Go (game)

Go is an abstract strategy board game for two players, in which the aim is

to surround more territory than the opponent. The game was invented in ancient China more

than 2,500 years ago, and is therefore believed

to be the oldest board game continuously played today.

It was considered one of the four essential arts of the cultured aristocratic Chinese scholars in

antiquity. The earliest written reference

to the game is generally recognized as the historical annal Zuo Zhuan.

Despite its relatively simple rules, Go is very complex, even more so than chess. Compared to chess,

Go has both a larger board with more scope for play and longer games, and, on average,

many more alternatives to consider per move. The playing pieces are called "stones".

One player uses the white stones and the other, black.

The players take turns placing the stones on the vacant intersections of a board

with a 19×19 grid of lines. Beginners often play on smaller 9×9 and 13×13 boards,

and archaeological evidence shows that the game was played in earlier centuries on a board

with a 17×17 grid. However, boards with a 19×19 grid had become standard

by the time the game had reached Korea in the 5th century CE

and later Japan in the 7th century CE. Once placed on the board, stones may not be moved,

but stones are removed from the board when "captured". Capture happens when a stone

or group of stones is surrounded by opposing stones on all orthogonally-adjacent points.

The game proceeds until neither player wishes to make another move;

the game has no set ending conditions beyond this. When a game concludes,

the territory is counted along with captured stones and komi to determine the winner.

Games may also be terminated by resignation. As of mid-2008, there were well

over 40 million Go players worldwide, the overwhelming majority of them living in East Asia.

the International Go Federation has a total of 75 member countries

and four Association Membership organizations in multiple countries.

Overview

[^] Go is an adversarial game with the objective of surrounding a larger total area of the board

with one's stones than the opponent. As the game progresses,

the players position stones on the board to map out formations and potential territories.

Contests between opposing formations are often extremely complex and may result in the expansion,

reduction, or wholesale capture and loss of formation stones.

[^] A basic principle of Go is that a group of stones must have at least one "liberty"

to remain on the board. A "liberty" is an open "point" bordering the group.

An enclosed liberty is called an "eye", and a group of stones with two or more eyes is said

to be unconditionally "alive". Such groups cannot be captured, even if surrounded. A group

with one eye or no eyes is "dead" and cannot resist eventual capture. The general strategy is

to expand one's territory, attack the opponent's weak groups, and always stay mindful of the

"life status" of one's own groups. The liberties of groups are countable. Situations

where mutually opposing groups must capture each other or die are called capturing races,

or semeai. In a capturing race, the group with more liberties will ultimately be able

to capture the opponent's stones. Capturing races and the elements of life

or death are the primary challenges of Go.

A player may pass on determining that the game offers no further opportunities

for profitable play. The game ends when both players pass, and is then scored. For each player,

the number of captured stones is subtracted from the number of controlled points in "liberties"

or "eyes", and the player with the greater score wins the game. Games may also be won

by resignation of the opponent.

Finer points

In the opening stages of the game, players typically establish positions in the corners

and around the sides of the board. These bases help

to quickly develop strong shapes which have many options for life and establish formations

for potential territory. Players usually start in the corners,

because establishing territory is easier with the aid of two edges of the board.

Established corner opening sequences are called "joseki" and are often studied independently.

"Dame" are points that lie in-between the boundary walls of black and white,

and as such are considered to be of no value to either side. "Seki"

are mutually alive pairs of white and black groups where neither has two eyes. A "ko"

is a repeated-position shape that may be contested by making forcing moves elsewhere.

After the forcing move is played, the ko may be "taken back" and returned to its original position.

Some "ko fights" may be important and decide the life of a large group,

while others may be worth just one or two points. Some ko fights are referred to as "picnic kos"

when only one side has a lot to lose. The Japanese call it a hanami ko. Playing

with others usually requires a knowledge of each player's strength, indicated by the player's rank.

A difference in rank may be compensated by a handicap—Black is allowed to place two

or more stones on the board to compensate for White's greater strength.

There are different rule-sets, which are almost entirely equivalent, except

for certain special-case positions. Aside from the order of play and scoring rules,

there are essentially only two rules in Go: Almost all other information about how the game is

played is a heuristic, meaning it is learned information about how the game is played, rather

than a rule. Other rules are specialized, as they come about through different rule-sets,

but the above two rules cover almost all of any played game.

Although there are some minor differences between rule sets used in different countries,

most notably in Chinese and Japanese scoring rules,

these differences do not greatly affect the tactics and strategy of the game. Except where noted,

the basic rules presented here are valid independent of the scoring rules used.

The scoring rules are explained separately. Go terms

for which there are no ready English equivalent are commonly called by their Japanese names.

Basic rules

[^] Two players, Black and White,

take turns placing a stone of their own color on a vacant point of the grid on a Go board.

Black plays first. If there is a large difference in skill between the players,

the weaker player typically uses Black and is allowed to place two or more stones on the board

to compensate for the difference. The official grid comprises 19×19 lines,

though the rules can be applied to any grid size. 13×13 and 9×9 boards are popular choices

to teach beginners, or for playing quick games. Once placed, a stone may not be moved

to a different point. Vertically

and horizontally adjacent stones of the same color form a chain that cannot subsequently be

subdivided and, in effect, becomes a single larger stone. Only stones immediately connected

to one another by the lines on the board create a chain;

stones that are diagonally adjacent are not connected. Chains may be expanded

by placing additional stones on adjacent intersections, and can be connected together

by placing a stone on an intersection that is adjacent to two or more chains of the same color.

