Jōseki
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In Go, joseki (定石, jōseki) are studied sequences of moves in the corner areas of the Go board, in which the result is considered balanced for both black and white sides. 定 (jō) means "fixed" or "set" and 石 (seki) means stones, giving the literal meaning "set stones", as in "set pattern". Joseki is 定式 in Chinese.
The "fairness" often refers to an equitable trade-off between securing territory in the corner versus making good thickness toward the sides and center. In application these concepts are in fact very dynamic, and often joseki are deviated from depending on the needs of the situation, and the opportunities available. While learning joseki is a tool to defend against a local loss, players always seek to take advantage of weaknesses in the opponent's shapes, often deviating from the joseki.
Usually joseki happen in corners in the opening stage. However, there are also joseki for the middle game.
The current body of joseki are not fixed, but patterns that have gained acceptance in professional games. That is, they form a consensus judgement that might change in the future, or with certain caveats. Hence the basic definition may be misleading for new players in that joseki can be misconstrued as foolproof and unalterable, and are otherwise optimal for all situations. Many joseki are in fact useful only for study within an artificially confined corner, and in real play are only considered good form when used in proper combination with other plays on the board (ie. other joseki and fuseki moves).
Knowing a particular joseki simply means that one knows a sequence of moves, resulting in a balance or fair trade-off between their positions. This is in practice much easier than appraising how joseki relate to the rest of the board—hence knowledge of joseki is regarded as shallow, when compared with the ability to integrate a strategy into a complex game landscape.
There is a go proverb that states that "learning joseki loses two stones in strength," meaning that rote learning of sequences is not advantageous. Rather learning from joseki should be a player's goal. Hence the study of joseki is regarded as a double-edged sword and useful only if learned (not by rote) by understanding the principles behind each move. Every joseki should be used as a specific tool that leaves the board in a particular shape.
Just as using an improper tool in machinery can be devastating, choosing the wrong joseki can easily be worse than improvising one's own moves. In his book A Way of Play for the 21st Century, Go Seigen compared choosing the proper joseki to choosing the proper medicine: Pick the right one, and you feel better. Pick the wrong one, and you die.(par.) Rui Naiwei similarly remarked that playing joseki is easy [but] choosing the right one [in a game] is hard.(par.)
A joseki may fall out of use for various reasons, some of which may often seem minor to the amateur player, and professionals may consider one variation suboptimal for a very specific reason —one which strong amateurs are not likely to exploit. There is no definitive guide to what is joseki; the situation with joseki dictionaries is similar to that of natural language dictionaries, in that some entries are obsolete and the listing is not likely to be complete.
While some claim that studying joseki is an important part of developing one's strength as a player, more regard the study of life and death to be more important.
Basic joseki
Corner joseki conventionally start with one player occupying a corner point, in an empty 10×10 area of the board, and the other player replying with an approach move (Japanese kakari). The initial play in the corner is almost always on a 3-3, 3-4, 3-5, 4-4 or 4-5 point. Other plays that have been experimented with include 5-5, 6-3 and 6-4, all of which sacrifice territory for influence.
Of those plays, the classical 3-4 point (komoku) and more contemporary 4-4 point (hoshi) are the most used. The standard approaches are at 5-3 or 5-4 to the 3-4 point, and at 3-6/6-3 to the 4-4 point. The number of subsequent variations is then quite large (of the order of ten reasonable plays for the next one).
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| A Go opening with two common Joseki in North-East and South-West |
Also useful is the tenuki concept of breaking away from a sequence, to play elsewhere, before the 'official' endpoint of the joseki. After a joseki sequence has ended, a play returning to the same area may be termed a follow-up play. There is no formal theory for these, though numerous set sequences can be seen in professional play.
It is imperative that players should not play joseki merely from rote memorization but adapt according to the overall board situation. It's important to keep in mind that go is a game involving marginal analysis and joseki are merely heuristics of sound play. Playing joseki blindly will not improve one's game.