The rules of Go are quite simple and elegant. They can be summarised as follows:
In practice, some variation of these rules are normally used. These variations, which are fully explored below, are mere elaborations and abbreviations and do not change the character of the game.
A Go board (goban in Japanese) is a grid with 19 horizontal and 19 vertical lines, forming 361 intersections. For beginning players or short games, smaller boards of 13x13 or 9x9 intersections are sometimes used, without otherwise changing the rules.
Playing pieces consist of two sets of stones (go-ishi), one black set and one white set. The number of stones is indefinite (the rules assume an endless supply) but 181 black stones and 180 white stones are sufficient to cover the 361 intersections of the board, so these amounts are usually found in a full set.
Since the number of stones is large, they are stored in bowls (go-ke), one for each player; this usually has a lid which, upturned before play, is used to hold captured stones.
Go is a game for two players. One player uses the black stones, the other white. The board starts empty. Black moves first (this gives a slight advantage, so the weaker player traditionally plays Black; a handicap can be used to give Black several starting moves, see below).
The players alternate making moves. Making a move consists of putting a single stone on one of the intersections (the intersections at the edges and corners of the grid are part of the board and plays there are also valid). Once played, a stone does not move, and remains at the same point unless it is captured. A player is allowed to pass instead of making a move. A player may also resign on his move, conceding victory to the opponent.
Stones of the same color that are directly adjacent (along the lines of the board) are said to be connected into a group. Stones on the board have a certain number of adjacent empty intersections, called liberties. When a stone, or a connected group of stones, has no remaining liberties, it is captured. The complete group is taken off the board, and added to the opponent's prisoner pile.
To prevent endlessly repeating positions, the rule of ko (a Japanese word for "eternity") prevents any play that would repeat the previous board position. In practice, this occurs most often when a stone has just been captured, and the stone which made the capture is left with only one remaining liberty. If recapturing that stone would recreate the same board position from the previous move, the position is called ko, and the recapturing move is illegal. The player may not recapture immediately and must play elsewhere. On a subsequent move, the board position will be different, and the stone may then be captured if it is still possible.
If a stone is played such that it has no remaining liberties (or is part of a group with no remaining liberties) and does not gain liberties by immediately capturing an opponent's group, this is termed suicide since the stone, and any stones it was connected to, would be thus reduced to zero liberties and captured. Though it is usually a mere tactical blunder to do so, many rule sets prohibit a suicide play, making it an invalid move.
When both players have passed, the game has ended. Dead stones (those that remain on the board but cannot avoid capture) are now removed as if they were captured. Most rule sets allow disputes over whether stones cannot avoid capture to be resolved simply by continuing play until both players are agreed on the status of the groups. The Japanese rules, instead, have a long list of exceptions and precedents that are referred to in tournament play. Most players remain unaware of these complications in the Japanese rules for the vast majority of their games.
After dead stone removal, counting begins to determine which player is the winner with the greater share of the board. There are two methods of counting. In the Japanese counting method (called territory counting), each player scores the number of empty intersections he has enclosed, and subtracts the number of captures taken from him (this is done easily by placing the captures taken from a player into their empty intersections to reduce the score). In the Chinese counting method (called area counting), captures are not counted, but a player scores for every intersection that he controls -- that is, all points where he has placed a stone or that are completely surrounded by his stones.
Whichever counting method is used, the player with most points wins. In normal circumstances, the Chinese and Japanese counting methods give the same winner.
To equalize games between players of different skills, handicaps are used. These are considered a part of the game, and unlike in many games they do not distort the nature of the game. Players at all levels of strength employ handicaps to make the game more balanced.
Handicaps are given by having Black (which in this case is the weaker player) play 2 or more stones (traditionally up to 9 on the full 19x19 board) as his first move. (In a game with a difference of only 1 rank, Black plays first as usual but White's komi is reduced; see below.) These initial stones must be played on the 9 marked intersections under Japanese rules; some other rules allow free placement of the initial handicap stones.
Black's initial advantage of moving first can be compensated by komi: a fixed number of points, agreed before the game, added to White's score at the end of the game. In an "even", or non-handicap game, the correct value of komi (to properly compensate for Black's advantage) is controversial, but common values are 5.5 or 6.5 -- the fractional value avoids a tied game. In a handicap game, komi is usually set to 0.5 (i.e. White wins if the game is tied). A handicap game with a handicap of 1 starts like an even game, but White receives only 0.5 komi (i.e. a White player who is stronger by one rank is handicapped only by Black's first-move advantage).
Although the rules of Go are very simple, the game itself can be extremely complex. Go is a thinking game like chess, checkers, and reversi, although its depth exceeds even those games. It is a highly strategic game, where decisions in one part of the board are influenced by a seemingly unrelated situation in distant parts of the board.
The game emphasises the importance and tensions of balance on multiple levels. To secure an area of the board, it is good to play moves close together; but to cover the largest area one needs to spread out. To ensure one does not fall behind, aggressive play is required; but playing too aggressively leaves weaknesses undefended that can be exploited. Playing too low (close to the edge) secures insufficient territory; yet playing too high (far from the edge) allows the opponent to invade. Many people find the game attractive for its reflection of the polarities found in life.
Although attempts have been made to program computers to play Go, success in that area has been moderate at best. Even the strongest programs are no better than an average club player, and would easily be beaten by a strong player even getting a nine stone handicap. This is attributed to many qualities of the game, including the "optimising" nature of the victory condition, the virtually unlimited placement of each stone, the large board size, and the high degree of pattern-recognition involved. For this reason, many in the field of artificial intelligence consider Go to be a better measure of a computer's capacity for thought than chess.
