Go

Overview

Go is a two-player strategic board game that originated in China more than 4000 years ago.

Objective

The objective of Go is to have a higher score than your opponent at the end of the game. Your score depends on the amount of territory that you have surrounded and the number of stones that you have on the board.

Board

The board is a square grid of 19 horizontal and 19 vertical lines forming 361 intersections, also called points. Smaller boards (like 13x13 and 9x9) are also used by beginners or for quick games.

Liberties and Territory

The concepts of group, liberty and territory are central to the game. In the following definitions, "adjacent" always means "orthogonally adjacent". A set is "maximal" if it's as big as it can be, i.e. if it's not a subset of a larger set.

  • A group is a maximal set of one or more like-colored adjacent stones:

All white stones here form a single group.
The black stones form two distinct groups.

  • A liberty is an empty point adjacent to a group:

The marked points are all the liberties of these four groups.

  • A territory of a player is a maximal set of adjacent empty points such that all stones adjacent to it are of that player's color:

The set of 6 empty points in the top left corner is a territory of White's.

The set of 3 empty points on the right side is a territory of Black's.

The set of 6 empty points in the bottom left corner is not a territory
of either player's because it's adjacent to stones of both colors.

Play

The game begins with an empty board.

One of the players uses black stones and the other player uses white stonesBlack plays first, then turns alternate.

On their turn, a player must place a stone on an empty point or pass.

There are two restrictions applied to stone placements: the ko rule and the prohibition of suicide moves.

Capture

After a placement, all enemy groups with no liberties are removed from the board:

The black group in the top left corner has only one liberty. Placing a white stone there results in capturing the entire black group because it removes its last liberty.

 

Placing a white stone at the marked point does not capture the black group because it still has one liberty. However, White can capture the black group on their next turn by occupying its second liberty with a white stone.

 

Suicide moves

A player may not place a stone such that its group is left with no liberties, unless placing this stone results in a capture of enemy stones.

 

Black cannot place a stone where the transparent black stones are shown because such moves will leave a black group without liberties.

 

Placing a white stone at the top of the board where the transparent white stone is shown leaves the 3-stone white group without liberties. However, this move is allowed because it captures the black group. After removing the captured black stones from the board, the 3-stone white group at the top gets liberties.

 

Ko

You may not, as a result of a placement on the board, recreate the board position that existed before your opponent's last move.

This situation occurs when a single stone is captured and the opponent has the option to place a new stone on the same point, capturing the same stone that forced the previous capture. This creates a 2-move cycle:

White's placement results in the capture of a single black stone. If Black were to place a black stone on the same point, the stone that White placed on their turn would be captured and the board would return to a previous position. The ko rule prevents such repetition.

Under area scoring, 3-move cycles are also forbidden. Some rulesets forbid cycles of any length by means of a superko rule, but this rule is not used here.

Komi, Handicap and Button

To compensate for the advantage of the first move, the player who uses white stones can get some additional points, called komi. The value of the komi is agreed upon by both players before starting the game. The commonly used values lie between 5.5 and 7.5. Non-integer values are normally used in order to avoid ties.

If there is a large difference in skill between the players then the handicap can be used to compensate for the difference: the weaker player plays with the black stones and he is allowed to place several stones before his opponent makes his first move. The number of handicap stones is agreed upon by both players before starting the game. The commonly used values lie between 2 and 9.

The button a special token which is worth 0.5 points. It can be used to break ties as an alternative to non-integer komi. At his turn, instead of making a board play or passing, a player may take the button, provided that none of the players has taken it before. Taking the button lifts any restriction on taking a ko.

End of Game

The game ends when both players pass consecutively.

At this point, the players decide which stones would be certainly captured if the game were to continue. Those stones are said to be dead. They must be removed and added to the pile of captured stones before counting the final score. The stones that remain on the board are said to be alive. If the players don't agree which stones are dead and which are alive, the game resumes. When both players pass consecutively again, the final score is determined.

There are two different scoring systems: area scoring and territory scoring.

Under area scoring, a player's score is the sum of the number of empty points in their territories and the number of stones of their color on the board. If the game is resumed upon a disagreement over which stones are dead, the position resulting from the sequence of moves played after the resumption is used for scoring.

Under territory scoring, a player's score is the sum of the number of empty points in their territories and the number of enemy stones that they have captured during the game. If the game is resumed upon a disagreement over which stones are dead, the final position prior to the resumption must be restored afterwards in order to score the game, as placing further stones in that position harms the players' scores. Some special rules are typically used in this system to govern play after a resumption, and territories surrounded by groups in seki (Japanese for "mutual life") are normally excluded from the score. However, those special rules are out of the scope of this article and are not used here.

Finally, in either system, the value of komi is added to White's score, and 0.5 points are added to the button owner if applicable.

The player with the higher score wins the game.

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