This has been argued here if black's 20th move checkmate counts as a checkmate. Here is the answer:

http://ikorid-cv.blogspot.com/2011/05/clarification-on-ice-age-chess-rules.html

I think 3rd check is equivalent to mate and mate is finishing game so no threefold repetition.

grenv: No dissent here, it is exactly what I meant.  

Justaminute: But then it is very possible the third check occurs in the same time as the third repetition of the position.  

I believe it should work this way:

position also includes number of checks each player has made.

3x repetition of position would have to take that into account.

therefore the only way this is a draw is 3x repetition of a position that isn't check... and only if there has been no intervening check between positions.

Anyone disagree? I invite dissent.

Justaminute: Actually currently the most common cause is that two chess bigshots find it in their common interest to have a draw outcome, then go through the motion of having such a repetition early in the game to avoid the wrath of the referees.

nabla:
I know. I was pointing out the usual way of arriving at three fold repetition of position is through perpetual check which would result in a win for the checker before three fold repetition of position occurred in three check chess. The other most common way of repeating a position in normal chess is in the endgame where there is limited material and fixed pawn positions. It is quite possible for three fold repetition of position to occur in these circumstances in three check chess but I suspect it is even rarer than normal chess. I was wondering whether anyone had a practical example or was this just a theoretical discussion. A theoretically drawn endgame in normal chess like bishop and king v king is perhaps where it would occur.

Justaminute: There is no rule about repetition of moves in chess, only about three repetitions of the same position.

rabbitoid: You are right of course, I had forgotten about that ! A word in the rules wouldn't hurt though.  

rabbitoid: I would be interested to know if this ever occurred in practise though. You can’t get it by perpetual check as the third check occurs after only 1.5 repeats of moves. The other way this usually occurs is in long drawn out endgames where one player is trying to make something of a small advantage. It could happen in these circumstances but I would think it was very unlikely.

nabla: As I recall the repetition rules were never implemented in the code here, you have to ask the admin (Fencer) explicitly to end a game.
The reasons were: first, it's very complicated to implement, since repetitions may concern moves that are not consecutive; second because that's how it's done in regular chess tournaments anyway: a player has to inform a referee that a 3rd repetition has occurred. And now you have discovered a third reason: the rule doesn't necessarily make sense for variants :)

ughaibu: The obvious answer is that the same position with a different number of checks given beforehand is in fact not the same position, so the repetition rule should not apply. Good question though since it is unclear whether the written rules mean that, and whether the implementation matches that. 

What's the result, in Three Checks Chess, if the third check repeats the position for the third time?

DarwinKoala: yes, but diagonal counts as touching too

Resher:

Perhaps I also misunderstand "touch" - I assume that to be an adjacent square?

DarwinKoala: Each piece has to touch another piece, but not necessarily an opponent's piece (except black's first and last moves).  I needed to read the rules carefully to understand this - I agree it could be expressed a little more clearly.

Cheversi (aaru vs. AlterMann)

I thought that a placed piece has to touch an opponents piece?

Thad: Using just one monitor can be done, but it would be a major hassle just making sure you didn't accidentally show your opponent your view of the game. You might even be able to use the same browser, but I'm thinking probably not with cookies and passwords and all.

I used two separate computers logged into two accounts on It's Your Turn (IYT) about seven years ago to try it. Unfortunately my opponent didn't like Dark Chess and I had no one else to try it with. This game is on IYT as Walter_Montego versus Walter Montego. I had accidentally created the account when I didn't notice the underline was missing and somehow ended up with two accounts. It is the only game I ever used the second account for.

If you don't think it would be a hassle getting up from the monitor after you make your move, making sure to blank the screen, and then have your opponent take over for his turn, and them keep switching back and forth the whole game, I am not sure what you would think is tedious. It'd be a lot easier to use two monitors hooked to one computer. Your idea will work. Give it a try and see how well it goes.

Walter Montego: Can you play using two different browsers signed in to two different accounts on one computer to play live? Players would have to manually manage minimizing their window when they finished their turn, but it'd be better than nothing and not too tedious.

DarwinKoala: It is true that we can play Dark Chess using BrainKing, but that is not what I want nor what I am talking about. I have tried to use BrainKing to play Dark Chess live. It is tedious, though it works. Besides having to log into the site and having to use an internet connection to do so, and hopefully the site is up and running when you want to try it, there's plenty of other things that just having the program on my own computer would solve. It'd be lots more user friendly in many ways. And it would not tie up BrainKing resources just for the one game either. Extra monitors will work, but even one extra monitor should be big enough to show all three views of a game in progress. And I can envision a type of overlaying the display with all three views superimposed on each other too.

Since I did miss the obvious thing about this that rabbitoid points out and that is such a program does indeed exist. The very program that BrainKing uses. Perhaps Fencer could let me have a copy of it or a modified version that I could use on my own computer to play Dark Chess at home? I had not thought of this. I will write him and ask in a few days so as to give him some time to respond to it here in the discussion board. It would have to come ready to use as I do not know how to program computers or change program code to do it myself.

