Go
History: Go is a traditional game from the East. It originated in China over 2,500 years ago and was introduced to Japan in 800 AD, becoming very popular in both countries. It is an influential game, with simple rules but with remarkable strategic complexity. In ancient times, it was one of the four arts taught to Chinese nobles (the others were music, calligraphy and painting). The game unfolds like a war that takes place in different battles, conflicts of local patterns with consequences across the board. The knowledge accumulated over the centuries, both in thematic literature and in tactical and strategic knowledge, rivals that of chess.
Material: A square board with 19 rows by 19 columns (played on intersections). It is also common to play on 9×9 and 13×13 boards for faster and less strategic games. A sufficient number of white and black pieces (about 150 for each color).
Definitions:
Group — one or more pieces of the same color adjacent vertically or horizontally.
Freedom of a group — the sum of empty intersections adjacent (vertically or horizontally) to the pieces of the group.
The following diagram shows a group of seven black pieces (with freedom equal to seven) and a group of six white pieces (with freedom equal to one).
The pieces always move forward, to an empty square, either on their column or on one of their diagonals. The diagram on the left shows where each piece could move if it were their turn to play.
Territory — a set of intersections surrounded by pieces of the same color and eventually by the boundaries of the board.
In the following example we observe three territories: A black territory with one intersection (at a9); another, also black, with nine intersections; and a white one with four intersections.
The number of pieces needed to create a territory is smaller in the corners of the board and larger in the center, as shown in the diagram.
Rules
By tradition, Black starts. On each turn, each player places a piece of their color on an empty intersection.
If, as a consequence of placing the new piece, any opposing group is left without freedoms, that group is captured and removed from the board, with the respective pieces being designated as prisoners.
A piece cannot commit suicide, that is, it cannot be placed in such a way that the group to which it belongs is left without freedoms, unless this move captures an opposing piece.
Ko rule — a player cannot repeat the board position from the previous turn.
Objective
When both players have passed consecutively, the game ends. The pieces that are in enemy territory and that cannot be captured are removed. Each player adds up the number of pieces he has and the number of intersections in the territories he controls. The player with the highest number of pieces wins. In the event of a tie, White (i.e. the second player) wins.
Notes
The more freedom a group has, the stronger it is. A group that only has one freedom is at risk of capture. Players must avoid this precarious situation with regard to their groups. In the following example, the white group has four freedoms. If a white piece is placed on c3, the group now has three freedoms. If a white piece is placed on f4, the group now has five freedoms (a better move than the first).
Regarding the suicide rule, it is allowed to place a piece in a location without liberties as long as there are captures. Thus, after removing the captured pieces, the group to which the piece belongs regains liberties. In the following diagram, a black piece can be played on i5, because the group of six white pieces is captured.
The suicide rule enables the existence of an essential pattern in Go: live groups. A group is said to be live if its structure is such that the opposing player cannot capture it under any circumstances.
The following example shows some living groups. In each of them there are two separate territories. Placing a piece in one of the territories is not possible (it would be suicide). For the opponent to be able to capture one of these groups he would have to play two pieces at the same time, which is illegal. These two territories are called eyes, and a group is said to be alive if it manages to form two eyes.
The following diagram shows two groups that are not alive.
The black group in the upper right corner only has one liberty on i9. If White plays there, he captures the group.
The white group below has two liberties. However, if Black moves to f1, the white group is in danger. Even if White captures f1 (by playing g1), the group is then captured by a new black move to f1.
The Ko rule is used to avoid situations in which both players would be forever repeating the same sequence of moves.
In the following example, consider that Black has just played on e4, capturing a white piece on e5. By the Ko rule, White cannot capture e4 by playing on e5, because that would mean repeating the position from the previous turn. So he must play on another intersection.
Note that on the next turn this restriction no longer has effect. If Black does not protect himself by playing e5, White may do so (and at that point the Ko rule applies to Black).
The following diagram is an example of an endgame (both players have passed) on a small board. Pieces belonging to groups that are not alive and are in the opponent's territory are removed. In this example, there are three white pieces (i2, b7 and b8) and two black pieces (f8 and h6).
The respective territories are counted. Each player has two. The white territory on the left is worth 4, while the one on the right is worth 13. The black territory on the left is worth 8, while the one on the right is worth 12. There are also territories that belong neither to Black nor to White (for example, d3 or d9), which are not counted. In addition, we count the number of pieces each player has: Black has 20 pieces and White has 19. Therefore, White's score is equal to 19 (pieces) + 17 (territories) = 36 points. Black's score is equal to 20 (pieces) + 20 (territories) = 40 points. Black wins!
Since Black has a certain advantage in starting, it is common to give White an extra point (which in the East is called Komi). A common value for the Komi is 5.5 (the decimal part helps to avoid ties).
Additional information: