Opponent mirrors your moves

[PeterPeter]

Have you ever played a weaker player who thought the best way to play was to exactly mirror your moves?

How would you break the symmetry? Play on tengen, then start a fight there?

How long would you allow them to copy you before deviating from your usual game plan?

[SoDesuNe]

If you are White then just play your game. If he mirrors to the end, he will lose due to Komi, I think.
If you are Black, the easiest way to break symmetry is to play Tengen. "When" to break symmetry is a good question. I prefer to break it pretty soon because I am more comfortable then.

[mipli]

I personally dislike playing against opponents who play mirror go, but that is probably mostly because I'm not used to handling it. Met someone in a tournament that played mirror go, and I ended up playing on tengen after about 15 moves to stop it. It had been a moyo-ish opening up to that point, so tengen wasn't a horrible move but there was certainly better moves on the board.

If you look at Sensei's they have a page about mirror go. There you can also see a list of some pro games that was mirror go.

[PeterPeter]

If you are White then just play your game. If he mirrors to the end, he will lose due to Komi, I think.

Surely there comes a point where the board is so crowded that you capture a big group of his which stops him from playing the same move? As in, capture your opponent's group just in time to save your own.

I would get fed up long before that point, and probably resign and never play that person again. When playing a weaker player, it just seems like a cheap trick.

[PeterPeter]

Met someone in a tournament that played mirror go, and I ended up playing on tengen after about 15 moves to stop it. It had been a moyo-ish opening up to that point, so tengen wasn't a horrible move but there was certainly better moves on the board.

Who won the game? Was his strategy successful?

[amnal]

Surely there comes a point where the board is so crowded that you capture a big group of his which stops him from playing the same move? As in, capture your opponent's group just in time to save your own.

Well...the real idea here is to not change your play due to mirror go, but keep playing good moves such that you don't think breaking the mirror is good for the opponent. They almost certainly will break the mirror eventually, at which point it's a normal game.

This is the main general alternative to a tengen-oriented strategy where you let him mirror for a short time whilst constructing a position where tengen is a good move, at which point you play it. Of course, you have to be careful not to play it too late as well, or your opponent may find playing tengen to be better than continuing the mirror . Plus, this only works if you're black and white is mirroring; black can play tengen as his first move if he wants to mirror.

Other than this, there are a few specific strategies involving making the mirror an actively bad move that loses a fight somehow. For instance, if you are white and black starts on tengen then mirrors, you can play a sequence of moves around tengen that lead to capturing black's group a move before he captures your own. The problem with this kind of strategy is that the opponent can probably break the mirror and make your anti-mirror moves bad - they're too specific to be generally good moves. You can play to contrive a situation where it works fine and even breaking the mirror is good for you, but this is hard to read and set up properly - an example would be the idea of setting up opposing ladders but you still have to be careful to do it such that the opponent can't just ignore the mirror and play a better move. In real life if your opponent is carefully and actively reading, he's unlikely to let it get this far and will break the mirror earlier.

Mirror go has a professional history, it's not just an amateur trick. For instance, Fujisawa Hosai (the first 9 dan when the modern system was implemented, who famously lost against Go Seigen the second 9 dan in a long pair of jubangos) was well known for it. That's not to say it's necessarily good, but it's a strategy with potentially a lot of depth behind it.

[mipli]

Met someone in a tournament that played mirror go, and I ended up playing on tengen after about 15 moves to stop it. It had been a moyo-ish opening up to that point, so tengen wasn't a horrible move but there was certainly better moves on the board.

Who won the game?

In the end I won that game, but I think that was more due to the fact that I handled the invasions and reductions better. The tengen stone probably helped a bit since he used moves to capture it, but I won't claim that it was a very important stone or that it was a very important part of my strategy. I think I thought of the tengen stone as more of a strange probing move, I figured he had to think about as much as I did and since it stopped the mirror go I was content with that.

Was his strategy successful?

It was successful in that it annoyed me, and made me play a move I would normally not play (tengen early in the opening). But since I won the game in the end the strategy can't be said to have been a great success for him.

[SoDesuNe]

As long as neither of you played Tengen, he/you cannot capture a group without letting you/him also capturing one. That's mirror Go ; ) If Black has already a stone on Tengen then attaching is the simplest to break the mirroring.

But yeah, I also think mirroring is cheap.

[Dusk Eagle]

Some of my most fun games on KGS have been when I decided to play mirror Go against my opponent (normally when I'm black, taking tengen as the first move). Some people are fine with it, and it results in quite an interesting game. Others get irrationally upset, and provide some of the most memorable games I've played ever played online. I had one guy start insulting me after almost every move, escape, come back, insult me more, play moves that I had no choice but to mirror while threatening that I better not mirror him again, etc. Eventually the fight reached the center and he played a dubious move, and I broke the symmetry and won. He didn't say "thanks" afterward.

I guess I just kind of enjoy doing perfectly valid strategies that cause certain types of people to get easily upset. I do the same thing in random Team Fortress 2 games quite a bit (e.g. play medic and fight the enemy rather than healing my team).

I think if you're a half-decent Go player you should be able to handle your opponent playing mirror Go against you, unless you think the game of Go is fundamentally flawed. I personally have some prepared countermeasures if anyone ever plays mirror Go against me, though just coming up with something on the spot works as well. It's a valid way to play, so if you want to be strong you should be able to handle it.

[PeterPeter]

Mirror go has a professional history, it's not just an amateur trick. For instance, Fujisawa Hosai (the first 9 dan when the modern system was implemented, who famously lost against Go Seigen the second 9 dan in a long pair of jubangos) was well known for it.

