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This is an in-depth discussion of the rules of go. There has been a certain amount of variation in the rules of go over time, and from place to place. This article discusses those sets of rules broadly similar to the ones currently in use in East Asia. (Traditional Himalayan variants, including Tibetan go, differ more markedly and are discussed instead in the article Go variants.) Even among these, there is a degree of variation. Notably, Chinese and Japanese rules differ in a number of respects. The most significant of these is the scoring method, together with attendant differences in the manner of ending the game.[1] While differences between sets of rules may have moderate strategic consequences on occasion, they do not change the character of the game. The different sets of rules usually lead to the same game result,[2] so long as the players make minor adjustments near the end of the game. This article first presents a simple set of rules which are, except for wording, identical to those usually referred to as the Tromp-Taylor Rules,[3] themselves close in most essential respects to the Chinese rules. These rules are then discussed at length, in a way that does not assume prior knowledge of go on the part of the reader. The discussion is for the most part applicable to all sets of rules, with exceptions noted. Later sections of the article address major areas of variation in the rules of go, and individual sets of rules.
[edit] Basic rulesA set of rules suitable for beginners is presented here. In some respects, these differ from the rules most commonly used. However, the basic rules are simply stated, and provide a convenient basis on which to discuss differences in rule sets. The rules are studied more fully in Explanation of the basic rules below. Two statements of the same basic rules, differing only in wording, are given here. The first is a concise one due to James Davies. The second is a formulation of the basic rules used for expository purposes in this article. Except for terminology, the basic rules are identical to the Logical Rules first proposed in their current form in September 1996 by John Tromp and Bill Taylor.[4][5] They are also quite close to the Simplified Ing Rules of the European Go Federation, the only exception being the method of ending the game. [edit] Concise statementThese rules appear in "The Rules and Elements of Go" by James Davies.[6] They assume familiarity with the equipment used to play go, for which one may refer to Elements of the game below. Notes: The words move and territory are used differently here than elsewhere in this article; play and area, respectively, are used instead. A clarification to Rule 5 is added in parentheses.
These rules rely on common sense to make notions such as "connected group" and "surround" precise. What is here called a "solidly connected group of stones" is also called a chain. [edit] Reference statementThe basic rules are here formulated in a more detailed way in order to ease their presentation in the section #Explanation of the basic rules below. (Each rule and definition links to a detailed explanation in that section.) An optional rule prohibiting suicide is included as Rule 7A. Players and equipment
Positions
Play
End
[edit] Comparative features of the basic rulesThe essential features of these basic rules relative to other rule sets are summarized here. Each of the differences is discussed in greater detail in a later section of the article. What variation exists among rule sets concerns primarily Rules 7A, 8, 9 and 10.
[edit] Explanation of the basic rulesThe object of the game of go is, in rough terms, to control more territory at the end of the game than one's opponent does. [edit] Elements of the game[edit] PlayersRule 1. Go is a game between two players, called Black and White. The choice of black or white is traditionally done by chance between players of even strength. The method of selection is called nigiri. One player, let's call him/her Player A, takes a handful of stones; Player B then places either one or two stones on the board, indicating "even" or "odd." Player A counts the number stones in his/her hand to determine whether there is an odd or even number. If the number stones matches the other player's selection of "even" or "odd," Player A will play the black stones; if not, he/she will take the white stones. When players are of different strengths, the weaker player takes black. Black may also pre-place several handicap stones before play begins, to compensate for the difference in strength (see below). [edit] Board Rule 2. Go is played on a plane grid of 19 horizontal and 19 vertical lines, called a board. Definition: A point on the board where a horizontal line meets a vertical line is called an intersection. Two intersections are said to be adjacent if they are distinct and connected by a horizontal or vertical line with no other intersections between them. The condition that the intersections be "distinct" is included to ensure that an intersection is not considered to be adjacent to itself. Intersections are also called points. There are 361 points on a regular 19 × 19 board. For simplicity, we will illustrate the rules mostly using 5 × 5 boards. Each of the following diagrams shows two points on a 5 × 5 board:
