Although the normal size of a Go board is 19 by 19 lines, it is possible to use smaller sizes. Beginners can learn the basics on a 9 by 9 board, and a quick game can be played on a 13 by 13 board without losing the essential character of the game. The examples in the rules will be given on a 9x9 board. The Arena however features only the 19x19 board.

The Go board
A game starts on an empty board. Note that the "I" is missing in the coordinate system.
Each player has a sufficient supply of stones, one taking black, the other white. Players take turns, placing one of their stones on a vacant point at each turn. Black moves first. Moving is not compulsory: a player may pass his turn without losing the right to move on his next turn. The game ends if both players pass on successive turns.
Once played, stones are not moved, but they may be captured.

Object
Ultimately the object of the game is to conquer more territory than the opponent, so obviously after a game has ended, there's counting involved. There are two systems for counting: Japanese and Chinese. The difference may be one point.

Counting
  • The Japanese count the vacant points surrounded by a player plus the stones has captured. As a shortcut, these captured stones are placed inside the opponents territory, to reduce it with a corresponding number of points.
  • The Chinese count both a player's stones and the vacant points of territory surrounded by him. Captured stones do not matter since placing them inside the opponent's territory doesn't alter the score.

Counting

Japanese count
Black has surrounded 15 points of territory, 10 in the lower right corner and 5 towards the top of the board. Black's territory includes a point formerly occupied by a stone he has captured.
White's territory is 17 points, but black will place his one prisoner inside it, reducing it to 16 points. Thus white wins with one point.

Chinese count
Black has surrounded 15 points of territory and 25 men, totalling 40.
White has surrounded 17 points of territory and 24 men, totalling 41.
Thus white wins with one point.

Groups and liberties
Stones of one color that are horizontally or vertically connected are called a group. The vacant points that are horizontally and vertically adjacent to a stone or a group of stones, are called liberties. An isolated stone or group of stones is captured when all of its liberties are occupied by enemy stones.

Atari

On the left three black stones, all in atari, that is: under threat of immediate capture. On the right white can capture the black group by placing a stone inside it, taking its last liberty. It doesn't matter that the white stone has no liberties at the moment of placement because it effectuates the capture and thus creates liberties for itself.

Suicide
Suicide is illegal. If in the above example black would have an outside liberty, white could not legally occupy the center. The only occasion on which a stone may be placed on a point where it has no liberties, is if it effectuates a capture that ensures its life after the captured stones have been taken off the board.

Eyes

Groups may live unconditionally. In the above diagram both black groups (actually the bottom one consists of a group and a single stone) are safe because white cannot commit suicide.

Eyes

Groups may also be dead unconditionally. In the above diagram the black group at the bottom is in danger of being captured. To ensure that his group has two eyes, Black needs to play at the marked point. If White plays at the marked point, the black group will no longer be able to make two eyes, and cannot avoid eventual capture. White can always fill in the outside liberties and then fill the points next to the marked point.
The black group at the top left is already alive even though there is a white stone inside one of its eyes. Since White can never capture the black stones, the white stone caught inside the group can't be saved.

In the course of a real game, players are not obliged to complete the capture of a dead group once it is clear to both players that it is dead. In this case, once White has played at the marked point, the situation may be left as it is until the end of the game. Then, the dead black stones are simply removed from the board and (in Japanese count) counted together with the capturing player's other prisoners.


A player may not recreate a position that occured previously with the same player to move.

Ko

The simplest example is depicted above: at the top white can capture, creating the postion at the bottom, where black can capture, creating the postion at the top, where white can capture ...
So in top position, assuming black's last move was elsewhere, white can capture the black stone and black cannot immediately recapture, but must make a move elsewhere, giving white the opportunity to "close the ko" (placing a stone where he just captured the black stone). If white refrains from closing the ko after black's move elsewhere, black again is at liberty to recapture, and then it will be white who in turn must make his next move elsewhere.

Far more complicated situations may arise if several of the above situations arise simultaneously at different places on the board. The leading principle remains that a player may not recreate a position that occured previously with the same player to move.

Seki
Local stalemates may appear and they're called seki.

Seki

Usually a group which can't make two eyes will die unless one of the surrounding enemy groups also lacks two eyes. This often leads to a race to capture, but can also result in a stand-off situation, known as seki, in which neither group has two eyes, but neither can capture the other due to a shortage of liberties.
Two examples of seki are shown in the diagram above. Neither player can afford to play at the marked points since to do so would enable the other to make a capture.

Even though the groups involved in a seki may have an eye, as a general rule none of the points inside a seki count as territory for either player.

End of a game
The game ends by agreement. When neither player believes that he can make more territory, capture more stones, or reduce his opponent's territory by playing on, he will pass instead of making a move on the board. Two consecutive passes end the game.

Handicap system
As remarked in the introduction, one of the best features of the game of Go is its handicap system. A weaker player may be given an advantage of anything up to nine stones. These are placed on the board as his first move. The applet will accept more than one stone as black's first move.

Through the grading system, any two players can easily establish the difference in their strength, and therefore how many stones the weaker player should take in order to compensate for this difference. Since a player's grade is measured in terms of stones, the number of stones for the handicap is simply the difference in grade between the two players.

There is an established pattern for the placement of handicap stones on the marked points of the board. For handicaps of two or three stones, where the stones can't be placed symmetrically, the convention is that the far left corner is left vacant.