The essential rules of Go are as follows:
There are a small number of slight variations for these rules. These variations, are subtle, mainly affect the scoring method, and do not much change the character of the game. For a more detailed treatment, see Go rules.
In a game between players of unequal strength, a handicap is used: the weaker player starts out with a number of stones on the board. See Go handicap for the details.
Although the rules of Go are very simple, the game itself can be extremely complex. Go is a complete-knowledge, deterministic, strategy game like chess, checkers, and reversi, although its depth exceeds even those games. Its large board and lack of restrictions allows great scope in strategy, as decisions in one part of the board are influenced by a seemingly unrelated situation in distant parts of the board, and moves made early in the game can shape the nature of conflict hundreds of moves later.
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.
There is a theory that says no game has ever been played twice. This may be true: On a 19x19 board, there are about 4.63x10170 possible positions and about 10360 possible games. (By contrast, the number of legal positions in chess is estimated to be between 1043 and 1050, and physicists estimate that there are not more than 1090 protons in the entire universe.)
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. See computer Go article for detail.
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.
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 can be had, but are considered inferior to glass as they are generally much lighter, 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.
The bowls for the stones are of a simple shape, like a flattened sphere with a level underside. The lid is loose-fitting and is upturned before play to place opponent's stones captured during the game. The bowls are usually made of turned wood, although small lidded baskets of woven bamboo or reeds make an attractive cheaper alternative.
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 called a Heso recessed into the board. Tradition holds that this is to give a better resonance to the stone's click, but the more conventional explanation is to allow the board to expand and contract without splitting the wood. A board is seen as more attractive when it is marked with slight dents from decades -- or centuries -- of stones striking the surface.
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 commonly 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. In Japan, amateur ranks are only recognised up to 8 dan (before the year 2001, only amateur ranks up to 7 dan were recognised).
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. Players asked to give their rank will therefore often qualify it with "in my country" or "on this Internet server".
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 these three countries are of comparable strength, although Korean players dominate the major international titles. All three countries 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.
See also: Go players
History The origins of the game are unknown, but the oldest surviving references come from China in the 6th century BC. Except for changes in the board size and starting position, has essentially kept the same rules since that time, which quite likely makes it the oldest board game still played today. It had reached Japan by the 7th century, and gained popularity at the imperial court in the 8th. By 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 in Japan, Honinbo Sansa[?], was made head of a newly founded Go academy (the Honinbo school, the first of several competing schools founded about the same time), which developed the level of playing greatly, and introduced the martial-arts style system of ranking players. The government discontinued its support for the Go academies in 1868 as a result of the fall of the Tokugawa shogunate.
In honour of the Honinbo school, whose players consistently dominated the other schools during their history, one of the most prestigious Japanese Go championships is called the "Honinbo" tournament.
Historically, Go has been unequal in terms of gender. However, the opening of new, open tournaments and the rise of strong female players, most notably Rui Naiwei[?], has in recent years legitimised the strength and competitiveness of emerging female players.
Elwyn Berlekamp[?] and David Wolfe[?] have developed a mathematical theory of the late endgame in Go based on the combinatorial game theory of John Horton Conway. Whilst not of general utility in most play, it greatly aids the analysis of certain classes of positions.