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Mirror go is a type of elementary Go opening strategy. It refers to all go openings in which one player plays moves that are diagonally opposite those of this opponent, making positions that have a rotational symmetry through 180° about the central 10-10 point (tengen in Japanese). The Japanese term for mirror go is manego.
There are actually two types of strategy of this kind, and several ways the symmetry can or should be broken for tactical or strategic reasons.
- In games without komidashi compensation points given, Black starts at 10-10 and imitates White subsequently. Black hopes that the possession of the central point will give an edge in an otherwise symmetrical position.
- In games with komidashi, White imitates Black from the start. Black can eventually occupy the central point, but White will then hope to show that the game can be made close enough on the board so that the compensation points give a win.
In itself, mirror go is a deeply flawed strategy. It may be refuted in a number of ways: using ladder tactics, using ko fights, or most convincingly for a novice by using contact plays to demonstrate that possession of the central point can just as well lead to shortage of liberties. This has not prevented the circulation of folklore stories about mirror go as some kind of sure-win strategy, eagerly taken up by novice players. Hikaru No Go episode 8 featured a naive attempt to trick Akira Toya using mirror go, which Toya easily rebuffed using the contact play strategy.
Mirror go can be refuted in boards of odd size rather more easily than those of even size. This is a probable explanation of why odd sizes are always used. (On even-sized boards ladders and kos still can be used.)
After sporadic use down the years, mirror go was brought back into some fashion by the shinfuseki period, in which the tengen opening was explored. Subsequently Fujisawa Kuranosuke used it often as White, aiming for large-scale battles.
- It is said that Hideyoshi Toyotomi used it against Honinbo Sansa, in Go Monthly Review 1963/9, p.66. Hence a nickname taiko go, taiko being an honorific for Hideyoshi.