I think I'm starting to see after reading these replies and some of the linked material. Basically the things that confused me most about the rules of go when I first looked at it are playing a role in creating the attack surface: How do we decide to stop the game? How do we judge whether this (not completely surrounded) stone is dead? Why don't we play it out? Etc.

Most rulesets allow you to "play it out" without losing points. Humans don't do it because it's boring and potentially insulting or obnoxious.

Judging whether something "is dead" emerges from a combination of basic principles and skill at the game. Formally, we can distinguish concepts of unconditionally alive or "pass-alive" (cannot be captured by any legal sequence of moves) and unconditionally dead (cannot be made unconditionally alive by any sequence of moves), in the sense of Benson's algorithm (https://en.wikipedia.org/wiki/Benson%27s_algorithm_(Go) , not the only one with that name apparently). But players are more generally concerned with "cannot be captured in alternating play" (i.e., if the opponent starts first, it's always possible to reach a pass-alive state; ideally the player has read out how to do so) and "cannot be defended in alternating play" (i.e., not in the previous state, and cannot be made so with any single move).

Most commonly, an "alive" string of stones either already has two separate "eyes" or can be shown to reach such a configuration inevitably. (Eyes are surrounded points such that neither is a legal move for the opponent; supposing that playing on either fails to capture the string or any other string - then it is impossible to capture the string, because stones are played one at a time, and capturing the string would require covering both spaces at once.)

In rarer cases, a "seki" (English transliteration of Japanese - also see https://senseis.xmp.net/?Seki) arises, where both player's strings are kept alive by each others' weakness: any attempt by either player to capture results in losing a capturing race (because the empty spaces next to the strings are shared, such that covering the opponent's "liberty" also takes one from your own string). I say "arises", but typically the seki position is forced (as the least bad option for the opponent) by one player, in a part of the board where the opponent has an advantage and living by forming two eyes would be impossible.

Even rarer forms of life may be possible depending on the ruleset, as well as global situations that prevent one from reducing the position to a sum of scores of groups. For example, if there is no superko restriction, a "triple ko" (https://senseis.xmp.net/?TripleKo) can emerge - three separate ko (https://senseis.xmp.net/?Ko) positions, such that every move must capture in the "next" ko in a cycle or else lose the game immediately.

It gets much more complex than that (https://senseis.xmp.net/?GoRulesBestiary), although also much rarer. Many positions that challenge rulesets are completely implausible in real play and basically require cooperation between the players to achieve.

Sorry this is mostly way over my head, but perhaps you can explain something to me that puzzled me when I looked at go 50 odd years ago now.

(Please note, I absolutely do understand life requires two eyes, and why that is so, but my knowledge doesn't extend much further than that).

So hypothetically, if we get to the point where play normally stops, why can't I put a stone into my opponent's territory? I am reducing his territory by 1 point. So he will presumably object and take my "dead" stone off, first restoring the balance and then penalising me one point by putting the newly captured stone in my territory. But can't I insist that he actually surrounds the stone before he takes it off? That would take four turns (I would pass each time) costing him 4 points to gain 1. There must be a rule to stop this, but is it easily formally expressed? Or is it a) Complicated or b) Require some handwaving ?

> So hypothetically, if we get to the point where play normally stops, why can't I put a stone into my opponent's territory? I am reducing his territory by 1 point. So he will presumably object and take my "dead" stone off, first restoring the balance and then penalising me one point by putting the newly captured stone in my territory. But can't I insist that he actually surrounds the stone before he takes it off? That would take four turns (I would pass each time) costing him 4 points to gain 1. There must be a rule to stop this, but is it easily formally expressed? Or is it a) Complicated or b) Require some handwaving ?

There are multiple scoring systems (American, Chinese, and Japanese and a couple of others).

* In Chinese scoring, stones do NOT penalize your score. So they capture your stone and gain +1 point, and lose 0 points.

* In American scoring, passing penalizes your score. So you place a stone (ultimately -1 point), they place 4 stones (-4 points), but you pass a further 4 points (4x passes == -4 more points). This ends with -4 points to the opponent, but -5 points to you. Effectively +1 point differential.

* In Japanese scoring, the player will declare your stone dead. Because you continue to object the players play it out. Once it has been played out, time is rewound and the state of the stones will be declared what both players now agree (ie: I need 4 stones to kill your stone, if you keep passing I'll kill it).

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So your question is only relevant to Japanese scoring (in the other two systems, you fail to gain any points). And in Japanese scoring, there is the "time rewind" rule for post-game debate. (You play out positions only to determine alive vs dead if there's a debate. This is rarely invoked because nearly everyone can instinctively see alive vs dead).

IE: In Japanese scoring, the game has ended after both players have passed. Time "rewinds" to this point, any "play" is purely for the determination of alive vs dead groups.

In all three cases, playing out such a position is considered a dick move and a waste of everyone's time.

Thanks - I've had to do basically this exact explanation countless times before. It'd be nice if there were an obvious place to refer people for this info.

That said, when I teach beginners I teach them Chinese scoring and counting. If they understand the principles it'll be easy enough to adapt later, and it doesn't change strategy in a practical way. They can play it out without worry, it makes more sense from an aesthetic standpoint ("you're trying to have stones on the board and keep them there, or have a place to put them later" - then you only have to explain that you still get points for the eyes you can't fill).

It's also IMX faster to score on 9x9: you can shift the borders around (equally benefiting both players) to make some simple shapes and easily see who has the majority of the area and you aren't worrying about arranging territories into rectangles.

Than you, a longstanding mystery (for me) solved!

Amusingly, the endgame ritual is the same for all styles.

You play every good move. Then you traditionally play the neutral moves (+0 points to either player) to make counting easier. Then the game ends as both players pass.

In Chinese, American, or Japanese scoring, this process works to maximize your endgame score.