In the first two diagrams, the points are adjacent; in the third they are not. Though 19 × 19 boards are standard, go can be played on another size board. Particularly common sizes for quick games are 9 × 9 and 13 × 13. (See also "Board size" below.) Beginners might prefer to play on a 9 × 9 board to start. The nature of the game remains similar enough to make this worthwhile, yet the games are shorter. For beginners, playing longer games is less important than playing a greater number of games. [edit] StonesRule 3. Go is played with playing tokens known as stones. Each player has at his disposal an adequate supply of stones of his color. Traditionally, Black is given 181 stones, and White, 180, to start the game. This is almost always sufficient, but if it turns out to be insufficient, extra stones will be used. [edit] PositionsRule 4. At any time in the game, each intersection on the board is in one and only one of the following three states: 1) empty; 2) occupied by a black stone; or 3) occupied by a white stone. A position consists of an indication of the state of each intersection. Specifying a position involves only the current state of the board. It requires no indication of whose turn it is, nor any information relating to previous moves or states of the board. This definition of "position" is used in Rule 8 ("positional superko"). The diagram shows a possible position:
Naturally, two stones are said to be adjacent if they occupy adjacent intersections. Similarly, a stone and an intersection are adjacent if the stone occupies an intersection adjacent to that intersection. [edit] Connected stones and pointsDefinition. In a given position, two stones of the same color (or two empty intersections) are said to be connected if it is possible to pass from one to the other by a succession of stones of that color (or empty intersections, respectively) in which any two consecutive ones are adjacent. The concept of connected stones is used to describe (via the concept of liberties, defined below) the conditions in which stones are captured by a move. The concept of connected empty points is used only at the end of the game, to define a player's score. In the following position, the stones 1 and 7 are connected by the sequence of black stones 1, 2, ..., 7, in which each stone (other than 1) is adjacent to the stone before it. The empty points a and k are connected by the sequence of empty points a, b, ..., k, in which each point (other than a) is adjacent to the one before it. In fact, it is easy to see in this position that all the black stones are connected to each other and that all the empty points are connected to each other.
Let us examine the previous position and determine which stones and empty points are connected.
In the diagram, stones and empty points are marked with the same number or letter, respectively, whenever they are connected to each other. A chain is a set of one or more stones (necessarily of the same color) that are all connected to each other and that are not connected to any other stones. Although it is not necessary to define the word chain in order to state the rules, the concept is important for an understanding of the game. For example, Black and White each have four chains in the diagram above. Black has one three-stone chain, one two-stone chain, and two one-stone chains. White has one four-stone chain and three one-stone chains. It follows from the definitions that any stone on the board belongs to exactly one chain. Furthermore, saying that two distinct stones of the same color are connected is the same as saying that they belong to the same chain. [edit] LibertiesDefinition. In a given position, a liberty of a stone is an empty intersection adjacent to that stone or adjacent to a stone which is connected to that stone. We study some examples.
In the above position, the points a, b, c, d, e, are the liberties of the black stone at 1.
The result would have been the same if we had determined the liberties of Black 2, or of any other stone belonging to the black chain.
In this position:
Since any two stones belonging to the same chain have the same liberties, we often speak of the liberties of that chain. For example, in the first diagram, the points a, b, c, d and e are the liberties of the lone black chain. In the second diagram, the liberties of the black chain in the lower right are c, d and h. [edit] Play[edit] Initial positionRule 5. At the beginning of the game, the board is empty. [edit] Alternation of turnsRule 6. Black moves first. The players alternate thereafter. What players may do when they move is the object of Rules 7 and 8. [edit] MovingRule 7. On his turn, a player may either pass (by announcing "pass" and performing no action) or play. A play consists of the following steps (performed in the prescribed order):
A move is defined as a play or a pass. Thus, on each turn a player moves once. A player may pass on any move. Usually, passing is beneficial only at the end of the game, when all territory has been claimed and further moves would be useless, or even harmful to a player's position. The following three sections discuss the successive steps of a play in greater detail. Let us observe immediately however that, in view of Steps 2 and 3, all stones remaining on the board after any move must have at least one liberty. [edit] Placing a stone on the boardStep 1 of a play. The player places a stone of his color on an empty intersection (chosen subject to Rule 8 and, if it is in effect, to Optional Rule 7A). As indicated by the reference to Rules 8 and 7A (respectively the superko rule and prohibition of suicide, to be discussed later), there are some restrictions on the choice of point at which to play. The following diagrams show a possible sequence of moves at the beginning of the game:
The following diagrams show how Black might play later in the same game:
Numbers are often used, as here, to indicate new moves in printed diagrams. Once a stone has been played, it remains on the board, in the same location, until the end of the game or until it is captured (removed from the board as part of Step 2 or Step 3 of a play). [edit] CaptureStep 2 of a play. (After playing his stone) a player removes from the board any stones of his opponent's color that have no liberties. We say that the stones removed from the board have been captured by the player moving. We now give some examples in which the capture rule is applied. The diagrams below show the capture of a white stone by Black. To begin with, the white stone has a single liberty at a. By playing a stone at a, Black removes the last remaining liberty of the white stone. It is subsequently removed from the board.
