1998-12-02 last update, 1996-10-08 first day; Robert Jasiek
Copyright: All rights of the author are preserved according to the international law.

Ko Classification


This page classifies go rule sets as to their ko rules. All sets are treated if they are in use, were in use, might be useful in practice or are of theoretical interest. A set might be omitted if its sources are hardly available (e. g. ancient variants or internet implementations).

Highly different ko rules have been evolved. One might expect ko rules to be simple and logical. (Simple = easy to understand and short; logical = clear text for uneqivocal determination of allowed moves and no infinite game after any allowed move.) However, many ko rules have other features, e. g. tradition or reflection of high level go terms. Different ko rule sets have different definitions of kos or none. A ko may be a basic ko consisting of exactly two board points or some bigger ko that even might not be connected.

The main classification reflects logical purposes of ko rules. The rule sets are listed in the order of the classification and their features are described in detail.


Fundamental parts of ko rule sets

Each ko rule set is considered to consist of the following fundamental parts, denoted by abbreviations:

1 Unwritten

1.1 Ancient

1.2 Tibet

1.3 Modern

2 Voidness

2.1 Mathematical

2.1.1 Conway
2.1.2 Universalists

2.2 Non-mathematical

2.2.1 Japanese
2.2.2 Chinese

2.3 Repetition Japanese

3 Repetition

3.1 Single

3.1.1 Primitive Ko
3.1.2 Positional Super Ko
3.1.3 Situational Super Ko
3.1.4 Natural Situational Super Ko

3.2 Multiple

3.2.1 Fixed Ko

3.3 Modified

4 Prohibition

4.1 Primitive

4.2 Simple

4.3 Disturbance

4.3.1 Ing Successors
4.3.2 Ing Rules


The meanings of the features are: history = evolution, use, predecessors; philosophy = main conceptual intentions of ko rules; exceptions = main special rulings; length = text length of ko rules; logic = yes: mathematical proof given / can be given, no: illogical, maybe: possibly mathematical proof might be given; terms = grade of highest logical level of used terms (order: none [= board, board point, stone, colour], basic ko, ko string, ko coupling, ko types, life+death); complexity = (referring to a given board position of a board of n points; worst of all "reasonable" - worth playing - cases: ) "kos:" for determination of kos on the board; "moves:" for determination of all allowed moves / of lengths of move-sequences leading to repeated board ; variants = yes: exist, no: do not exist, future: might be expected to come.

1.1 Ancient unwritten


1.2 Tibetan Unwritten


1.3 Modern Unwritten


2.1.1 Conway Rules of Mathematical Voidness


Example text: A move which repeats a prior position is legal. [interpretative rewording; redundant]

2.1.2 Universalists of Mathematical Voidness


Example text: A board play which repeats a position two moves ago is illegal. Other board plays may repeat a prior position. [interpretative rewording; second sentence redundant]

2.2.1 Japanese Non-mathematical Voidness

Japanese 1949

Taiwanese 1952

World Amateur Go Championship 1980

Japanese 1989

Example text: A shape in which the players can alternately capture and recapture one opposing stone is called a ko. A player whose stone has been captured in a ko cannot recapture in that ko on the next move. When the same whole-board position is repeated during a game, if the players agree, the game ends without result. In the confirmation of life and death after the game stops [...], recapturing in the same ko is prohibited. A player whose stone has been captured in a ko may, however, recapture in that ko again after passing once for that particular ko capture.

2.2.2 Chinese Non-mathematical Voidness

Chinese 1988

2.3 Repetition Japanese Voidness

WWGo 1996

3.1.1 Primitive Ko Repetition

Most Primitive Go Rules 1989/90

Example text: No move may repeat a prior position. [Slightly reworded. Further paragraph: A player loses if all his stones on the board disappear or if he has no legal move to make.]

3.1.2 Positional Super Ko Repetition

Ikeda 1968-69

Ing 1974

New Zealand [1975 ?]


Tromp-Taylor 1996

Example text: A board play may not repeat an earlier grid coloring. [original text includes pass and move in a turn rule]


3.1.3 Situational Super Ko Repetition

Lasker 1945

New Zealand [1978 ?]

AGA 1991

Example text: It is illegal to play in such a way as to recreate a previous board position from the game, with the same player to play.

Tromp-Taylor 1995

3.1.4 Natural Situational Super Ko Repetition

Example text: A player may not use a board play to recreate a position if he has used one to create it.

3.2.1 Fixed Ko Repetition

Example text: A board play may not recreate a pair of positions.

3.3 Modified Repetition

The Basic-Fixed Rules

Rules: basic ko rule and fixed ko rule otherwise.

4.1 Primitive Prohibition

One-coloured Primitive Prohibition

Example text: Only once a stone may be played on each board point.

Two-coloured Primitive Prohibition

Example text: Only once a stone of each colour may be played on each board point.

4.2 Simple Prohibition

The Basic Ko Rules

Example text: Ko definition: Two board points are a ko if on them a move of one player followed by a move of the other player repeats the configuration of stones. Basic ko rule: A stone in a ko that has captured a stone in it may not be recaptured immediately. Prohibition rule: From all board positions with the same set of board points of all kos and with the same configuration of stones on the board without the set each player may only once play on each board point.

The Prohibition Rules

The Configuration Rules

4.3.1 Ing Successors

The Ko Coupling Rules

Example text: [...] Disturbance rule: During a ko stone move-sequence in a ko position a player becomes the disturber of the ko position by playing a ko stone in it first. As soon as the ko position with the stones on it has been repeated since the moment just before the occurance of the current disturber he is not allowed to play a ko stone in the ko position. With the end of the ko stone move-sequence in the ko position a disturber ceases to exist. [...] [...]

The New Ko Rules

4.3.2 Ing rules

Ing 1986

Ing 1991