This document contains the shorter rules of Go for the AGA (April 1991).
1) The Board and Stones: Go is a game of strategy between two sides usually played on a 19xl9 grid (the board). The game may also be played on smaller boards, 13xl3 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.
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.
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.
5) Capture: A liberty of a stone is a vacant, horizontally or vertically adjacent intersection. A single stone in the middle of an empty board has four liberties: the vacant intersections above, below, left and right of the stone. The intersections diagonal to the stone are not adjacent and are not counted as liberties of the stone. A single stone on a side intersection has a maximum of three liberties; a single stone in the corner has a maximum of two liberties.
Stones of the same color are said to be connected if they are adjacent along horizontal or vertical lines on the board (each occupies a liberty of the other). Two stones are part of the same string if they are linked by a chain of connected stones of the same color. The liberties of a string of stones are the liberties of all the individual stones in that 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, leaving no liberties, 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.
6) Repeated Board Position (Ko): It is illegal to play in such a way as to exactly recreate a previous full board position from the game, with the same player to move. The most typical example is a situation where the players can each alternately capture and recapture a single stone. This is known as ko. ("Ko" is the Japanese Buddhist word for eternity.) After the first capture, the player moving next may not recapture immediately, as this would repeat the board position; instead, that player must play elsewhere on the board (or pass).
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.
8) Illegal Moves: An illegal move is one violating the rules. If a player makes an illegal move, it shall be taken back, treated as a pass, and a pass stone exchanged.
9) Ending the Game: Two consecutive passes normally 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.
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. Although players' scores may differ under the two methods, the difference in their scores, and 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. An empty point is surrounded by stones of a single color if one can't reach any stone of the opposing color from that point by moving only to adjacent empty points. There are rare situations (Japanese seki) in which empty points are left at the end of the game which are not entirely surrounded by stones of a single color, and which neither player dares to fill.
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.
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.