At the edge of the board and especially in the corners, stones have fewer liberties to start with and are more easily captured.
Next, White captures a chain of four black stones by playing at a.
Black captures the white chain by playing at a.
Black captures the marked white chain at the edge of the board by playing at a. Then White captures the black stone in the corner by playing at b.
Here, White captures the three marked black chains by playing at a.
[edit] Self-captureStep 3 of a play. (After playing his stone and capturing any opposing stones) a player removes from the board any stones of his own color that have no liberties. Optional Rule 7A. A play is illegal if one or more stones would be removed in Step 3 of that play. The removal of one or more stones in Step 3 is called self-capture, or suicide. Before discussing self-capture further, let us note that most rule sets give effect to Optional Rule 7A, which prohibits it. This means that, in those rule sets, any play which under the basic rules would require a self-capture to be performed is illegal. For further information, see "Suicide" below. We begin with an example which, it is emphasized, does not involve self-capture. When Black plays at a, the capture of the marked white stones results in the black chain at the bottom right acquiring liberties. This move is legal (with the same result) whatever the rules.
The previous example shows that it is important that Step 2 of a play (capture) precedes Step 3 (self-capture). If the order were reversed, then self-capture would occur here. It is not difficult to convince oneself that if a play results in the capture of opposing stones, self-capture does not occur. We now present some examples of plays in which self-capture occurs. These moves would be illegal under the optional rule prohibiting suicide. In this example, if Black plays at a, then the stone played by him is removed immediately. This move has the same effect on the position as a pass, though it would not allow White to end the game by passing next (Rule 9). The move is in any event illegal by Rule 8. (This is the positional superko rule. This move might be legal under other versions of the superko rule. See "Repetition" below.)
In the next example, Black plays at a, resulting in the self-capture of the marked black stones.
[edit] KoRule 8. A play is illegal if it would have the effect (after all steps of the play have been completed) of creating a position that has occurred previously in the game. Though a pass is a kind of "move," it is not a "play." Therefore, Rule 8 never bars a player from passing. Before going further, we state a consequence of Rule 8 called the ko rule: Consequence (ko rule). One may not play in such a way as to recreate the board position following one's previous move. The word ko, pronounced with a long "o", is taken from Japanese (劫, kō; usually written with katakana: コウ) and can mean both "threat" and "aeon" (from the Buddhist kalpa time unit).[26] Rule 8 is known as the positional superko rule. The word "positional" is used to distinguish it from slightly different superko rules that are sometimes used. While the ko rule is observed in all forms of go, not all rule sets have a superko rule. The practical effects of the ko rule and the superko rule are similar; situations governed by the superko rule but not by the ko rule arise relatively infrequently. For further information, see the section "Repetition" below. The superko rule is designed to ensure the game eventually comes to an end, by preventing indefinite repetition of the same positions. It differs in nature from the threefold repetition rule of chess, which does not ban repetition, but on the contrary allows it as one method of forcing a draw. The ko rule has very important strategic consequences in go. Some examples follow in which Rule 8 applies. These examples cover only the most important case, which is the same under all varieties of ko rule. The first diagram shows the board immediately after White has played at 1, and it is Black's turn. Black captures the marked white stone by playing at a. If White responds by capturing at b with 3, the board position is identical to that immediately following White 1. White 3 is therefore prohibited by the ko rule.
Another example of ko follows. Here, Black 3 is illegal by the ko rule.
As noted in the section "Self-capture," Rule 8 prohibits the suicide of a single stone. This is something of a triviality since such a move would not be strategically useful. Taking it for granted that no suicide of a single stone has occurred, a moment's thought will convince the reader that the ko rule can be engaged in only one situation: Restatement of the ko rule. One may not capture just one stone, if that stone was played on the previous move, and that move also captured just one stone. Furthermore, this can occur only when one plays in the location at which one's stone was captured in the previous move. The two points where consecutive captures might occur, but for the ko rule, are said to be in ko. For example, in the first two diagrams above, the points a and b are in ko. The next two examples involve capture and immediate recapture, but the ko rule is not engaged, because either the first or second capture takes more than one stone. In the first diagram below, White must prevent Black from playing at a, and does this with 1 in the second diagram. Black can capture the three stones in White 1's group by playing at b. He does this with Black 2 in the third diagram. White may recapture Black 2 by playing at a again, because the resulting position, shown in the fourth diagram, has not occurred previously. It differs from the position after White 1 by the absence of the two marked white stones.
