AGA Rules Committee
September 1, 1991

Official AGA Rules of Go

{The following are the American Go Association Rules of Go for amateur play. Unless specifically stated otherwise, these rules are in effect at all AGA sanctioned events. The rules themselves are given in boldface; comments and examples are given in ordinary type [and surrounded with curly braces or parentheses]. Illustrative figures are [interspersed].}

Any paraphrase of these rules which is identical in content is acceptable as a statement of the AGA Rules of Go so long as it makes reference to the more complete Official Rules given below. (By "identical in content" we mean that the result of applying these paraphrased rules should give the same result as would the Official Rules in every situation.)

{These rules are supplemented by the Official AGA Tournament Regulations governing time control, player conduct, the role of monitors, etc.}

1) The Board and Stones: Go is a game of strategy between two sides usually played on a 19x19 grid (the board). The game may also be played on smaller boards, 13x13 and 9x9 being the two most common variants. The board is initially vacant, unless a handicap is given (see Rule 4). The two sides, known as Black and White, are each provided with an adequate supply of playing tokens, known as stones, of the appropriate color.

{For recording purposes, the horizontal lines on the board are designated 1,2,3,...,19, starting from the bottom as seen by Black. The vertical lines are designated A,B,C,...,T (skipping 'I'), starting from the left as seen by Black. Points on the board are identified by their coordinates, c.g. A-1, C-3, T-19, etc.}

2) Play: The players alternate in moving, with Black playing first. In handicap games, White moves first after Black has placed his or her handicap stones. A move consists in playing a stone of one's color on an empty intersection (including edges and corners), or in passing. Certain moves are illegal (Rules 5 and 6), but a pass is always legal (Rule 7). Points are awarded for controlling space in a manner described below (Rule 12). The object of the game is to end with the greater total number of points.

3) Compensation: In an even (non-handicap) game, Black gives White a compensation of 5 1/2 points for the advantage of the first move. This compensation is added to White's score at the end of the game. In handicap games, Black gives White 1/2 point compensation. This avoids draws.

4) Handicaps: The game may be played with a handicap to compensate for differences in player strengths. The weaker player takes Black, and either moves first, giving only 1/2 point compensation to White, as in Rule 3 (this is known as a "one stone handicap"), or places from 2 to 9 stones on the board before the first White move.

{The nine intersections corresponding to the horizontal lines 4, 10, and 16 and the vertical lines D, K, and Q are called star points, and are ordered as follows: (See Figure 1.)

1st star point Q-16 5th star point Q-10
2nd star point D-4 6th star point D-10
3rd star point Q-4 7th star point K-16
4th star point D-16 8th star point K-4
9th star point K-10 (center point)

Figure 1

The handicap stones are traditionally played as follows:
2 stones on the 1st and 2nd star points
3 stones on the 1st through 3rd star points
4 stones on the 1st through 4th star points
5 stones on the 1st through 4th star points and the center point
6 stones on the 1st through 6th star points
7 stones on the 1st through 6th star points and the center point
8 stones on the 1st through 8th star points
9 stones on the 1st through 9th star points

Unless otherwise specified, handicap stones shall be placed in this fashion. Handicaps greater than nine stones and handicaps on boards with fewer than 19 lines are not standardized.}

If the players have agreed to use area counting to score the game (Rule 12), White receives an additional point of compensation for each Black handicap stone after the first. (Black would otherwise gain an additional point of area for each handicap stone.)

5) Capture: Stones of the same color are said to be connected if they are adjacent along horizontal or vertical--not diagonal--lines on the board. A string of connected stones consists of those stones which can be reached from a given stone by moving only to adjacent stones of the same color. A string of connected stones is surrounded by stones of the opposite color if it has no empty points horizontally or vertically--not diagonally--adjacent to any of its member stones. (Such adjacent empty points are known as liberties of the string.)

After a player moves, any stone or string of stones belonging to the opponent which is completely surrounded by the player's own stones is captured, and removed from the board. Such stones become prisoners of the capturing player. It is illegal for a player to move so as to create a string of his or her own stones which is completely surrounded (without liberties) after any surrounded opposing stones are captured.

{This means that it is possible to fill an empty space within an opponent's group and capture even if the player's own stone or stones would momentarily be surrounded by the group being captured. See Figure 2. But self-capture is illegal.

Figure 2. White 1 captures the Black stones


6) Repeated Board Position (Ko): 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.

{The most typical example is a situation where the players can each alternately capture and recapture a single stone. This is known as "ko" (See Figure 3). After the first capture, the player moving next may not capture immediately, as this would repeat the board position; instead, that player must play elsewhere on the board (or pass). The player who first captured may then "fill" the ko (or otherwise resolve it), or play elsewhere as well (often in response to the other player's previous move.) If the board position has changed, and the ko has not yet been resolved, the opponent is then free to capture, and it is the original player who may not then immediately recapture. This process is known as a ko fight, and the moves played away from the ko itself are known as ko threats.

Figure 3. Ko

Rarely, multiple kos or other repetitive situations will arise; the principle for handling them is always the same: the players must avoid repeating the full-board position, so they are periodically, and alternately, forced to play away from the repetitive situation before responding.}

7) Passing: On his or her turn, a player may pass by handing the opponent a stone, referred to as a pass stone, rather than playing a stone on the board.

