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 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.