In the first diagram below, it is White's turn. White must prevent Black from connecting the marked stones to the others by playing at a. The second diagram shows White's move. White is threatening to kill the marked black stones by playing at b. In the third diagram, Black plays at b to prevent this, capturing White 1. However, by playing at a again, White can capture Black 2's group. This is not barred by the ko rule because the resulting position, shown in the fourth diagram, differs from the one after White 1 by the absence of the marked black stones. This kind of capture is called a snapback.
The next example is typical of real games. It shows how the ko rule can sometimes be circumvented by first playing elsewhere on the board. The first diagram below shows the position after Black 1. White can capture the marked black stone by playing at a. The second diagram shows the resulting position. Black cannot immediately recapture at b because of the ko rule. So he instead plays 3 in the third diagram. For reasons that will become clear, Black 3 is called a "ko threat."
At this point, White could choose to connect at b, as shown in the first diagram below. However, this would be strategically unsound, because Black 5 would guarantee that Black could eventually kill the white group altogether, no matter how White played.
Instead, White responds correctly to Black 3 with 4 in the first diagram below. Now, contrary to the situation after White 2, Black can legally play at b, because the resulting position, shown in the second diagram, has not occurred previously. It differs from the position after Black 1 because of the presence of Black 3 and White 4 on the board. Now White is himself prohibited from recapturing at a by the ko rule. White has no moves elsewhere on the board requiring an immediate reply from Black (ko threats), so White plays the less urgent move 6, capturing the black stone at 3, which could not have evaded capture even if White had waited. In the next diagram, Black connects at a before White has a chance to recapture. Both players pass and the game ends in this position.
[edit] EndRule 9. The game ends when both players have passed consecutively. The final position (the position later used to score the game) is the position on the board at the time the players pass consecutively. Since the position on the board at the time of the first two consecutive passes is the one used to score the game, Rule 9 can be said to require the players to "play the game out." Rule 9 differs significantly from the various systems for ending the game used in practice, which are discussed in the section "Counting phase" below. These generally allow the game to end as soon as it is obvious to the players which stones would remain on the board if the game continued. The precise means of achieving this varies widely by rule set, and in some cases has strategic implications. These systems often use passing in a way that is incompatible with Rule 9. Under Chinese rules, and more generally under any using the area scoring system of these basic rules, a player who played the game out as if Rule 9 were in effect would not be committing any strategic errors by doing so. He would, however, likely be viewed as unsportsmanlike for prolonging the game unnecessarily. On the other hand, under a territory scoring system like that of the Japanese rules, playing the game out in this way would in many cases be a strategic mistake. [edit] TerritoryDefinition. In the final position, an empty intersection is said to belong to a player's territory if 1) all stones adjacent to it or to an empty intersection connected to it are of his color; and 2) there is at least one such stone. Note: Unless the entire board is empty, the second condition - that there be at least one stone of the kind required - is always satisfied and can be ignored. A point can never belong to both players' territories. On the other hand, it may well happen that an empty intersection belongs to neither player's territory. In that case the point is said to be neutral territory.[27] There are rarely any more than a handful of neutral points at the end of a game; in the majority of cases, there are none at all. Japanese and Korean rules count some points as neutral where the basic rules, like Chinese rules, would not. For more on this, see the section "Seki". In order to understand the definition of territory, it is instructive to apply it first to a position of a kind that might arise before the end of a game. Let us assume that a game has ended in the position below[28] (even though it would not normally occur as a final position between skilled players).
The point a is adjacent to a black stone. Therefore a does not belong to White's territory. However, a is connected to b (by the path shown in the diagram, among others), which is adjacent to a white stone. Therefore a does not belong to Black's territory either. In conclusion, a is neutral territory. The point c is connected to d, which is adjacent to a white stone. But c is also connected to e, which is adjacent to a black stone. Therefore c is neutral territory. Similarly, the points f and g are neutral territory. On the other hand, h is adjacent only to black stones and is not connected to any other points. Therefore, h is black territory. For the same reason, i and j are black territory, and k is white territory. In the previous position, White might play 1 in the next diagram. If the game ended in this position, the marked intersections would become White's territory, since they would no longer be connected to an empty intersection adjacent to a black stone.
