Mirror Go, also commonly referred to by the Japanese term manego, refers to the practice where one player plays each move at a point that "mirrors" his opponent's previous move. The "mirror", in this case, is a a point reflection, equivalent to a 180-degree rotation around the center point (tengen).
The theory behind Mirror Go is that a move that mirrors the opponent's move can't be worse off. Players who mirror the moves can abort the mirroring process when they feel their opponent has made a bad move, giving an advantage.
In modern games, White is more likely to perform the mirroring as Black is burdened by having to overcome White's komi advantage. In games without komi, it is more viable for Black to play on tengen and then mirror.
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Black opens on tengen, then mirrors White
Bad shape for Black
White mirrors Black
For Black to be able to play mirror go, he needs to open on tengen, which removes the only point that cannot be mirrored. From thereon out, Black can mirror moves as long as he sees fit. Black's usual plan will be to carefully assess each white move, and deviate from mirroring when he feels it is a bad move.
This is the opening of the first game ever played between Go Seigen and Kitani Minoru. Go Seigen opened on tengen with , then mirrored Kitani's moves until move 64. Go was criticized for this by many commentators, who felt that it made a mockery of the game. For further diagrams and analysis, see Kitani Go First Game.
See countering mirror go.
When Black plays mirror go, it is important whether or not there is komi. With komi, White can comfortably play normal moves, as continued mirroring by Black will lead to a draw and the board, and thus a victory for White with the komi. Without komi, Black's mirroring is more dangerous.
In either case, White will have to be careful to prevent the tengen stone from becoming too useful. With the tengen stone in place, most ladders will favour Black, so white will have to try and avoid situations where ladders might allow black to gain an advantage. White will also have to make sure that tengen does not become a well-positioned move. Playing a central moyo strategy, for example, would give Black the advantage of having the tengen stone already in place.
Ideally, White would like to create a situation where Black's tengen stone becomes a liability, rather than an asset. The next diagrams show a simple example of how Black's tengen stone can become an liability
In the first diagram, we can see how mirroring results in bad shape for black. By the second diagram, the tengen stone has been completely neutralized. If black stubbornly keeps mirroring, the result will be disaster. In the third diagram, Black is one move too late, and will lose five stones in the center.
Of course, is not usually a viable move to punish mirror go. Black can stop mirroring and attack the white stone immediately, satisfied that his strategy has paid of by compelling White to play a bad move. Instead of playing immediately, White will generally have to find a strategy where a fight develops toward the center, so that or similar moves can be played naturally.
After the introduction of komi, mirror go became a more viable strategy for White. The idea being that White tries to keep the game in balance by mirroring, then winning due to the komi. Several professional players have experimented with it, notably Fujisawa Hosai, who played it quite often.
In this game between Fujisawa Hosai and Yamabe Toshiro, Fujisawa played mirror go for 47 moves, deviating from the symmetry on move 48. Fujisawa was notorious for often using mirror go, and was disdained for it by many of his contemporaries. The above game, as well as another one against Yamabe, convinced Fujisawa that mirror go was a flawed strategy. For more analysis of this game, see Some reflections on mirror go at GoGoD's New in Go
See countering mirror go.
There are several counter strategies that Black can try against White's mirroring strategy. Although such counter strategies normally will work, Black must be careful not to play any obviously bad moves along the way, as that will allow White to deviate from the mirror strategy and take control of the game.
The simplest counter strategy, of course, if for Black to play tengen. It is the only move on the board that White cannot mirror, and White will have to deviate. But this strategy is only viable when tengen is a good move for Black. If it isn't, then Black has played a bad move and White will happily deviate to take control of the game.
A more advanced strategy is to set up opposing ladders. If these ladders run into each other, Black's first move advantage will be decisive. This strategy has been used in professional play several times. An example can be found in this game between Fujisawa Hosai and Sugiuchi Masao.
Other strategies are possible, but all of them generally depend on generating a fight that reaches the center, where Black's first move advantage can be brought into play decisively. The game above between Fujisawa and Yamabe is an excellent example of this (see analysis)
Although uncommon, mirror go is certainly not unheard of in professional play. Some players try it once or twice, some even use it regularly. Some examples:
- Go Seigen played mirror go against Kitani Minoru in 1929 in the first game ever, see Kitani Go First Game.
- Fujisawa Hosai played mirror go against Sugiuchi Masao in 1961 (Meijin, round 1). See Hosai Masao - Two Ladders Mirror Go.
- Hanawa Yasutoki played it against Nakayama Noriyuki in 1977. This game is analysed in The Treasure Chest Enigma.
- Wang Lei Sr. played mirror go against Yu Bin in the 2005 Chinese Mingren (round 3). The game can be found on go4go.