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=== Play === ==== Initial position ==== '''Rule 5. At the beginning of the game, the board is ''empty''.''' ==== Alternation of turns ==== '''Rule 6. Black moves first. The players alternate thereafter.''' What players may do when they move is the object of Rules 7 and 8. ==== Moving ==== '''Rule 7. On their 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):''' * Step 1. Placing a stone of their color on an empty intersection (chosen subject to Rule 8 and, if it is in effect, to Optional Rule 7A). * Step 2. Removing from the board any stones of their opponent's color that have no liberties. * Step 3. Removing from the board any stones of their own color that have no liberties. 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, bearing in mind that, in view of Steps 2 and 3, all stones remaining on the board after any move must have at least one liberty. ==== Placing a stone on the board ==== '''Step 1 of a play. The player places a stone of their 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: {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| u| u| u| ur | l| | | | r | l| | | | r | l| | | | r | dl| d| d| d| dr|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | | b1| r | l| | | | r | l| | | | r | dl| d| d| d| dr|24}} | | {{Go board 5x5 |ul| u| u| u| ur | l| | | b| r | l| | w2| | r | l| | | | r |dl| d| d| d| dr|24}} |-valign="top" |Start | |Black plays | |White plays |} The following diagrams show how Black might play later in the same game: {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| u| u| w| ur | l| b| | b| r | l| | w| | r | w| | | | r | dl| d| d| b| dr|24}} | | {{Go board 5x5 | ul| u| u| w| ur | l| b| b1| b| r | l| | w| | r | w| | | | r | dl| d| d| b| dr|24}} | |-valign="top" |Before | |After |} 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). ==== Capture ==== '''Step 2 of a play. (After playing their stone) a player removes from the board any stones of their opponent's color that have no liberties.''' The stones removed from the board are said to have been ''captured'' by the player moving. 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. {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | l| b| w| A| r | l| | b| | r | dl| d| d| d| dr|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | l| b| w| b1| r | l| | b| | r | dl| d| d| d| dr|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | l| b| | b| r | l| | b| | r | dl| d| d| d| dr|24}} |-valign="top" |Before | |Black plays | |After removal |} At the edge of the board and especially in the corners, stones have fewer liberties to start with and are more easily captured. {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| u| u| u| ur | b| | | | r | w| b| | | r | lA| | | | r | dl| d| d| d| dr|24}} | | {{Go board 5x5 | ul| u| u| u| ur | b| | | | r | w| b| | | r | b1| | | | r | dl| d| d| d| dr|24}} | | {{Go board 5x5 | ul| u| u| u| ur | b| | | | r | l| b| | | r | b| | | | r | dl| d| d| d| dr|24}} |-valign="top" |Before | |Black plays | |After removal |} Next, White captures a chain of four black stones by playing at ''a''. {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| w| u| u| ur | w| b| A| | r | w| b| b| w| r | l| w| b| w| r | dl| d| w| d| dr|24}} | | {{Go board 5x5 | ul| w| u| u| ur | w| b| w1| | r | w| b| b| w| r | l| w| b| w| r | dl| d| w| d| dr|24}} | | {{Go board 5x5 | ul| w| u| u| ur | w| | w| | r | w| | | w| r | l| w| | w| r | dl| d| w| d| dr|24}} |-valign="top" |Before | |White plays | |After removal |} Black captures the white chain by playing at ''a''. The black stone is not captured, because the white stones are removed first, providing it with two liberties. {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| u| u| u| ur | l| | | | r | l| | | b| b | l| | b| w| w | dl| d| b| w|drA|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | | | r | l| | | b| b | l| | b| w| w | dl| d| b| w| b1|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | | | r | l| | | b| b | l| | b| | r | dl| d| b| d| b1|24}} |-valign="top" |Before | |Black plays | |After removal |} 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''. {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | l| w| w| w| r | l| | b| b| w | dl| b| wT| wT|drA|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | l| w| w| w| r | l| | b| b| w | dl| b| wT| wT| b1|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | l| w| w| w| r | l| | b| b| w | dl| b| d| dB| b1|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | l| w| w| w| r | l| | b| b| w | dl| b| d| w2| b1|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | l| w| w| w| r | l| | b| b| w | dl| b| d| w2| dr|24}} |-valign="top" |Before | |Black plays | |After capture | |White plays | |After capture |} Here, White captures the three marked black chains by playing at ''a''. {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| u| b| b| ur | w| w| | w| r | w| bT| w| bT| w | bT| A| bT| bT| w | bT| bT| w| w| dr|24}} | | {{Go board 5x5 | ul| u| b| b| ur | w| w| | w| r | w| bT| w| bT| w | bT| w1| bT| bT| w | bT| bT| w| w| dr|24}} | | {{Go board 5x5 | ul| u| b| b| ur | w| w| | w| r | w| | w| | w | l| w1| | | w | dl| d| w| w| dr|24}} |-valign="top" |Before | |White plays | |After capture |} ==== Self-capture ==== '''Step 3 of a play. (After playing their stone and capturing any opposing stones) a player removes from the board any stones of their 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''. Most rulesets give effect to Optional Rule 7A, which prohibits it. This means that, in those rulesets, any play which under the basic rules would require a self-capture to be performed is illegal. For further information, see {{section link||Suicide}} below. First 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. {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| u| u| u| ur | l| | | | r | b| b| b| b| b | b| wT| wT| wT| wT | b| wT| dA| b| b|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | | | r | b| b| b| b| b | b| wT| wT| wT| wT | b| wT| b1| b| b|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | | | r | b| b| b| b| b | b| | | | r | b| d| b1| b| b|24}} |-valign="top" |Before | |Black plays | |After capture |} 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. If a play results in the capture of opposing stones, self-capture will not occur. A newly placed stone only removes a liberty from chains which it is adjacent to; thus, a chain can only be captured if it contained a stone adjacent to the newly placed stone. The removal of that adjacent opposing stone creates a liberty for the newly placed stone, which is now a part of the chain(s) of its own color that lost a liberty when it was placed. Thus, all of those chains, which are now one chain, still have at least one liberty. Now 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 them 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 {{section link||Repetition}} below.) {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | b| | b| | r | w| w| | | r |dlA| w| d| d| dr|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | b| | b| | r | w| w| | | r | b1| w| d| d| dr|24}} | | {{Go board 5x5 | ul| u| u| u| ur | l| | b| | r | b| | b| | r | w| w| | | r |dlA| w| d| d| dr|24}} |-valign="top" |Before | |Black plays | |After self-capture; violates Rule 8 |} In the next example, Black plays at ''a'', resulting in the self-capture of the marked black stones. {|border="0" cellspacing="0" cellpadding="0" |-valign="top" | {{Go board 5x5 | ul| u| w| u| ur | l| | b| | r | l| b| w| w| w | l| w| bT| A| w | b| d| w| bT| bT|24}} | | {{Go board 5x5 | ul| u| w| u| ur | l| | b| | r | l| b| w| w| w | l| w| bT| b1| w | b| d| w| bT| bT|24}} | | {{Go board 5x5 | ul| u| w| u| ur | l| | b| | r | l| b| w| w| w | l| w| | A| w | b| d| w| d| dr|24}} |-valign="top" |Before | |Black plays | |After self-capture |} ===={{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|ε«}}, {{Transliteration|ja|kΕ}}; usually written with [[katakana]]: {{nihongo2|γ³γ¦}}) 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}} | | {{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}} | | {{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 | |Black captures | |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}} | | {{Go board 5x5 | ul| u| u| u| ur | l| b1| b| | r | l| | w| | r | l| | | w| b | dl| d| w| d| w2|24}} | | {{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 | |White captures | |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}} | | {{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}} | | {{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}} | | {{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 | |White sacrifices | |Black captures | |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}} | | {{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}} | | {{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}} | | {{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 | |White sacrifices | |Black captures | |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}} | | {{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}} | | {{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 | |White captures | |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}} | | {{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 | |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}} | | {{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}} | | {{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}} | | {{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 | |Black recaptures legally | |Ko bars recapture at ''a'' | |Black connects |}
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