a 5th repetition or a 75th move runs out of time, are they awarded the draw under Art. 6.9?
Which leads right into the next question: if the arbiter doesn't notice the draw condition right away, are any moves made on the board after the 5th repetition or the 75th move considered 'valid' and real? Or would the draw be considered retroactive to the moment the 5th repetition or 75th move was hit, potentially nullifying a subsequent checkmate? On the one hand, 9.6.1 says "at least five times" and 9.6.2 says "at least 75 moves" - which seems to quietly suggest that any subsequent moves are indeed real and valid.
On the other hand, the 75 move rule says "if the last move resulted in checkmate, that shall take precedence" while the fivefold repetition rule conspicuously omits any mention of checkmate. Why would there be such an obvious discrepancy if subsequent moves are real and valid? The discrepancy points toward checkmate only counting on the exact 75th move, with any subsequent checkmate being ignored.
The 9.6.1 and 9.6.2 draws are a complete mess. They somehow get more confusing the more you try to understand them.

user15671: Both opponents have resigned in OTB game.

user-1: 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.

user924: 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.

user8201: Good question!
As of Jan 2018, the FIDE rules are a lot clearer than they used to be. However there are some minor issues.
(1) The wording is arguably unclear that passing is illegal!
Firstly, it's not stated that "making a move" means "moving one piece". This should be specified (and not in Article 4, which should really be a Competition Rule). Castling is properly defined as a king move, no issue there.
Secondly, it's not clear that the riders (queen, rook, bishop) can't move by staying on their current squares. E.g. 3.2 "The bishop may move to any square along a diagonal on which it stands." is missing the word "other". And I'm not sure that 3.1 "It is not permitted to move a piece to a square occupied by a piece of the same colour." is strong enough.
(2) I am generally 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. But in any case, illegality of position is only used in one obscurely narrow context:

Rapid Play A.4.4 If the arbiter observes both kings are in check, or a pawn on the rank furthest from its starting position, he shall wait until the next move is completed. Then, if an illegal position is still on the board, he shall declare the game drawn.

Maybe "illegal position" is defined just to allow it into the arbiters' vocabulary. The arbiters are given wide discretion under FIDE Laws.
However the Laws allow play in illegal positions. It's only a requirement for checkmate, for example, that the last move be legal. This is correct because it may be very hard to determine whether a position is legal. Also, there is a lot of interesting chess in illegal positions (e.g. wPb2, wBa1) so there's no reason to exclude them.
(3) 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 & stalemate, when by definition no legal moves remain). But that's not really an ambiguity, just a lack of standard terminology.
(4) 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? 9.2.2.2 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:
(5) Both players resign (race condition).
(6) 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 judgment rightly holds sway.

user14079: 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.

315: <p>Let an <em>ambiguous</em> rule be defined to be a rule experienced, reasonable, well-informed opponents</p>

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

<p>Answers to earlier questions (<a href="https://chess.stackexchange.com/q/12613/315">here</a> and <a href="https://chess.stackexchange.com/q/19710/315">here</a>) have found the FIDE's 50-move rule to remain ambiguous <em>according to the foregoing definition.</em> 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 <a href="https://www.fide.com/fide/handbook.html?id=208&amp;view=article" rel="noreferrer">laws of chess,</a> or are there others?</p>

<p>If there are others, what are they, please?</p>

<p><strong>FACTS VERSUS OPINION</strong></p>

<p>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.</p>

<p>I do not ask which interpretation is right.</p>
 28162: <p>The longer you study the rules about game endings, the more ambiguities you find. As other answers have pointed out, three of the huge ones are:</p>
<ol>
<li>The effect of writing down a checkmating move to claim a 50 move draw (Art. 9.2.1.1)</li>
<li>Whether the starting position counts for repetition draws (Art. 9.2.2)</li>
<li>What exactly &quot;a pawn could have been captured en passant&quot; means in the repetition draw rule (Art. 9.2.2.1)</li>
</ol>
<p>Another question is whether resignation is legal in a situation where you cannot be checkmated by any series of legal moves. The glossary at the end of the FIDE Laws, which is considered an integral part of the Laws, defines &quot;resigns&quot; as follows:</p>
<p>&quot;Where a player gives up, rather than play on until mated.&quot;</p>
<p>On its face, this definition seems to imply that you can only resign if you can legally be checkmated. (Interestingly, the USCF rules also define resignation in a way that implies at least the possibility of checkmate). Now, maybe we shouldn't read any significance into this definition. Maybe it's just sloppiness and carelessness on the part of whoever wrote the glossary.</p>
<p>Or maybe not. In several other places, the FIDE Laws noticeably go out of their way to make sure that someone won't lose without a theoretical checkmate. Art. 6.9 states that you cannot lose on time if your opponent cannot legally checkmate you. Art. 7.5.5 protects you from losing when checkmate is impossible even if you make two illegal moves yourself. And the Quickplay Finish Guidelines go so far as to shield you from losing in situations where checkmate is merely <em>unlikely!</em></p>
<p>So, I would say there's certainly a reasonable case for taking the glossary definition of &quot;resigns&quot; at face value.</p>
<p>Next, there are many issues with the draws in Art. 9.6.1 for fivefold repetition, and Art. 9.6.2 for 75 moves. First, if it turns out that checkmate cannot legally occur for either side prior to hitting the 5th repetition, or the 75th move, does that mean it would be an immediate 'dead position' draw under Art. 5.2.2, on the grounds that checkmate is impossible by any series of legal moves? And by the same principle, if a player who could not legally be checkmated prior to a 5th repetition or a 75th move runs out of time, are they awarded the draw under Art. 6.9?</p>
<p>Which leads right into the next question: if the arbiter doesn't notice the draw condition right away, are any moves made on the board after the 5th repetition or the 75th move considered 'valid' and real? Or would the draw be considered retroactive to the moment the 5th repetition or 75th move was hit, potentially nullifying a subsequent checkmate? On the one hand, 9.6.1 says &quot;<em>at least</em> five times&quot; and 9.6.2 says &quot;<em>at least</em> 75 moves&quot; - which seems to quietly suggest that any subsequent moves are indeed real and valid.</p>
<p>On the other hand, the 75 move rule says &quot;if the last move resulted in checkmate, that shall take precedence&quot; while the fivefold repetition rule conspicuously omits any mention of checkmate. Why would there be such an obvious discrepancy if subsequent moves are real and valid? The discrepancy points toward checkmate only counting on the exact 75th move, with any subsequent checkmate being ignored.</p>
<p>The 9.6.1 and 9.6.2 draws are a complete mess. They somehow get more confusing the more you try to understand them.</p>
 15671: <p>Both opponents have resigned in OTB game.</p>
 -1: <p>Another guy mentioned "Both opponents have resigned in OTB game." as an answer.</p>