The game might end with the moves shown below. In the final position, the points marked a are black territory and the points marked b are white territory. The point marked c is the only neutral territory left.
The point in the lower right corner and the point marked a on the right side of the board would fall under the seki exception of Japanese and Korean rules, in which they would be considered neutral territory. (See the section "Seki" below.) [edit] AreaDefinition. In the final position, an intersection is said to belong to a player's area if either: 1) it belongs to that player's territory; or 2) it is occupied by a stone of that player's color. Consider once again the final position shown in the last diagram of the section "Territory." The following diagram illustrates the area of each player in that position. Points in a player's area are occupied by a stone of the corresponding color. The lone neutral point does not belong to either player's area.
[edit] ScoreDefinition. A player's score is the number of intersections in his area in the final position. For example, if a game ended as in the last diagram in the section "Territory," the score would be: Black 44, White 36. The players' scores add to 80. The difference between this and the 81 intersections on a 9 × 9 board is accounted for by the one point of neutral territory. The scoring system described here is known as area scoring, and is the one used in the Chinese rules. Different scoring systems exist. These determine the same winner in most instances. See the Scoring systems section below. [edit] WinnerRule 10. If one player has a higher score than the other, then that player wins. Otherwise, the game is drawn. In the previous example, Black wins by eight points. Margin of victory does not matter; winning by one point is as good as winning by 100 points. [edit] Scoring systemsThe most prominent difference between rulesets is the scoring method. There are two main scoring systems: territory scoring (the traditional Japanese method) and stone or area scoring (the Chinese method). Care should be taken to distinguish between scoring systems and counting methods. Only two scoring systems are in wide use, but there are two ways of counting using "area" scoring. [edit] Territory scoringIn territory scoring (including Japanese and Korean rules) a player's score is determined by the number of empty locations that player has surrounded minus the number of stones their opponent has captured. Furthermore, Japanese and Korean rules have special provisions in cases of seki, though this is not a necessary part of a territory scoring system. (See "Seki" below.) Typically, counting is done by having each player place the prisoners they have taken into the opponent's territory and rearranging the remaining territory into easy-to-count shapes. [edit] Stone or area scoringIn stone or area scoring (including Chinese rules), a player's score is determined by the number of stones that player has on the board plus the empty area surrounded by that player's stones. There are several common ways in which to count the score (all these ways will always result in the same winner):
[edit] Attempts at reconciling the two scoring systemsIf the game ends with both players having passed the same number of times, then the score will be identical no matter which method is used. AGA rules call for a player to give the opponent a stone when passing. This "passing stone" ensures a correct result under any counting method. [edit] Counting phaseCustomarily, when players agree that there are no useful moves left (most often by passing in succession), they attempt to agree which groups are alive and which are dead. If disagreement arises, then under Chinese rules the players simply play on. However, under Japanese rules, the game is already considered to have ended. The players attempt to ascertain which groups of stones would remain if both players played perfectly from that point on. (These groups are said to be alive.) In addition, this play is done under rules in which kos are treated differently from ordinary play. If the players reach an incorrect conclusion, then they both lose. Unlike most other rule sets, the Japanese rules contain lengthy definitions of when groups are considered alive and when they are dead. In fact, these definitions do not cover every situation that may arise. Some difficult cases not entirely determined by the rules and existing precedent must be adjudicated by a go tribunal. The need for the Japanese rules to address the definition of life and death follows from the fact that in the Japanese rules, scores are calculated by territory rather than by area. The rules cannot simply require a player to play on in order to prove that an opponent's group is dead, since playing in his own territory to do this would reduce his score. Therefore, the game is divided into a phase of ordinary play, and a phase of determination of life and death (which according to the Japanese rules is not technically part of the game). [edit] Optional rules[edit] CompensationTo allow players of different skills to compete fairly, handicaps and komi are used. These are considered a part of the game and, unlike in many other games, they do not distort the nature of the game. Players at all levels employ handicaps to make the game more balanced. [edit] KomiMain article: Komidashi In an "even", or non-handicap game, Black's initial advantage of moving first can be offset by komi (compensation points): a fixed number of points, agreed before the game, added to White's score at the end of the game. The correct value of komi (to properly compensate for Black's advantage) is controversial, but common values are 5.5, 6.5 or 7.5 -- the fractional value avoids a