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codeulike parent
They dont really spell it out but they mean "from this position the player has 218 possible legal moves to choose from"
stavros
Ah, wow, I read the article wrong all this time, thank you. I thought they meant "the maximum number of moves you can make to reach any chess position is 218", and I was wondering why the article made no sense to me.
tromp
It's conceivable that the maximum number of plies (half-moves) you need is 218. The best known lower bound on needed number of plies is 185 for "Harry Goldsteen's furthest position" https://timkr.home.xs4all.nl/chess2/diary.htm So perhaps the hardest-to-reach position manages to improve on that by an additional 33 plies.
AceyMan
The word 'available' inserted at the right spot would make all the difference in clarity here.
NooneAtAll3
imo "possible" would be better
tshaddox
I'd go with "legal," which also implies possible.
bscphil
I thought the same, but no doubt pawn promotion rules dramatically increase the depth needed to reach certain positions.
kelipso
Is that weird? I feel like it’s plausible though. Very rare to have chess games with more than 200 moves.
stavros
Yeah I kind of thought that it meant "you can get to any valid position in 218 moves max", it was hard to parse.
OwlGoesHoot
I read it as “there is no legal position for which the minimum number of moves necessary to reach it is greater than 218” but I also did not read the whole article before coming to check the comments
refulgentis
It’s also rare to have one with more than 50 moves. I’m curious if this class of observation will help establish a true bounds. Especially because we don’t have a definition of what it means - my instinct is to first do that, so “infinity” isn’t the obvious upper bound.
kelipso
Proving this feels more difficult than proving what’s in the OP article, because here you have to show path lengths between original position and all possible positions have a max length, while OP article had to show all positions have a max degree. Path length just seems like a harder problem compared to node degree.
LorenPechtel
The upper bound is a few thousand. A game is considered drawn if no pawn has moved and no piece has been captured for 50 moves. And there's also the threefold repetition rule: if the same exact position (counting things like castling eligibility etc) occurs three times it's drawn.
chipsrafferty
I think the upper bound is 6300, then. Each pawn can move 6 times, times 16 pawns, times 50 moves before a game ending draw, plus each capturable piece that can delay the game another 50 moves (15 on each side)
tln
What? Lots of serious chess games last more than 50 moves

Not sure if I'm reading your comment correctly

The upper bound of moves is 8848.5 https://www.reddit.com/r/chess/comments/168qmk6/longest_poss...

... under the rule that after 75 moves without a capture or pawn advancement the game is drawn

refulgentis
You read a comment saying it’s rare to have more than 200 moves, then a reply noting more than 50 is also rare, then suggested you were confused and maybe it was unreadable and asked “What?” because…some games have above 50 moves. shrugs

Thanks for note re: upper bound with 75 moves without pawn advancing constraint.

tln
More than 50 moves in a game is hardly rare, hence my surprised reaction
KingLancelot (dead)
jwpapi
lol same here.

This is not so interesting then…

wodenokoto
I thought it was "How many moves in a game does it take to reach this position"
winternewt
I thought it was "there's no position in chess that requires more than 218 moves to reach."
unkulunkulu
I thought “there is no chess problem that is both reachable from starting position and requires more than 218 moves to solve”
binarymax
There are no more than 218 ways to interpret this title
fer
It'll take at most 218 comments on HN to figure out the meaning of the submission title
matheusmoreira
It'll take at most 218 characters to express the fact I did not understand the submitted article's title.
leoff
I seriously thought they meant "it's not possible to have a chess game with more than 218 moves"
ojo-rojo
Yeah, I'm still confused.
fwip
There is no reachable[1] chess position[2] at which a player has more than 218 valid moves[3] to choose from.

[1] Able to happen while following the rules of chess

[2] The arrangement of chess pieces on the board

[3] A valid move is the motion of one piece to a place on the board, which doesn't break the rules of chess - e.g: "King to E4."

coolness
I also thought this but OP is right: https://dev.timenote.info/de/Nenad-Petrovic

> In 1964 Petrović constructed a position with 218 possible moves for White.

jmkd
Same, which also damages my personal policy of reading the link before any HN comments.
stephenhumphrey
Huh. I also have that personal policy. Yet this time I jumped first to the comments before reading the article. I’m not certain why. Perhaps I subconsciously intuited that the ambiguity in the headline might be resolved by some of you smart people. Brains are weird; mine is, anyway.
jl6
The right™ order is to check the comments before the article, but to read the article before commenting.
weinzierl
But this an interesting problem too. More specifically is there an upper bound for the number of moves in a legal chess game?
dmurray
I believe 8848.5 under modern rules (with the 75-move rule, as in FIDE rules since 2014). There's a reasonably rigorous demonstration here:

https://wismuth.com/chess/longest-game.html

LegionMammal978
Thanks for that link! I was looking for that answer a few years ago, but I couldn't find anyone who had carried it out all the way through (with the cost of "switching control" fully accounted for), nor many people who were even aware of the 75-move rule.
LorenPechtel
75 move rule?! Did they go and change the 50 move rule or something, because I've never heard of it.
dmurray
The 50 move rule is still there: either player has the right to claim a draw after 50 moves without a pawn move or a capture. So the game can end then if either player wishes it to (and it almost always will end, because at least one player can expect no better than a draw).