Walter Montego:
As rabbitoid noted: Brainking already handles the logic. "Lock Screen" works as a blanking tool when the player leaves the keyboard (this has an added overhead of either using own laptops or setting up user accounts (or a uer account) so users can control passwords.

Dual monitors can be used to display both views of the board to spectators - thus they can see it how the players see it!!

Walter Montego: That program exists. It's called brainking.com.

Pegniar: Thank you for the link. A small correction is needed as it is Kevin Hill that invented the Embassy Chess and you have it as K. Hill.

I am curious if you will make a program to play Dark Chess? I mean a program that will let two people play Dark Chess, not a program to play Dark Chess against, though that'd be cool too though to make it fair you'd have to have the board set up elsewhere and tell the machine what it sees as I do not trust the inner workings of the machine to not cheat and be looking at the position. It is this that makes Dark Chess different than other forms of Chess, the players have restricted information and different views of the board. Seeing the other person's view ruins the game. That is why when you view a game in progress here on BrainKing the board is blank for everyone but the players. You'd have to have a secure blank button on the program I want created to cover such looking while an opponent left the room for some reason or even just an accidental viewing that neither player would want to have happen.

I would like to play Dark Chess live against a person in person. It'd take two monitors hooked to one computer. The computer would not have to know anything about playing, just keep track of the pieces and displaying the correct information to each player. If the monitors were placed back to back, you could almost have a game like regular Chess with the feel of playing Battleship and hidden information. A third monitor away from the players could show all three views of the board and let kibitzers follow along with the action. Embellishments to such a program would be different displays a player might want, 2D, 3D, Chess clock, speculative piece placement, and other things that I'm sure people that play the game have thought up, but just the basic game itself would be a great thing to have.

If you're interested in this and are not familiar with Dark Chess, please write me and we can set up a few games to let you see how it is played. There's many of us here that play Dark Chess that would like to be able to play the game at home or in a club, but there's not any available software for it, or the rules are not as they are here as it seems Dark Chess is hard to program just to display the game as compared to having the program play the game. "Lawless" has expressed much interest in someone making this a project. I'm sure we could help you test it out, make suggestions, and find bugs for you.

I'm glad to find a forum about chess variants. Please visit my page about chess variants. It contains both interesting historical variants and my own suggestions of "improvements" of chess (at least they are good for training purposes).
http://hem.passagen.se/melki9/chessvar.htm

Pedro Martínez: I agree with that. The implementation does seem backwards in this regard.

Seems like the ice age event happens simultaneously with the move... which is fine, but the rules should point that out.

joshi tm: Well, I would personally like Fencer to change the implementation of the game instead. In my opinion, checkmate should take precedence over the effects of an Ice Age. The Black delivered the checkmate with the 20th move, while the Ice Age occurred after the 20th move of the Black (as defined in the rules).

pedestrian: Whoa, overlooked that. What a crazy game, finally the mystery is solved.

Fencer can you add that to the rules section please?

joshi tm: The funny thing in Pedro's example is that White won because Black lost his king (it was isolated as a result of the ice age). That seems to indicate that the Ice Age happens before mate is registered.

That one does not count. It's again a direct hit checkmate, so there's no chance for the Ice Age to prevent mate anyway.

Thom27: ok well done anyway!

Justaminute: ooops! Indeed I made the game private, I didn't think about this. Is there a way to show the game anyway? If not, you have to trust me or try yourself...

(OTOH this is good luck: no one will see how I have bungled the presumably mating move #20)

Thom27: good, but the game seems to be private.

I have tested how it is implemented: Ice Age takes precedence over mate: game

(See also before move 20 of black )

wetware: The *test* for mate is after the 40th move.... but the *definition* of mate relies on knowledge of what the king *could* do next move.... therefore checkmating takes precedence in this case - it happens one move before the other event.

grenv: I'd still like to hear your answer to this question: to which position do you apply the test for mate?  Is it (my contention) to the position at the end of move 40--before the ice age?  Or is it to the position after the ice age adjustments--with White about to make move 41?  It's a clear question, I think...and crucial.

If you claim that the ice age adjustments between moves 40 and 41 must be taken into account, note those same ice age adjustments could iinclude a black king's being frozen to death at the same time that white's king is mated.  Or you'd be stuck claiming that you only consider the ice age consequences for some purposes that you choose, while ignoring other consequences of that identical event.

grenv: Oh, and one last comment... whether you know it or not, you are taking the possible next move into account when assessing if it's a checkmate.... otherwise it makes no sense. If you don't see that then it's no wonder you don't see my larger point.

You might google parallel lines in non-Euclidian geometry if you want some ideas on how interpretation of definitions should not be swayed by your experience and pre-conceived notions.

wetware:

from thefreedictionary.com - but all definitions are essentially the same

check·mate (chkmt)
tr.v. check·mat·ed, check·mat·ing, check·mates
1. To attack (a chess opponent's king) in such a manner that no escape or defense is possible, thus ending the game.

After move 40 an escape *is* possible if the ice age event would cause the king to avoid capture.

