A Go board (goban in Japanese) is a board with 19 horizontal and 19 vertical lines. These form a grid of 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 and one white. 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. Black moves first (this gives a slight advantage, so the weaker player traditionally plays Black).
The players alternate making moves. Making a move consists of putting a single stone on one of the intersections. Once played, a stone does not move, and remains at the same place unless it is captured. A player is allowed to pass instead of making a move.
Stones on the board have a certain number of adjacent empty intersections, called liberties. Stones of the same color that are directly adjacent (along the lines of the board) are said to be connected into a group. 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 ko rule (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 capture 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.
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, and counting begins.
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) as his first move. (In a game whith 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 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 one could play Go with a piece of cardboard for a board and a bag of plastic chips, Go players pride themselves on their Go sets. The traditional Go board (called 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 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.
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 made of 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) 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.
Another fact that is difficult to observe is that 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.
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, and a board is seen as more attractive when it is marked with slight dents from decades -- or centuries -- of stones striking the surface.
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.
The popular [SGF file format] is used to exchange Go lessons and recorded games. Several free reading and authoring programs are available. See http://www.cwi.nl/~jansteen/go/sgfeditors.html for a list.
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 limits 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 a balanced game between the players. Beating this handicap consistently is usually rewarded by a promotion to the next rank, and thus a change in the number of handicap stones required.
In China, Japan and Korea, there are two distinct ranking systems, one for professional players (who receive a fee for each game they play, and for other related activities) and one for amateur players. Amateur ranks are only recognised up to 8 dan (until the beginning of the 21st century, 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.
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