Countering mirror go
Mirror go played by White can be countered by Black playing on the tengen, although this is not necessarily the best way. Below are several examples of countering mirror go. The result is often absurd, but if the player who is mimicking aborts halfway, the other player would have succeeded in stopping the mirror go.
The more challenging task is to do so without giving your opponent any opportunity to take advantage of your mirror defense attempt (by suddenly responding with a better move than yours).
1. Shortage of liberties due to tengen
The opponent stone at tengen can be used to create shortage of liberties.
What's the best continuation if connects at 9?
2. Setting up two ladders
If no opponent stone at tengen is available to create shortage of liberties, then setting up two ladders is another technique.
Incidentally, this very technique is seen in a professional game: Hosai Masao - Two Ladders Mirror Go.
3. Setting up two kos
Here's another standard  anti-mane-go strategy that can be used instead. It assumes that some form of superko rule is in force. White starts by creating two kos; call these kos A and B, and their mirror images A' and B'. White captures in A, and Black mimics by capturing in A'. Then White captures in B, and Black captures in B'. Then White recaptures in A', and Black recaptures in A. Then White recaptures in B'. The superko rule forbids Black from recapturing in B, so he has to stop mimicking.
4. More strategic considerations
Not everything that looks like it, is actually mirror go. In a database search, there are nine pro games with this exact position. White won eight out of nine, and seven of those were without komi. Since 1985, the pros seem to have decided that this is not a viable strategy for black. (If you delete from the board and look at it as black to play, it's quite a common position--and people don't often play tengen here.)
It would be a mistake is to play an irrational move in an attempt to capture black in a cheap trick. In Example 2, white notices black's mirror go when black plays . White plays in an attempt to begin a capturing race in which is a burden to black. However, when black breaks mirror go by extending to a. Ladder strategies may share of the same bad taste.
One may still feel this is playable for White.
Proper strategy is to recognize the possibility of mirror go from the start and play a solid territorial game in which black's tengen stone feels inconsistent and does not aid black in fighting or building a moyo. In Example 3, white plays a solid territorial game that would feel like relying on komi even if black were not playing manego. While is a natural move, makes feel like a light refutation of ippoji. Black has played a territorial game that is inconsistent with his first move and has lost most of its advantage.
xela: What if white plays and then black decides not to play mirror go? Many people would prefer to have on the 4-4 or 4-5 point.
5. Real life examples
- The game Kato(w) versus Takemiya(b) played in the Meijin League in 1995 began with mirror play. After seven moves the position resulted as shown in the diagram:
Takemiya played , which would be considered soft shape, thinking that if the moves white a through black f followed the position would result with the black stone on the tengen would be developing Black's moyo while erasing White's moyo, as shown in the following diagram, so Kato would break off mirror play. In the game, Kato immediately broke off mirror play by playing at the circled point approaching the lower right corner.
- Karami has a pro example. I don't know if it was mirror go, but when Black finally played on tengen, the position was symmetrical. And the move was decisive. See http://gobase.org/replay/?gam=/games/japan/titles/minister/17/game-21.sgf at GoBase. Upto that tengen play, the game indeed was some interesting kind of mirror Go, that is, not move wise, but joseki wise.
 The Countering (1) example appears in Akira's game against Okamura in Hikaru no Go chapters 14-15.
 Robert Jasiek: For a strategy invented by me during the late 90s, I am impressed that it is already called "standard". Actually, the strategy also involves creation of kos of different values and winning the bigger ko.
@Robert: When did you start playig Go? You state yourself: "Started seriously in May 1991." As this strategy was applied long before, I would doubt it be you invention. Just one example occured in 1974 (see GoGoD for this case).
- Archaic: In the example where Black plays on Tengen first and he attempts to mirror White, if White simply tries to play a territorial game to destroy Black's attempt to mirror in pretty much every case the game will end up with both sides having equivalent territory under the Japanese ruleset and Black winning by +1 under the Chinese ruleset (although of course the traditional Japanese ruleset was probably the only similar ruleset used back then). Perhaps the author of that section can provide the actual diagrams as to where pros were able to refute black's mirror with a purely territorial game by possibly crawling towards the center and then forcing capture (which seems to be the safest method). The other avenue of approach I see working is where White attempts to play the most efficient territorial game possible, threatens capture near the end and forces Black to break off while seizing a greater amount of territory than Black in the process. Although, I'm not sure if these can be done in practicality without losing momentum. So, I've also tested rotating outwards towards the outer edges if the board as well to counter the other person's mirror without the use of tengen but it does not seem to be too effective and either leaves you with the sane position and score (# of captures as your opponent) or mostly just backfires since the opponent makes the last move which usually results in your capture. Otherwise, the only way to capture your opponent outright by rotating towards the outer edges is to put him in a position of suicide where you can capture him on the next move (and also since you can't perform a suicide on yourself anyway). So we're left with White breaking mirror through the crawling towards the center or the superko at 4 corners trick being the only viable capture tactics that White can use to counter Black's mirror in the days without komi. Using methods that involve crawling towards the center to counter your opponent's mirror may be the best and easiest way of play considering that the natural progression of a game in Go usually involves going from the corners to the center.