Superko rules prohibit recurrences of an earlier board position under certain circumstances. The recurrence of such positions could be caused by long cycles, such as triple ko, eternal life or round-robin ko.
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Superko rules come in various forms, depending on when they forbid recreating a board position; factors include:
- on whose turn the position was previously created;
- whether players have passed, immediately before or at some intervening time;
- the type of move recreating the position (see Chinese Superko below).
The main variants in use of the superko rule and other rules on repetitions are:
|Name of variant||Meaning||Used in||Comments|
|Positional Superko||Forbids a board play to repeat a board position||Chinese rules (in theory) |
|See Positional Superko / Example|
|Chinese Superko||A play may not repeat a board position by means of basic ko or sending two, returning one||Chinese rules (in practice)||This, according to the sixth meeting of the International Go Rules Forum,|
is what the Chinese rules were intended to specify.
|Situational Superko||Forbids a board play to repeat a board position with the same player to move (regardless of how it previously arose)||AGA rules |
New Zealand rules
|Natural Situational Superko||Forbids a player’s board play to repeat a board position that they created with an earlier board play||BGA Rules 2009||
Note that it could allow an immediate ko recapture if the first capture was preceded by a pass.
|No result||If a board position is repeated, the game has “no result”||Japanese rules |
|This usually means the game must be replayed.|
|Long cycle rule||Repetition of a board position ends the game.||Computer Olympiad rules||(PJT: ¿used with basic ko rule? |
The game result depends only on the number of stones captured by either since the first occurrence of the position.
|Threefold repetition||Third or later occurrence of a position is required to declare end of the game, the result is draw/loss/win depending on stones captured in the loop.||Renaissance rules||A single repetition of a position not necessarily means an endless cycle, see for example 1-eye-flaw. Further repetitions are also increasingly easier to recognize in practice.|
|Minimal superko||Forbids a board play to repeat the last position both players passed in successively.||Renaissance rules||Slight complement to the basic ko rule.|
A possible reasoning behind a few of the above is a generic long cycle rule. The following variants have also been used in theoretical discussions:
- Active Superko (first description here by Rubilia): a player who makes the same play (including pass, in the same position) for the second time wins.
- Sound Superko (term suggested by Robert Pauli): forbids “completing a cycle” (in his terminology) that does not include a pass.
- Strict Superko (term suggested by Robert Pauli): forbids “completing a non-trivial cycle” (in his terminology).
The most commonly used example why a superko rule is necessary is triple ko. See below for a more simple case that also illustrates the rule:
This situation is a seki in the corner. Assume Black just passed, so that this position has occurred with either side to play. If White plays at a, Black plays b. With a superko rule in place, White cannot retake at , so Black can take the corner. Of course, this comes from a White initial mistake, but see Rules Beast 1 for a case where White breaks the seki and wins the fight...
- Pinwheel Ko — A network of kos rotating round the board
- 1 Eye Flaw — Superko prevents non-perpetual repetition
- Rules Beast 1 — A position where superko surprisingly requires an extra move in a seki to avoid a capture
- Molasses Ko — Superko may turn the game into No Pass Go
- Superko Puzzle 2 — In which an unconditionally live group is sacrificed to break a cycle
- Fixed ko rule: repeated positions may occur but you are not allowed to make the same move in them.
- Short cycle rules?: The result depends on the number of moves between the repeats. If there are 3 or fewer moves between the repeats the game is a loss for the player who makes the repeat. If there are more moves between the repeats the game is a draw.
- Long cycle seki rule: The groups involved in a long cycle are treated as alive in seki
- Colour-blind superko: Ignore the colours, suggested as a ko-rule for multi-colour go.
- Pair ko rule: Forbids repetition of pairs of moves rather than positions, which drastically reduces the amount of information that players need to remember from previous positions.
- Quadruple Ko Fight – a superko experiment
- http://home.snafu.de/jasiek/superko.html – lists five variants and the advantages and disadvantages of the major three (positional / situational / natural situational)
- Article 7: The repetition of the same pattern shall be prohibited unless the right to an alternate move is disregarded.
In modern terms, "disregarding the right to an alternate play" means passing. When a pass lifts a ko or superko ban, this is sometimes called "pass for ko threat". There are different versions of the superko rule in which an intervening pass allows a repetition. Yasunaga's seems to say that any intervening pass lifts the restriction on repetition, but it may mean a pass by the player who makes the repetition. The interaction with passes can pose problems with ending the game, but not taking passes into account can lead to strange results. My home page, when I get it up, will have a discussion, and my solution to the problem, based on Combinatorial Game Theory. Meanwhile, for my solution, see Spight Rules.
Yasunaga proposed a three-consecutive-pass rule, which is how my solution will usually work. Ing uses a four-pass rule.