tied game. In a handicap game, komi is usually set to 0.5 (i.e. White wins if the game is tied). A handicap game with a handicap of 1 starts like an even game, but White receives only 0.5 komi (i.e. a White player who is stronger by one rank is handicapped only by Black's first-move advantage). Before the 20th century, there was no komi system. When the great Shusaku was once asked how an important game came out, he said simply, "I had Black," implying that victory was inevitable. As more people become aware of the significance of Black having the first move, komi was introduced. The amount of komi has been increased periodically based on analysis of game results indicating >50% wins for Black; each time, after a period of adjustment Black has again begun winning >50% of games. When it was introduced in Japanese Professional games, it was 4.5 points. However, Black still had a better chance to win, so komi was increased to 5.5 points in 1974. In 2002, the Japanese Go Association again increased the komi value to 6.5. [edit] HandicapMain article: Go handicaps Handicaps are given by allowing the weaker player to take Black, and declaring White's first few moves as mandatory "pass" moves. In practice, this means that Black's first move is to place a set number of stones (usually the number is equal to the difference in the players' ranks) on the board before allowing White to play. Traditionally, the hoshi ("star points") -- strategically-important intersections marked with small dots -- are used to place these handicap stones. On the 19 × 19 board, there are nine star points: at the four 4-4 points in the corners, at the four 4-10 points along the sides, and one at the 10-10 point (the centre of the board, or tengen in Japanese). Smaller boards such as the 13 × 13 and 9 × 9 also have star points. The 13 × 13 has 9 at the 4-4 points, 4-7 points, and the center. The 9 × 9 board has only 5 points: the 3-3 points and the center. When Black is only one rank weaker (also known as one stone weaker, due to the close relationship between ranks and the handicap system) he or she is given the advantage of playing Black, perhaps without komi, but without any mandatory White passes. For rank differences from two through nine stones, the appropriate number of handicap stones is used. Beyond nine stones, the difference in strength between the players is usually considered great enough that the game is more a lesson, with White teaching Black, so nine stones is the nominal upper limit on handicap stones regardless of the difference in rank (though higher numbers of stones, up to 41 stones in some cases, can be given if the teacher wants more of a challenge). [edit] Thinking timesSee the Time control section of the main Go article. [edit] VariationsGo was already an ancient game before its rules were codified, and therefore, although the basic rules and strategy are universal, there are regional variations in some aspects of the rules. [edit] SekiTo define this notion, we must begin with two definitions: Terminology:
These definitions are given only loosely, since a number of complications arise when attempts are made to formalize the notion of life and death. A group of stones of one color is said to be alive by seki (or in seki) if it is not independently alive, yet cannot be captured by the opponent.
For example, in the diagram above, the black and white groups each have only one eye. Hence they are not independently alive. However, if either Black or White were to play at the circled point, the other side would then capture their group by playing in its eye. In this case both the black and white groups are alive by seki. In the diagram above, the circled point is not surrounded by stones of a single color, and accordingly is not counted as territory for either side (irrespective of rule set). In more complex cases, as here,[29]
a vacant point may be surrounded by a group of a single color which is in seki. According to Japanese and Korean rules, such a point is nonetheless treated as neutral territory for scoring purposes. Generally, the Japanese and Korean rules only count a vacant point as territory for one color if it is surrounded by a group or groups of that color that are independently alive. [edit] RepetitionThe major division in rules to prevent repetition is between the simple ko rule and the super ko rule: the simple ko rule (typically part of the Japanese ruleset) prevents repetition of the last previous board position; while the superko rule (typically part of Chinese derived rule sets, including those of the AGA and the New Zealand Go Society) prevents repetition of any previous position. In both cases, the rule does not however prohibit passing. The super ko rule is differentiated into situational super ko (SSK, in which the "position" that cannot be recreated includes knowledge of whose turn it is) and positional super ko (PSK, which ignores whose turn it is). Natural situational super ko (NSSK) is a variant in which what matters is not whose turn it is, but who created the position (i. e., who made the last move other than a pass.) The Ing rules feature a complicated distinction between "fighting" and "disturbing" ko. Situations other than ko which could lead to an endlessly repeating position are rare enough that many frequent players never encounter them; their treatment depends on what rule set is being used. The simple ko rule generally requires the inclusion of additional rules to handle other undesirable repetitions (e.g. long cycles which can lead to no result where the game must be replayed). The first position below is an example of a triple ko, taken, with minor changes, from Ikeda Toshio's On the Rules of Go.[30]
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