After 75 moves, however, it's not optional, the game has ended. It's still a draw if the game subsequently "ends" in checkmate or a loss on time, though maybe not the players sign the score sheet, move on to the next round, etc.

sim7c00
how can the longest game have half a move???
InitialLastName
"Moves" are counted for both players. Half a move means White moved but Black hasn't yet.
sim7c00
thank you!
pxx
An A press is an A press. You can't say it's only a half.
unkulunkulu
there certainly is if you consider 50 moves rule.

And you can derive an easy upper bound from that as 50x8x8x2 (basically each 50 moves you make a pawn move)

if you only consider 3 moves repetition and not 50 move rule then this is harder and the number becomes one of those crazy combinatorical numbers.

CrazyStat
> And you can derive an easy upper bound from that as 50x8x8x2 (basically each 50 moves you make a pawn move)

This is not high enough, because the 50 move rule also resets when a piece is captured.

Certhas
Actually no. 50×(16×6 + 32) = 50×(16× 8) works I think. Every 50 moves, move a pawn or capture. There are 16 pawns. Each pawn can be moved 6 times, so there are 16×6 pawn moves available. In addition there are 32 captures available.
jibal
The 50 move rule is a rule of chess so it must be considered.

The 3 repetition rule is an opportunity for one of the players to declare a draw, but games can continue beyond that. The mandatory draw rule is 5 repetitions. In any case, the 50 move rule is far more limiting as to the number of moves in a game, since repetitions are necessarily neither pawn moves nor captures (the whole point of the 50 move rule being limited to those is that they are irreversible).

Scarblac
The 50 moves rule doesn't have be considered as it is optional. The players may claim, but they don't have to. So the game can continue.

The 75 move rule is the exact same thing but mandatory. That has to be considered.

(same thing is true for 5 times repetition vs 3 times).

Captures also reset the counter, not only pawn moves.

kevindamm
Not explicitly, but when you consider the "cannot repeat the same board layout three times" rule, the number of moves possible in a game does have a limit.
jibal
The 3 repetition draw rule has no bearing on the number of possible chess positions. And for the number of possible moves in a game the 50 moves with no capture or pawn move rule is a much more stringent limit.

BTW, the 3 repetition rule only comes into play is one of the players invokes it ... games can legally have more than 3 repetitions, but not more than 5 repetitions.

kevindamm
I didn't know it required a player to invoke it, I was basing the statement on chess implementations I've read (and written) where it kicks in automatically... but the 5-time limit you mention still supports my case that there's an upper limit. As long as the number of pieces remains the same, there are a finite number of arrangements for them so eventually (after a finite number of moves) a position would be repeated enough times. If a piece is captured (or converted) it resets this but still yields a finite number of new arrangements. Eventually you either cannot avoid the repetition, or a win condition is met, or a draw for insufficient material.

Compare this to, say, the L game, where the number of moves is unbounded.

jibal
Your "case" that there's a limit isn't in question ... as I said, the 50 move rule is a far more stringent limitation. And those 50 moves cannot include repetitions--they are captures and pawn moves, which are irreversible.

If you read my comment that you responded to carefully, you will find that it is precise and accurate--as I said, the repetition rule has no bearing on the number of positions.

This horse is dead, so I'm moving on.

Scarblac
The 50 moves rule also needs to be claimed by one of the players.

However there is a 75 move rule and a 5 time repetition rule that are both automatic (don't need to be claimed).

brumar
Same. I was astonished it was remotely possible to do this.
jibal
To do what?
brumar
To find "How many moves in a game does it take to reach this position". Which was a wrong interpretation.
BrenBarn
Yes, very strange that the phrase "possible moves" never occurs in the article. The key word is "possible". The article consistently just uses the phrasing "have moves" but this is not an obvious way of phrasing things to the average person (although I think it's more common in chess lingo).
unkulunkulu
in chess lingo the most common is “legal moves”
fsckboy
in chess lingo, most common is "moves"; only in a weird circumstance (beginners?) would you need to say "legal".