{Normally neither player would choose to pass if there were any worthwhile moves to be made on the board (even if they did not have to give up a pass stone). Thus, the exchange of a pass stone with the opponent also serves as a signal that the player passing believes that the game is over. Of course, the opponent is free to continue to play if he or she believes that there are worthwhile moves left to make, and the player who passed is free to respond.}

8) Illegal Moves: An illegal move is one violating the rules. If a player makes an illegal move--such as moving twice in a row (i.e., before the opponent has made a response), attempting to play on an occupied intersection, self-capture, or retaking a ko so as to repeat the full board position, the player must take back his or her move (both moves, if he or she moved twice in succession), it shall be treated as a pass, and a pass stone exchanged.

An illegal move must be noted as such by the opponent before he or she makes his or her move. When a player moves, he or she is tacitly accepting the opponent's previous move as valid. In particular, if it is discovered that an earlier move by one of the players was illegal, the game must nevertheless be continued as it stands unless both players agree to restore the earlier board position and proceed from that point.

9) Ending the Game: Two consecutive passes signal the end of the game. After two passes, the players must attempt to agree on the status of all groups of stones remaining on the board. Any stones which the players agree could not escape capture if the game continued, but which have not yet been captured and removed, are termed dead stones. If the players agree on the status of all such groups, they are removed from the board as prisoners of the player who could capture, and the game is scored as in Rule 12. If there is a disagreement over the status of some group or groups, play is resumed as specified in Rule 10.

10) Disputes: If the players disagree about the status of a group of stones left on the board after both have passed, play is resumed, with the opponent of the last player to pass having the move. The game is over when the players agree on the status of all groups on the board, or, failing such agreement, if both players pass twice in succession. In this case any stones remaining on the board are deemed alive. Any stone or group of stones surrounded and captured during this process is added to the capturing player's prisoners as usual.

{It is recommended, particularly if the players do not share a common language, that the following procedure be used to determine agreement on the status of groups. After two consecutive passes, the next player touches each connected string of opposing stones on the board which he or she believes to be dead. If the opponent disagrees, he or she also touches the same string. When a player is done indicating groups he or she believes are dead, he or she passes, passing a stone to the opponent as usual, and the opponent follows the same procedure. At any point, a player may resume play rather than continuing to indicate dead groups or passing. If both players pass and there was no disagreement indicated, the game is over, and all groups which the players have indicated as dead are removed from the board. If they both pass while a disagreement still exists, all stones remaining on the board are alive, and the board is counted as it stands. (The burden is thus effectively on the player who would be disadvantaged by such a result to resume play in the event of a disagreement.)

11) The Last Move: White must make the last move--if necessary, an additional pass, with a stone passed to the opponent as usual. The total number of stones played or passed by the two players during the entire game must be equal.

12) Counting: There are two methods for counting the score at the end of the game. One is based on territory, the other on area. The players should agree in advance of play which method they will use. If there is no agreement, territory counting shall be used.

(Although players' scores may differ under the two methods, the difference in their scores, and hence the game result, will be the same.)

Territory: Those empty points on the board which are entirely surrounded by live stones of a single color are considered the territory of the player of that color.

(At the end of the game, the empty points remaining on the board fall into regions. A region is the smallest set of empty points containing a given empty point and any empty points adjacent to any empty point in the set. That is, a region consists of those empty points which can be reached from a given empty point by moving only to adjacent empty points. A region is entirely surrounded by stones of a single color if the only stones adjacent to empty points in the region are of that color. There are situations (Japanese seki) in which a region of empty points is left at the end of the game which is not entirely surrounded by stones of a single color, and which neither player would fill because to do so would bring dire consequences. See Figure 4. When counting by territory, it is also possible that there will be some neutral points left between live groups belonging to the two players which have not been filled, although it is customary to fill all such points before scoring the game.)

Figure 4. Seki

Area: All live stones of a player's color left on the board together with any points of territory surrounded by a player constitute that player's area.

Neutral Points: Any empty points left on the board at the end of the game which are not completely surrounded by either player's stones are known as neutral points, and are not counted toward either player's territory or area. (There will rarely be any such points.)

Counting by Territory: When counting by territory, players add up their total territory less any prisoners held by the opponent (including dead stones removed at the end of the game). The player with the greater total (after adjusting for any compensation offered according to Rule 3) is the winner.

(It is customary for the players to fill in their opponent's territory with their prisoners, and to then rearrange their territories to facilitate counting. These are merely mechanical conventions to simplify counting.)

Counting by Area: When counting by area, the players add up their total area. Prisoners are ignored. The player with the greater total area (after adjusting for any compensation offered according to Rules 3 and 4) is the winner.

(In fact, since the total of the two players' areas will sum to 361, less any neutral points left on the board in seki, it is generally only necessary for one of the two players to count their area; if it exceeds 180, (or 180 adjusted for half of any neutral points in seki), they are the winner. It is customary for the player doing the counting according to this method to fill in his or her territory and then rearrange the stones into convenient heaps. Again, these are merely mechanical conventions to simplify the counting process.)

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