Editing Rules of Go (section)

===={{anchor|Ko}} Ko and Superko====
{{main|Ko fight}}
'''Rule 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. One consequence of Rule 8 is the so-called ''ko'' rule:

'''Consequence (ko rule). One may not play in such a way as to recreate the board position following one's previous move.'''

Whereas Rule 8 prohibits repetition of any previous position, the ko rule prohibits only ''immediate'' repetition.

The word ''ko'', pronounced with a long "o", is taken from [[Japanese language|Japanese]] ({{nihongo2|劫}}, {{Transliteration|ja|kō}}; usually written with [[katakana]]: {{nihongo2|コウ}}) and can mean both "threat" and "aeon" (from the Buddhist ''[[Kalpa (aeon)|kalpa]]'').<ref>[http://senseis.xmp.net/?KoEtymologyDiscussion Ko Etymology Discussion] {{Webarchive|url=https://web.archive.org/web/20051216102035/http://senseis.xmp.net/?KoEtymologyDiscussion |date=2005-12-16 }} at Sensei's Library</ref>

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 rulesets 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 {{section link||Repetition}} below.
 
The superko rule is designed to ensure the game eventually comes to an end, by preventing indefinite repetition of the same positions. While its purpose is similar to that of the [[threefold repetition]] rule of Western chess, it differs from it significantly in nature; the superko rule bans moves that would cause repetition, whereas Western chess allows such moves as one method of forcing a draw. It is more similar to the prohibition of moves which would repeat the position in Chinese Chess (Xiangqi). The ko rule has important strategic consequences in go.

Some examples follow in which Rule 8 applies. These examples cover only the most important case, namely the 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.

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Go board 5x5
| ul|  b|  w|  u| ur
|  b| wT|  A|  w|  r
|  l|  b|  w|   |  r
|  l|   |  b| w1|  r
| dl|  d|  d|  d| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| ul|  b|  w|  u| ur
|  b|  B| b2|  w|  r
|  l|  b|  w|   |  r
|  l|   |  b| w1|  r
| dl|  d|  d|  d| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| ul|  b|  w|  u| ur
|  b| w3|   |  w|  r
|  l|  b|  w|   |  r
|  l|   |  b| w1|  r
| dl|  d|  d|  d| dr|24}}
|-valign="top"
|Black to move
|&nbsp;&nbsp;
|Black captures
|&nbsp;&nbsp;
|Illegal recapture
|}

Another example of ko follows. Here, Black 3 is illegal by the ko rule.

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Go board 5x5
| ul|  u|  u|  u| ur
|  l| b1|  b|   |  r
|  l|   |  w|   |  r
|  l|   |   |  w|  b
| dl|  d|  w| bT| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| ul|  u|  u|  u| ur
|  l| b1|  b|   |  r
|  l|   |  w|   |  r
|  l|   |   |  w|  b
| dl|  d|  w|  d| w2|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| ul|  u|  u|  u| ur
|  l| b1|  b|   |  r
|  l|   |  w|   |  r
|  l|   |   |  w|  b
| dl|  d|  w| b3| dr|24}}
|-valign="top"
|White to move
|&nbsp;&nbsp;
|White captures
|&nbsp;&nbsp;
|Illegal recapture
|}

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, 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''. Black 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.

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Go board 5x5
| ul|  w|  w|  b|  b
|  w|   |  w|  w|  b
|  l|  w|  w|  b|  b
| lA|  b|  b|   |  b
| wT| wT|  b|  b|  b|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| ul|  w|  w|  b|  b
|  w|   |  w|  w|  b
| lB|  w|  w|  b|  b
| w1|  b|  b|   |  b
| wT| wT|  b|  b|  b|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| ul|  w|  w|  b|  b
|  w|   |  w|  w|  b
| b2|  w|  w|  b|  b
| lA|  b|  b|   |  b
| dl|  d|  b|  b|  b|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| ul|  w|  w|  b|  b
|  w|   |  w|  w|  b
| lB|  w|  w|  b|  b
| w3|  b|  b|   |  b
| dl|  d|  b|  b|  b|24}}
|-valign="top"
|White to move
|&nbsp;&nbsp;
|White sacrifices
|&nbsp;&nbsp;
|Black captures
|&nbsp;&nbsp;
|Recapture legal
|}

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 capture 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''.

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Go board 5x5
|  w|  w|  w|  u| ur
| bT| bT|   |  w|  w
|  w| bT|  A|  b|  b
|  w|  w|  b|  b|  r
| dl|  d|  b|  d|  b|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
|  w|  w|  w|  u| ur
| bT| bT|  B|  w|  w
|  w| bT| w1|  b|  b
|  w|  w|  b|  b|  r
| dl|  d|  b|  d|  b|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
|  w|  w|  w|  u| ur
| bT| bT| b2|  w|  w
|  w| bT|  A|  b|  b
|  w|  w|  b|  b|  r
| dl|  d|  b|  d|  b|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
|  w|  w|  w|  u| ur
|  l|   |  B|  w|  w
|  w|   | w3|  b|  b
|  w|  w|  b|  b|  r
| dl|  d|  b|  d|  b|24}}
|-valign="top"
|White to move
|&nbsp;&nbsp;
|White sacrifices
|&nbsp;&nbsp;
|Black captures
|&nbsp;&nbsp;
|White snaps back
|}

=====Ko threats=====
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 Black instead plays 3 in the third diagram. For reasons that will become clear, Black 3 is called a "ko threat".

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w|  r
|  b|  A| bT|  w|  r
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w|  r
|  b| w2|  B|  w|  r
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w| b3
|  b| w2|  B|  w|  r
|  b|  b|  w|  w| dr|24}}
|-valign="top"
|White to move
|&nbsp;&nbsp;
|White captures
|&nbsp;&nbsp;
|Black plays away from the ko
|}

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 capture the white group altogether, no matter how White played.

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w| b3
|  b| w2| w4|  w|  r
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w| b3
|  b| w2| w4|  w| b5
|  b|  b|  w|  w| dr|24}}
|-valign="top"
|White 4 is an error
|&nbsp;&nbsp;
|Black takes advantage
|}

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 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.

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w| b3
|  b| w2|  B|  w| w4
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w|  r
|  b|  b|  w|  w| b3
|  b|  A| b5|  w| w4
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w| w6
|  b|  b|  w|  w|  r
|  b|  A| b5|  w| w4
|  b|  b|  w|  w| dr|24}}
|&nbsp;&nbsp;
|
{{Go board 5x5
| b1|  u|  b|  w|  w
|  l|  b|  b|  w| w6
|  b|  b|  w|  w|  r
|  b| b7| b5|  w| w4
|  b|  b|  w|  w| dr|24}}
|-valign="top"
|White replies to threat
|&nbsp;&nbsp;
|Black recaptures legally
|&nbsp;&nbsp;
|Ko bars recapture at ''a''
|&nbsp;&nbsp;
|Black connects
|}