Go is played on a board with a 19 x 19 grid, by two players, one using a set of (about) 180 white counters, the other a set of (about) 181 black counters. These counters are called stones.
Adjacent intersections are those intersections connected by lines of the grid, with no intervening intersections.
Two stones of the same colour are connected if they are on adjacent intersections or if they are both connected to a third stone.
A liberty of a stone is an unoccupied intersection adjacent to that stone or to any stone connected to that stone.
Territory of a player (at the end of the game) consists of all points occupied by that player's stones plus all unoccupied points adjacent to that territory.
A play consists of placing a stone (of that player's own colour) on an unoccupied intersection, then removing any of the opponent's stones that then have no liberties (if any), and then removing any of that player's own stones that then have no liberties (if any).
A move consists of
A game of go begins with an empty board, and the players take turns to move (beginning with black).
The game is finished when both players agree that there are no more worthwhile moves. 'Dead' stones may then be removed from the board by mutual agreement. If they cannot agree which stones are dead they must play on. If they cannot then agree who shall move next, all stones stay on the board (are alive) and are counted.
In an even game, 7 points (komi) are added to the white players territory.
In a handicap game, white passes the first n - 1 moves where n is the size of the handicap. There is no komi.
The winner is the player with the most points at the end of the game.
Go can be played on different sized boards than 19 x 19 with a corresponding different number of stones. 9x9 boards are often used for fast games or by learners.
The New Zealand rules of go use recursive definitions for describing connections, liberties and territory. As an example, the definition of territory states that a player's stones are territory for that player and all empty points adjacent to that player's territory are also territory for that player. That means that empty points adjacent to that player's stones are territory and empty points adjacent to these are also territory. In effect if we can trace a path from an empty point to a black stone by moving to adjacent empty points only, then that point is black territory.
In diagram 1 the positions marked with the same letter are adjacent. Intersections marked with different letters are not.
In diagram 2 the stones marked with the same numbers are connected. Stones marked with different numbers are not. Note that some connected stones are not adjacent.
In diagram 3 the stone marked with '1' has 5 liberties. They are made up of the two unoccupied intersections adjacent to it plus those adjacent to the other stones connected to it. Note that the liberty marked with 'a' is only counted once even though it is adjacent to two stones of the 'group'. The stones marked '2' have no liberties and should be removed from the board. If the black stone marked '3' was the last stone played then the white stones marked with the triangles should be removed as they have no liberties; the black stones can then stay on as they have the liberties vacated by the captured stones.
In diagram 4 the thirtysix points marked '1' are territory for black and the fourtythree points marked '2' are territory for white. The points marked 'a' and 'b' are territory for both players and can be ignored. (Note that either player could play on the point 'b' to deny their opponent one point of territory but that neither could play at 'a' without letting their opponent capture some of their stones). This was an even game so we add seven points to white and see that white has fourteen points more than black; thus white is the winner.
In diagram 5 we see some examples of repeating situations.
In the top left if black plays '1', capturing a white stone, white wants to respond by playing at the empty point just created at 'a', capturing black '1'. This is not allowed (see definition of move) as it would repeat the situation after white's last move. This position is known as 'ko'.
In the bottom left if black '1', white '2', black '3', white 'a', black 'b', and white is unable to capture at 'c' as that would repeat the starting position.
In the bottom right if black '1', white '2', black '3' we get a situation where white would like to recapture at 'a' but this would repeat the starting position, and so is not allowed.
In the top right black '1', white '2', black '3', white '4', black 'b', white 'a', black 'd' creates the position where white cannot recapture at 'c' (without repeating the board position).
In all the above cases white must play somewhere else to change the board position (preferably inviting a black response) and if black does not play there again (usually to connect the ko) white may recapture in a future move.
In diagram 6 black '1', white '2', black '3', white '4', black 'a', white 'b', black 'c' makes a situation where white cannot recapture at 'd' but a white move at '2' is allowed followed by black '1' and then white 'd'. Black cannot recapture at '3' or at 'c' (as that would repeat a previous board position); so moving first in this position gains nothing (and could lose everything).
In diagram 7 black '1', white '2', black 'a' repeats a previous board position but with the other player to move so black is allowed to play at 'a' in this position.
A pass is usually used to signify that the game is over. If the opponent then also passes the players can agree that the game is finished and which stones are to be removed.
Before counting at the end of the game it is necessary to remove any of the opponent's stones which can be captured. The rules allow for these 'dead' stones to be removed by mututal agreement. If the opponent does not agree to their stones being taken off the board in this way then it is necessary to play the moves to capture the stones. Where there is disagreement about who should have the first move in such a situation it is usual to allow the person whose stones are in dispute to have the first move to defend them. (They may of course choose to pass).
If a player attempts to repeat a previous board position it is up to their opponent to spot this before playing the next move (they cannot complain later). The invalid move should then be removed and that player's move be treated as a pass.
In other cases of moves that do not conform to the rules (eg. a player making two moves in a row or moving an already played stone) the game should be restored to the point where the illegal activity occurred with the offending player's last move being treated as a pass. If it is not possible to restore the board the offender should forfeit the game. In all cases the tournament referee (if there is one) should be called to oversee this process before any stones are moved.
In a handicap game the black player may prefer to use the traditional starting points shown in diagram 8. In two - four, six and eight - nine stone handicaps the points are played in order. In a five stone handicap the points 'a' - 'd' are played as well as 'i'. In a seven stone handicap the points 'a' - 'f' are played as well as 'i'. There is no traditional point for playing the one stone handicap but it is usual for black to play in the top right quadrant.
The size of the handicap is up to the players concerned or the tournament director but where both players have NZGS ratings the best handicap is the difference of their ratings rounded up to a whole number. If the difference is less that 0.5 they may also choose to play an even game.
The rules should be used in a spirit of fairness and cooperation. Where disputes arise they should first of all be settled by the players themselves. A higher authority (in the form of tournament referee or higher rated player) should be consulted in case of disagreement. In all cases, once the game is finished and the result confirmed by both players nothing can change the result.