Honestly, you people think this is opinion - it isn't... we are trying to interpret the rules as written. I believe my interpretation is the only possible interpretation.

*Opinion* might be that this rule isn't a good rule and should be rewritten.

grenv:
"point 1... Move 40 is defined as being different in the rules... assuming that it should behave the same is plainly wrong."

Plainly (please read for yourself), there is no such definition of moves 20, 40, etc., being inherently "different".  But by rule something special does happen after each of those moves (assuming, of course, that the game does not actually end on any of those moves.)  I do hope you agree that a game can be ended by a black move 20, 40, etc.

"point 2... Actually my sequence is the same as yours, I just have a different definition of checkmate... my definition is;
"if the white king is in check and cannot legally move out of check on his next move, then he is checkmated." "

Checkmate ends a game.  There is no "next move".  Plainly, the mating move is the last move of a game.  So any definition that demands a reference to a "next move" must be doing so in a hypothetical way, as it refers to something counterfactual--it will not occur.  In my opinion, it's better to avoid that in a definition if that can reasonably be done.

Your definition appears to be:
"if
the white king is in check and could not legally move out of check next
move in an identical situation in a different variant, then he is
checkmated"

Plainly not.  My test for mate after Black's move 40 would be the same as the test applied after moves 21 through 39.  I think that would be the same evaluation applied in conventional chess.

And please tell me when you perform the test for mate: before the ice age?

pedestrian:
I agree playing a game would be the simplest way to clarify things. There is no right answer, only opinion. I expect it would take a long time to get a change made anyway. I doubt it is a high priority, particularly as Kleinme can't find a single case where this happened in practice. I can't start a new game at the moment but I'm hoping someone else willl.

grenv: There are three separate issues here. 

wetware:
point 1... Move 40 is defined as being different in the rules... assuming that it should behave the same is plainly wrong.

point 2... Actually my sequence is the same as yours, I just have a different definition of checkmate... my definition is;
"if the white king is in check and cannot legally move out of check on his next move, then he is checkmated."

Your definition appears to be:
"if the white king is in check and could not legally move out of check next move in an identical situation in a different variant, then he is checkmated"

I like my definition better.

grenv: I think that the crux of our disagreement is this: when does a test for mate occur?  I contend that a move that would--in a given position--suffice to produce mate on any move numbered 21 through 39, would also produce mate on move 40.

I think the move 40 sequence is: (1) black plays a move, (2) one tests for mate, (3) if not mate, an ice age occurs, (4) one tests for frozen king(s).
I think your move 40 sequence is: (1) black plays a move, (2) an ice age occurs, (3) one tests for mate or frozen kings.

In your sequence only, it has become simultaneously possible for the white king to be mated and for the black king to be frozen--yet another situation not mentioned in the game description at chessvariants.org

nabla: In conventional chess, can I be checked (or even mated) by a pinned piece?  Or is that another optical illusion?

Justaminute: You are right, there are basically two ways to solve such questions.

1) Defining what is meant by check and checkmate in every new variant (even in Loop Chess, you will want to make clear that a ortho-checkmate from a distance can by parried by dropping a piece).

2) Going back to the axioms and work out what chess really means by check and checkmate, then keep those axioms as they are in every new variant bar some exceptions (three-check chess would be one).

I am not saying that 1) is stupid. Actually it is the way things are most often done. But even if you want every variant to include a (generally redundant) part about check and checkmate in its rules, it doesn't hurt to know that there is a default value given by 2).

Justaminute / Nabla: I believe that in Atomic chess you should be considered to be in check if a move of your opponents could blow up the king.... i think that is a mistake in the implementation... however it doesn't really matter or change the game too much.

In this case you're asking the wrong question. Checkmate occurs at the end of move 40 - However - the definition of checkmate is that the king cannot move out of check on his next move. Since the next move is move 41, the ice age event intervenes and must be taken into account...

I don't see how the interpretation could possibly be any other way unless it is explicit in the rules (which it isn't).

You can argue that it is a lop-sided game as a result if you like, but I don't think you can argue the interpretation;.

nabla: You only need to alter the rules to the extent of creating the new game, superfluous amendments don’t add anything except the ambiguity that you seek to avoid. You couldn’t pay three check chess without changing the definition of how the game ends. There is no need to do so in loop chess though. The only clarification of the rules that is needed in ice age chess is the assessment of checkmate occurs at the end of move 40 or the start of move 41. There is no need to refine checkmate in this game.

Bwild:

nabla: as soon as your opponent makes his 1st move

Justaminute: I don't see any reason to rewrite the FIDE rules.

Actually there is one. The FIDE rules are ambiguous when it comes to variants. When is a king "under attack" ? If I am playing my king in contact of the opponent's queen, but some extra rule forbids the queen to take my king (e.g. at that move my opponent is forced to move a knight), then is my king under attack or not ? If you would answer yes, I would argue that you are falling for an optical illusion.

Conversely, if my king is safe orthochess-wise, but can be captured through the way pieces are captured in that variant (for instance, in Atomic Chess, by an explosion), is my king under attack ?