Go strategy and tactics
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The game of Go has simple rules and can be learned very quickly, but players will soon seek guidance on the strategy and tactics of the game.
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Connection and Separation
Stones on the board are easier to defend in groups; connecting a group of stones makes it harder to capture, since the opponent would need to cover all the liberties of the group, capturing the group completely, rather than capturing single stones. A good tactic to employ, then, is to attempt to divide the opponent into separate groups, while keeping one's own stones connected. (Note that when Black starts with a large number of handicap moves, his stones are mainly useful for this purpose; the White player's stones are threatened immediately with separation, while Black has many potential connections to begin with.)
Life and Death
A key concept in the tactics of Go, though not part of the rules, is the classification of groups of stones into alive, dead or unsettled.
At the end of the game, groups that cannot avoid being captured during normal play are removed as captures. These stones are dead. Groups can reach this state much earlier during play; a group of stones can quickly run out of options and further play to save it is fruitless. Similarly, further play to kill such a group is often of no benefit (unless required to gain liberties for an own group), since if it remains on the board at the end of the game it is captured anyway. Thus groups can be considered "dead as they stand", or just dead, by both sides during the course of the game.
Groups enclosing an area completely can be harder to kill. Normally, when a play causes an area completely enclosed by the opponent to become filled, the group filling the area is captured since it has no remaining liberties (such a play is called "suicide", for obvious reasons). Only if the last play inside the area would kill the enclosing group, thus freeing one or more liberties for the group that filled the space, can the play be considered. This can only be achieved if the liberties on the outside of the enclosing group have been covered first. Thus, enclosing an area of one or more liberties (called an eye) can make the group harder to kill, since the opponent must cover all of its external liberties before covering the final, internal liberty.
From this, it is possible to create groups that cannot be killed at all. If a group encloses two or more separate areas (two or more eyes), the opponent cannot simultaneously fill both of them with a single play, and thus can never play on the last liberty of the group. Such a group, or a group that cannot be prevented from forming such an enclosure, is called alive.
Groups which are not definitely alive nor definitely dead are sometimes called unsettled groups. Much of the tactical fighting in Go focuses on making one's own groups live, by ensuring they can make two eyes, and on making the opponent's groups die, by denying them two eyes.
Reading
Determining ahead of time whether a group is currently alive, dead, or unsettled, requires the ability to extrapolate from the current position and imagine possible plays by both sides, the best responses to those plays, the best responses to those responses, and so on. This is called reading ahead, or just reading, and it is a skill that grows with experience. Many players study books of life and death problems to increase their skill at reading more and more complicated positions.
High and Low
In order to build a harmonious position, usually in the opening, one does not place all stones on the third line, nor all stones on the fourth line. "Harmonious" is here not just an aesthetic quality; rather it stands for a better level of overall (strategic) connection of the stones.
Thickness and Lightness
A position that cannot be attacked, that is, one which can easily get two eyes and for which there are not many moves that have to be answered, is called 'thick'. Thick positions are important as they radiate influence across the board. An error that is often made by weaker players is to make territory in front of their thick position; this is inefficient because the player is likely to get that territory anyway. Thickness is better used from a distance, as support for other actions. For example, if Black has a thick group and a weak group nearby, and White attacks the weak group, Black can have her weak group run towards her thick group. If she is successful, the strength of the thick group will protect the weak group. Or, if White tries to invade near a thick group, Black can try to push White towards her thick group. If she is successful, the strength of the thick group will help destroy the invasion.
A light group is also one that is hard to attack, but for a different reason. If a group has a large number of options, often including the sacrifice of part of it, it is called light. Because it is usually impossible to take away all or almost all options, attacking such a group is very hard for the opponent, attacking such a group will bring little advantage. A weak group which cannot be sacrificed at will is called heavy.
Attack and Defense
A large part of the middle game of a game of Go is usually spent by one player attacking the other player's weak group(s). What is important to remember is that in most cases the goal of an attack is not to kill the attacked group, but to gain territory or influence. The attack is more or less used to restrict the opponent's options and make it impossible for him to make territory or influence himself.