Use of computer networks to allow humans to meet, discuss games, and play one another, although generally considered inferior to face-to-face play, is becoming much more common. There are servers and software to facilitate this; see Additional Resources below for more information.
Go appears to stand apart among games in its rules and gameplay; it is difficult to find another board game which could be considered of the same "family" as Go. Naturally, though, on learning about the game, people will attempt to compare it with other games they may already have experienced. This is a non-exhaustive list of some games that are often compared with Go.
Although one could play Go with a piece of card for a board and a bag of plastic tokens, Go players pride themselves on their game sets. The traditional Go board (called a goban in Japanese) is solid wood, about 15-20cm thick, preferably from the rare golden-coloured Kaya tree, and stands on its own attached legs. Players sit on reed mats (tatami) on the floor to play. The stones (go-ishi) come in matching solid wood pots (go-ke) and are made out of clamshell (white) and slate (black) and are extremely smooth. The natural resources of Japan have been unable to keep up with the enormous demand for the native clams and slow-growing Kaya trees; both must be of sufficient age to grow to the desired size, and they are now extremely rare at the age required, raising the price of such equipment tremendously.
In clubs and at tournaments, where large numbers of sets must be maintained (and usually purchased) by one organisation, the expensive traditional sets are not usually used. For these situations, table boards (of the same design as floor boards, but only about 2-5cm thick and without legs) are used, and the stones are made of glass rather than slate and shell. Bowls will often be plastic if cheap wooden bowls cannot be had. Plastic stones are considered inferior to glass and most players find them too unpleasant to justify the difference in price.
The dimensions of the board (traditionally the grid is 45.45cm long and 42.42cm wide, with space beyond to allow stones to be played on the edges and corners of the grid) often surprise newcomers: it is not a perfect square, but is longer than it is wide, roughly in the proportion 12:11. Two reasons are frequently given for this. One is that when the players sit at the board, the angle at which they view the board gives a foreshortening of the grid; the board is slightly longer between the players to compensate for this. Another reason is that the Japanese aesthetic finds any structure which is perfectly symmetrical to be in bad taste, and the board is not made a perfect square for this reason.
Traditional stones are made so that black stones are slightly larger in diameter than white; this is probably to compensate for the optical illusion created by contrasting colours that makes the white stones appear larger on the board than black stones. The difference is slight, and since its effect is to make the stones appear the same size on the board, it can be surprising to discover they are not.
There is even an art to placing a Go stone, held between the tips of the outstretched index and middle fingers and striking the board firmly to create a sharp click. Many consider the acoustic properties of the wood of the board to be quite important. The traditional goban will usually have its underside carved with a pyramid recessed into the board; this is thought to give a better resonance to the stone's click. A board is seen as more attractive when it is marked with slight dents from decades -- or centuries -- of stones striking the surface.
In countries where Go is popular, ranks are employed to indicate playing strength. The Japanese formalised the teaching and ranking of Go, and modelled the system after their existing martial arts schools.
Players who are competent are ranked starting at 1 dan, through to 9 dan which is the highest rank (in the Japanese Edo period, only one player held this rank at a time, and was called Meijin). Approaching 1 dan, a player first progresses through kyu ranks, with 1 kyu being the rank below 1 dan, and a greater kyu ranking indicating a greater distance in strength from 1 dan. Since beginners will naturally progress through elementary concepts quickly, it is difficult to set a lower bound to the kyu ranks, but nominal starting points of 20, 25 or 30 are commonly used.
The distinction between each rank is, by definition, one handicap stone. In other words, the difference in rank between two players is theoretically equal to the number of handicap stones required for each player to have an even chance of winning. Beating this handicap consistently is the indicator that a player's strength has improved, and his rank should be adjusted upward by one stone, thus changing the number of handicap stones required.
In China, Japan and Korea, there are two distinct ranking sets, one for professional players (who receive a fee for each game they play, bonuses for winning, and fees for other related activities) and one for amateur players. Amateur ranks are only recognised up to 8 dan (before the year 2001, only amateur ranks up to 7 dan were recognised). The highly competitive qualifications for professional players have the side-effect that the two pools of players do not mix in a way that affects rankings; thus, the rankings for professional and amateur players have diverged to the point where amateurs at a given rank are actually weaker than the equivalent professional rank.
Similarly, other player pools that do not regularly mix (such as different countries, or online versus real-life player groups) often result in divergent playing strengths for the same rank level.
Although historically the strongest players in the world came from Japan, players from China in the 1980s and South Korea in the 1990s have reached the same or even a higher level. Nowadays, top players from the three countries are approximately of the same strength. All three have professional competitions where there is sometimes a high amount of prize money.
Players from other countries have traditionally been much weaker, except for some players who have taken up professional courses in one of the Asian countries. This is attributable to the fact that details of the game have been unknown outside of Asia for most of the game's history. A German scientist, Otto Korschelt, is credited with the first systematic description of the game in a Western language in 1880 AD; it was not until the 1950s that Western players would take up the game as more than a passing interest.
The game is close to 4000 years old and, except for changes in the board size, has essentially kept the same rules since that time, which quite likely makes it the oldest game. Invented in China, it reached Japan in the eighth century AD, where at first it was only played at the imperial court, and later by monks. At the beginning of the 13th century, the game was played in the general public in Japan. Early in the 17th century, the then best player, Honinbo Sansa, was made head of a newly founded Go academy, which developed the level of playing greatly, and introduced the system of ranking players. The academy was discontinued in 1868. In honor of Honinbo Sansa, the Japanese Go champions are still called "Honinbo".
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