<p>I want to comment on that but I don't have enough points.</p>

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

<p>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.</p>

<p>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.</p>

<p>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.</p>
 924: <p>I'm using <a href="http://www.fide.com/fide/handbook.html?id=208&amp;view=article" rel="nofollow noreferrer">the rules of january 2018</a>.</p>

<p>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.</p>

<p>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).</p>

<p>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.</p>

<p>Rule 4.1 says that you can't use two hands:</p>

<pre><code>4.1 Each move must be played with one hand only.
</code></pre>

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

<pre><code>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.
</code></pre>

<p>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:</p>

<pre><code>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.
</code></pre>

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

<p>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?</p>

<p>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.</p>
 8201: <p>Good question!</p>
<p><a href="http://www.fide.com/fide/handbook.html?id=208&amp;view=article" rel="nofollow noreferrer">As of Jan 2018</a>, the FIDE rules are a lot clearer than they used to be. However there are some minor issues.</p>
<p>(1) The wording is arguably unclear that passing is illegal!</p>
<p>Firstly, it's not stated that &quot;making a move&quot; means &quot;moving one piece&quot;. This should be specified (and not in Article 4, which should really be a Competition Rule). Castling is properly defined as a king move, no issue there.</p>
<p>Secondly, it's not clear that the riders (queen, rook, bishop) can't move by staying on their current squares. E.g. 3.2 &quot;The bishop may move to any square along a diagonal on which it stands.&quot; is missing the word &quot;other&quot;. And I'm not sure that 3.1 &quot;It is not permitted to move a piece to a square occupied by a piece of the same colour.&quot; is strong enough.</p>
<p>(2) I am generally happy with 3.10 (which arrived in 2014 I think), which defines legal moves, illegal moves and illegal positions.</p>
<p>Curiously, <em>legal</em> <em>positions</em> are not defined, but I can we can safely induce what they must be from the other three definitions. But in any case, illegality of position is only used in one obscurely narrow context:</p>
<blockquote>
<p>Rapid Play A.4.4 If the arbiter observes both kings are in check, or a pawn on the rank furthest from its starting position, he shall wait until the next move is completed. Then, if an illegal position is still on the board, he shall declare the game drawn.</p>
</blockquote>
<p>Maybe &quot;illegal position&quot; is defined just to allow it into the arbiters' vocabulary. The arbiters are given wide discretion under FIDE Laws.</p>
<p>However the Laws allow play in illegal positions. It's only a requirement for checkmate, for example, that the last <em>move</em> be legal. This is correct because it may be very hard to determine whether a position is legal. Also, there is a lot of interesting chess in illegal positions (e.g. wPb2, wBa1) so there's no reason to exclude them.</p>
<p>(3) 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 &amp; stalemate, when by definition no legal moves remain). But that's not really an ambiguity, just a lack of standard terminology.</p>
<p>(4) 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 &quot;genotypic&quot; versus &quot;phenotypic&quot;) is completely rational, but I think that some people expect castling and en passant to be handled in the same way.</p>
<p>A final corner-case cobweb in this area remains: suppose king and rook have not moved, and then the rook is captured. <a href="https://chess.stackexchange.com/questions/25113/can-you-castle-with-a-ghost-rook">Does the relevant castling right remain?</a> 9.2.2.2 says &quot;The castling rights are lost only after the king or rook is moved.&quot; This could actually have an impact.</p>
<p>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.</p>
<p>Two other lacunae, other posters have competently detailed, but I will list for completeness:</p>
<p>(5) Both players resign (race condition).</p>
<p>(6) No-one has the move at game start.</p>
<p>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 judgment rightly holds sway.</p>
 14079: <p>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.</p>

<p>Indeed, article 9.2.2 of the <a href="http://fide.com/fide/handbook.html?id=208&amp;view=article" rel="noreferrer">laws of chess</a> states that:</p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>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:</p>

<blockquote>
  <p>A player is said to ‘have the move’ when his opponent’s move has been ‘made’.</p>
</blockquote>

<p>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.</p>

<p>Law 1.2 just says:</p>

<blockquote>
  <p>The player with the light-coloured pieces (White) makes the first move</p>
</blockquote>

<p>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 ? </p>

<p>Well, that's where the ambiguity lies.</p>

<p>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.</p>