Territory and Influence
See Go concepts
Ko Fighting
Suppose that Black begins a ko by taking a stone of White's. White cannot immediately recapture; the rules state that he must, for the moment, play elsewhere. White may believe that good strategy requires he eventually recapture, but Black, on her next turn, will have the option of ending the ko, for example by filling in the spot White would use to recapture. To prevent Black from doing this, White can play a ko threat. A ko threat is a move that forces one's opponent to respond. A ko threat by White will force Black to choose between responding to the threat, and allowing White to recapture (thereby continuing the ko), or ending the ko, but having a poor position elsewhere on the board. Good ko threats are ones which threaten to do so much damage to one's opponent that they must be responded to.
If White chooses to play a ko threat, and Black responds to the threat instead of ending the ko, then White can recapture the stone that began the ko. This places Black in the same position that White was formerly in: Black can choose to give up the ko, or to find a ko threat. If Black and White alternate making ko threats with recapturing the ko, they are having a ko fight.
Eventually, one of three things will happen.
- One player will decide that winning the ko immediately is more important than responding to her opponent's latest ko threat. The player will move so that her opponent cannot recapture the ko, and her opponent gets to followup on his last ko threat, effectively making two moves in one area of the board.
- One player will run out of ko threats. That player will be forced to make a play that his opponent does not have to respond to immediately, and his opponent wins the ko immediately.
- One player will decide that playing elsewhere on the board is more important than continuing the ko. The opponent can either win the ko, or play elsewhere on the board.
Before deciding to start a ko, it is worthwhile evaluating what threats are available to both players, so that one can decide which side is likely to win the ko fight. Many of the playing skills come together in ko fighting (evaluating the value of moves; reading ahead to find likely moves of the opponent and best responses; choosing the best order of moves), and it is a topic of much discussion among players. This also causes many beginners to be fearful of fighting a ko, since they are not confident of their ability to evaluate threats.
The importance of a ko varies dramatically depending on the positions of the two players. Some kos offer very little gain for either player, such as three points or less. Others control the fate of large portions of the board, sometimes even the whole board, and the outcome of those kos can determine the winner of the game. For this reason, finding and using ko threats well is a very important skill.
One curiosity is the existence of multiple kos on the same board at the same time. A double ko is when two kos are being fought simultaneously. Such positions sometimes arise in actual play, but they are uncommon. A triple ko is when three kos are being fought simultaneously. In a triple ko, it is possible for the two players to continually take and retake the three kos without violating the ko rule. If both players judge this to be the best line of play, then the game would, theoretically, continue forever. When such a position occurs the game is called off and the opponents begin a new game. This is an extremely uncommon event, and many Go players play their whole lives without restarting a game due to a triple ko.
Sente and Gote
'Sente' and 'gote' are terms that are much used in Go theory. The player who has sente is the one whose move it is once a local continuation has been played out. The player who does not have sente has gote. A sequence is said to have been played in sente if the player who started it has sente afterward. Having or losing sente is important, as taking gote unnecessarily is equivalent to passing.
Opening (fuseki)
The first moves are usually played on or near the 4-4 star points in the corners, because in those places it is easiest to make territory. (In order to be totally secure alone, a corner stone must be placed on the 3-3 point. However, if a stone is placed at a 4-4 point and the opponent invades, the first player can build a surrounding wall as the second (invader) is forming a live group, thus exerting strong influence on a large area.) After that, standard sequences (joseki) are used to divide corners, and extensions along the side are made. Usually, the center area is kept empty the longest. Plays are usually on the third or fourth line—the second makes too little territory, while the fifth is too easily undermined by a play on the third. A play on the fourth line is directed more towards influence to the center, a play on the third line more towards making territory along the side.
Endgame (yose)
In the endgame, if the game is close, moves that are small are still worth some points, some more than others. One must choose which of these moves is more urgent to play based not only on the points it may gain, but on whether that move is sente. Yose refers to a specific kind of endgame play, which yields a reduction for your opponent. Generally, in the endgame, all the territory is staked out—there is no more to be gained. However, there are still points to be made, as well as possible ways of reducing small amounts of your opponents territory. A simple example would be a move that is dame (neutral point for you) but when its filled in, its sente, requiring white to fill a stone in his territory to answer. We say this is 'a one point reduction, with sente.'
To continue, see Go concepts.