It's fine for similarly rated players. I just don't like it when a weaker player uses it to play well above his strength for a large part of the game. Then it is like cheating. Like copying someone's paper in an exam .

[Dusk Eagle]

Not to insult anyone, but this discussion kind of reminds me of Day 9's discussion of the scrub vs the pro attitude (http://www.youtube.com/watch?v=9-9VLVkm8R4, starts around the 17 min mark but I don't have audio at the moment so I don't know the exact time). The gist of it is that the scrub holds on to unstated assumptions and expects their opponents to do the same, whereas the pro (which he uses in a slightly different sense than we do in the Go community) eliminates assumptions about how the game should be played and truly plays to win. If the "pro" believes that mirror Go offers them the best chance to win, then they'll play mirror Go without apology. [Edit: And if the pro is playing against someone who is mirroring them, they won't get upset that their unwritten assumptions are violated; they'll just search for the best moves.]

To make an analogy to Super Smash Bros Melee for those of you who have played that game, I used to get accused of "cheating" by certain weak players because when I knocked someone off the course, I would grab on to the ledge to prevent them from recovering. This is a very very basic tactic for anyone skilled at the game, but one that's not obvious to your ultra-casual player. The casual player holds this assumption that you should just let your opponent recover to the stage if they can and expects their opponent to return the favor, whereas a stronger player disregards that "etiquette" and does what gives them the best chance to win.

[Uberdude]

There are two quite different types of mirror Go, the one where black starts at tengen and then mirrors white, and the other where black plays normally and white mirrors black. Things are of course quite different if there is no komi, I will assume there is komi as has been normal for decades.

In the former, if white just plays normal good moves and black keeps on mirroring then if the centre becomes dame white will win due to komi. Thus the onus is on black to make the tengen stone count for more than komi. One strategy is to try use it as a ladder breaker/maker, another is to lead the game towards a moyo contest in which black can use his tengen stone to make his moyo bigger by more than komi.

In the latter, white is mirroring black and already has the komi advantage in the bank so if the board is split evenly white wins. If black just plays tengen then it stops white mirroring, but then it becomes like the first type and you have to make sure tengen counts for more than komi. Another standard idea as black is to make two ladders crash into each other in the centre which leads to disaster if white keeps mirroring, see the sgf below of a recent tournament game of mine in which my opponent started off mirroring but soon deviated when she presumably realised the trap (which has the nice property of arising from my standard 4-4 plus 3-4 opening). There are other similar ideas for black which, if white mirrors, lead to disaster but you need to be careful you don't play a bad move as part of the set up for the trap as white can always stop mirroring whenever she likes if you play a bad move.

http://www.lifein19x19.com/forum/download/file.php?id=2833

Also, another point about breaking the symmetry is it doesn't have to be tengen, but things nearby can also do it. The most famous exponent of mirror Go (as white), Fujisawa Kuranosuke/Hosai (the first 9 dan pro), was finally convinced it was duff by such a strategy, see these two very interesting GoGoD articles:

http://www.gogod.co.uk/NewInGo/MirrorGo_1.htm
http://www.gogod.co.uk/NewInGo/MirrorGo2_1.htm

As an example of white mirroring black, here's a 9x9 game where we agreed beforehand (as an experiment) that white would mirror black and black wasn't allowed to play tengen unless as part of a natural local sequence, so I had to find a way of making more than komi points with the symmetry collapse:

http://www.online-go.com/games/sgf/97260-27_Uberdude-vs-illluck.sgf

[skydyr]

If you want to study mirror go, whether to play it or to counter it, Fujisawa Kuranosuke, later known as Fujisawa Hosai, would frequently use it as white. There was a game in the 50s or early 60s in which his opponent, whom I can't recall, set out not only to beat him while he played mirror go, but to do it in a way that convinced Fujisawa of the futility of mirror go as a strategy. Apparently he succeeded, so again, this may be a good game to study. Hopefully someone here can recall which game it was.

[Uberdude]

If you want to study mirror go, whether to play it or to counter it, Fujisawa Kuranosuke, later known as Fujisawa Hosai, would frequently use it as white. There was a game in the 50s or early 60s in which his opponent, whom I can't recall, set out not only to beat him while he played mirror go, but to do it in a way that convinced Fujisawa of the futility of mirror go as a strategy. Apparently he succeeded, so again, this may be a good game to study. Hopefully someone here can recall which game it was.

Yamabe Toshiro. That is the topic of those GoGoD articles I linked to.

[Mef]

As long as neither of you played Tengen, he/you cannot capture a group without letting you/him also capturing one. That's mirror Go ; )

Actually you can have a one sided capture by making two rings of stones around the edge (alternatively 3 rings of stones around an arbitrary set of lines in the middle).

A quick and dirty example:

(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[7]KM[0.00]
PW[White]PB[Black]
;B[ga]
;W[ag]
;B[bf]
;W[fb]
;B[gb]
;W[af]
;B[be]
;W[fc]
;B[gc]
;W[ae]
;B[bd]
;W[fd]
;B[gd]
;W[ad]
;B[bc]
;W[fe]
;B[ge]
;W[ac]
;B[gf]
;W[ab]
;B[bb]
;W[ff]
;B[gg]
;W[aa]
;B[fg]
;W[ba]
;B[fa]
;W[bg]
;B[cf]
;W[eb]
;B[ea]
;W[cg]
;B[df]
;W[db]
;B[da]
;W[dg]
;B[ca]
;W[eg]
;B[ef])