[^] A vacant point adjacent to a stone is called a liberty for that stone.

Stones in a chain share their liberties. A chain of stones must have at least one liberty

to remain on the board. When a chain is surrounded by opposing stones so that it has no liberties,

it is captured and removed from the board.

Ko rule

An example of a situation in which the ko rule applies Players are not allowed

to make a move that returns the game to the previous position. This rule, called the ko rule,

prevents unending repetition.

As shown in the example pictured: Black has just played the stone marked 1, capturing a white stone

at the intersection marked with the red circle. If White were allowed

to play on the marked intersection, that move would capture the black stone marked 1

and recreate the situation before Black made the move marked 1.

Allowing this could result in an unending cycle of captures by both players.

The ko rule therefore prohibits White from playing at the marked intersection immediately.

Instead White must play elsewhere, or pass; Black can then end the ko by filling

at the marked intersection, creating a five-stone black chain. If White wants to continue the ko,

White tries to find a play elsewhere on the board that Black must answer; if Black answers,

then White can retake the ko. A repetition of such exchanges is called a ko fight.

While the various rule-sets agree on the ko rule prohibiting returning the board

to an immediately previous position, they deal in different ways

with the relatively uncommon situation in which a player might recreate a past position that is

further removed. See Rules of Go: Repetition for further information.

Suicide

[^] A player may not place a stone such that it or its group immediately has no liberties,

unless doing so immediately deprives an enemy group of its final liberty. In the latter case,

the enemy group is captured, leaving the new stone with at least one liberty.

This rule is responsible for the all-important difference between one and two eyes: if a group

with only one eye is fully surrounded on the outside, it can be killed

with a stone placed in its single eye. The Ing and New Zealand rules do not have this rule,

and there a player might destroy one of its own groups—"commit suicide".

This play would only be useful in a limited set of situations involving a small interior space.

In the example at right, it may be useful as a ko threat.

Komi

Because Black has the advantage of playing the first move,

the idea of awarding White some compensation came into being during the 20th century.

This is called komi, which gives white a 6.5-point compensation under Japanese rules.

Under handicap play, White receives only a 0.5-point komi, to break a possible tie.

Scoring rules

[^] Two general types of scoring system are used, and players determine which to use before play.

Both systems almost always give the same result.

Territory scoring counts the number of empty points a player's stones surround, together

with the number of stones the player captured.

Area scoring counts the number of points a player's stones occupy and surround. It is associated

with contemporary Chinese play and was probably established there

during the Ming Dynasty in the 15th or 16th century. After both players have passed consecutively,

the stones that are still on the board, but unable to avoid capture, called dead stones, are removed.

Area scoring : A player's score is the number of stones that the player has on the board,

plus the number of empty intersections surrounded by that player's stones.

Territory scoring : In the course of the game, each player retains the stones they capture,

termed prisoners. Any dead stones removed at the end of the game become prisoners.

The score is the number of empty points enclosed by a player's stones,

plus the number of prisoners captured by that player.

If there is disagreement about which stones are dead, then under area scoring rules,

the players simply resume play to resolve the matter.

The score is computed using the position after the next time the players pass consecutively.

Under territory scoring, the rules are considerably more complex; however, in practice,

players generally play on, and, once the status of each stone has been determined, return

to the position at the time the first two consecutive passes occurred and remove the dead stones.

For further information, see Rules of Go.

Given that the number of stones a player has on the board is directly related

to the number of prisoners their opponent has taken, the resulting net score,

that is the difference between Black's and White's scores, is identical under both rulesets. Thus,

the net result given by the two scoring systems rarely differs by more than a point.

Life and death

While not actually mentioned in the rules of Go,

the concept of a living group of stones is necessary for a practical understanding of the game.

Examples of eyes. The black groups at the top of the board are alive, as they have

at least two eyes. The black groups at the bottom are dead as they only have one eye.

The point marked a is a false eye. When a group of stones is mostly surrounded

and has no options to connect with friendly stones elsewhere,

the status of the group is either alive, dead or unsettled. A group of stones is said

to be alive if it cannot be captured, even if the opponent is allowed to move first. Conversely,

a group of stones is said to be dead if it cannot avoid capture,

even if the owner of the group is allowed the first move. Otherwise, the group is said

to be unsettled: the defending player can make it alive or the opponent can kill it,

depending on who gets to play first. An "eye" is an empty point or group of points surrounded

by one player's stones. If the eye is surrounded by Black stones, the "suicide rule" forbids White

to place a stone in a single-point eye surround

by Black unless the placement results in a capture of Black stones that creates a liberty

for White's new stone. Effectively, the capture rule is applied before the suicide rule,

and both are applied before White's play is completed. By the interplay of the capture

and suicide rules, survival for a group can be guaranteed only by having two or more eyes.

If two such eyes exist, the opponent can never capture a group of stones,

because one liberty is always remaining. One eye is not enough for life,

because a point that would normally be suicide may be filled by the opponent,

thereby capturing the group. In the "Examples of eyes" diagram, all the circled points are eyes.

The two black groups in the upper corners are alive, as both have at least two eyes.

The groups in the lower corners are dead, as both have only one eye.

The group in the lower left may seem to have two eyes,

but the surrounded empty point marked a is not actually an eye. White can play there