the "possible" qualifier would probably be used for an "english" reason rather than a "chess" reason, to suggest "future" moves as opposed to the moves already made to get to a position. it would be more likely for whatever reason to say "how many possible moves" than "how many future/hypothetical moves", i.e. the use of possible is not to rule out the idea of impossible, simply to mean how many "could you make now from a particular position" and/or i guess to suggest "possible initial moves" as opposed to future follow-on moves.

the ambiguity is not really in chess, it's in english (and probably every other language also)

BrenBarn
> the ambiguity is not really in chess, it's in english (and probably every other language also)

Sure, but the article is written in English. :-)

fsckboy
"moves" includes the idea of possible. "reachable" chess positions can only be the result of moves which are only those possible, and any follow-on moves would also only be those that are possible
BrenBarn
But if you say a position "has possible moves" (or "available moves") that unambiguously means moves that are possible from that position (i.e., future moves), whereas simply saying that a position "has moves" is ambiguous about whether those moves are in the past or the future.
arc-in-space
Ah, thank you. I was confused reading this article, thinking it's about the unique position that takes the most moves to reach.
WithinReason
Thanks, I misunderstood the entire article. Great writing!
sverhagen
How can those two things be true at the same time? Unless you appreciate misunderstanding what you read...?
burnished
What other reason would a person have for juxtaposing two statements?
absoflutely
You hit the nail on the head. They aren't both true which is why the second sentence is sarcasm.
Sharlin
How could someone ever write something while intending to convey the exact opposite meaning?
amitparikh
Right, and by "reachable" they mean it is theoretically possible to get to this board position through a normal (albeit obviously methodically chosen) series of moves.
Agingcoder
Yeah I gave up when I realized I didn’t understand what problem he was trying to solve
aqme28
I thought they meant that no game could go more than 218 moves. I can imagine some upper limit since three-fold repetition ends the game. But it’s a lot higher than 218.
hibikir
Another relevant rule is a draw after 50 moves without a capture or a pawn move. But yes, the maximum number of moves would be extremely large when both players are trying. Just think of a first 2 moved allowing the king out, an outrageous king march, followed by another pawn move...
amenghra
The confusion is perhaps caused by the word “reachable”. “No legal chess position with more than 218 possible moves” would have been more clear imho.
chankstein38
Thank you. I don't understand why people can't just explain what they're talking about before spending paragraphs talking about it. I thought this was like "After 218 moves there is no reachable chess position" which made no logical sense to me but I don't know enough about chess.
layman51
Oh, that makes more sense. That is an interesting thing to examine, but I wonder how useful it is. It reminds me of a tip I heard about improving at chess by actually counting all legal moves in a position so that you ensure you're not completely overlooking an option.
hx8
I had the same confusion, until they showed the existing 218 position and realized it was about maximizing white's legal moves.
amelius
If only they had appended "to choose from" to the article headline, it would have been clear from the start.
jibal
The caption on the first position is "Reachable chess position with 218 moves for White, published by Petrović in 1964."

And the title is unambiguous: "There is no reachable chess position with more than 218 moves" -- that cannot possibly mean "There is no reachable chess position that it takes more than 218 moves to reach". Also, lichess is a chess site, where people are certain to know that chess games can go way beyond 218 moves.

NooneAtAll3
the word "possible" is missing
nabla9
It should be obvious that there are no 8.7 × 10^45 possible chess moves from any chess position. All pieces have less than 32 moves per piece and 19 pieces means less than 608 moves.
electroly
No, they do mean possible moves and they don't mean maximum length game. There are on the order of 10^45 reachable chess positions. The article did not say that was the number of moves from one position. The article says 218 is the maximum number of moves from one reachable position--it's the whole point of the article!
jibal
The person you responded to knows that.
electroly
Their post was edited to remove the part I was responding to; the "do" in my post was directly correcting the "don't" in their original post, and vice versa.
davedx
But black has no king, so surely the game is over and there are no more legal moves?

Why does black have two pawns but no king?

I don’t understand any of this

electroly
Every board on the page, with the exception of the illegal "all queens" board, has a black king. The king is the one with the cross above the crown. In the first board, the 218 winner, it's at A1.
davedx
The first image, which I assume is the solution because the title is 'Reachable chess position with 218 moves for White, published by Petrović in 1964.', has no black king?
electroly
Look again. It's at A1. You may be confusing that symbol with a different piece.
davedx
Ugh. Sigh, I see it now - I thought that was the white king, because it has quite some white in it.

I will allocate 50% blame to my brain and eyes, and 50% to whoever designed a black king to have so much white in it :P Thanks!

jibal
No, it's 100% on you. If the piece on a1 is a white king, then what's the piece on f1? And why would there be no black king, when the subject is reachable (and thus legal) positions?

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