The web has many sites giving the rules of Go.
Unfortunately these pages are generally little use for answering the questions which one is likely to want to ask, e.g. "Do such-and-such rules allow suicide?". Exceptions are pages by David Fotland  and by Robert Jasiek , which provide clear comparisons of differences between various rulesets. This page attempts the same, for those rules which are most likely to be found in use in Europe.
In Britain, the Japanese rules have traditionally been used, except that triple-kos were treated like jigos, and komi was often 6 or 6½ points. Late in 2007 it was decided to adopt AGA (American Go Association) Rules  with 7½ komi. Some events may still use traditional Japanese rules.
If the organisers of a Go tournament in Britain, recognised by the BGA, do not specify otherwise then: the AGA Rules  are understood to apply, except that deliberate illegal play may lead to a forfeit. There is a BGA Interpretation  of some AGA Rules.
If the organisers of a Go tournament in Britain, recognised by the BGA, say that Japanese rules apply, then the traditional Japanese rules are used except that triple-kos and other such repeated positions count as jigos, and komi is 6, unless specified.
|Japanese, Korean||AGA||Chinese||SST (Ing)||New Zealand|
|Counting||Territory||Area or Territory||Area||Fill-in||Area|
|Does it cost to make a move inside my territory?||Yes||No||No||No||No|
|Can you count points in a seki?||No||Yes||Yes||Yes||Yes|
|Is suicide allowed?||No||No||No||Yes||Yes|
|What happens if the position repeats?||"No result". The game may be replayed||Repetitions are explicitly forbidden||Forbidden or may be a drawn game||Repetitions are restricted by the SST ko rule||Repetitions are explicitly forbidden|
|Status of "bent four in the corner"||Dead||Play it out||Play it out||Play it out||Play it out|
|Komi||6½||7½||7½ (3¾ by Chinese counting)||8||7|
|Placement of handicap stones||Fixed||Fixed||Free||Free||Free|
|Compensation for handicap stones||(No)||n-1 points if area count||n points||No||No|
Note: the various online Go servers make attempts at implementing one of these rulesets - usually Japanese or Chinese.
There are competent accounts of these five sets of rules on the web in Sensei's Library  and as follows:Tromp-Taylor  Rules, which are very similar to New Zealand rules, but they are not widely used.
Recommended sources for discussions of the rules of Go are pages by
With Japanese rules, you score the territory you have surrounded plus the prisoners you have captured: this is known as "territory scoring". With most other rules, you score the territory you have surrounded plus the points that you occupy ("area scoring"). The SST Rules implement area scoring by a fill-in counting method using sets with known numbers of stones. AGA Rules use pass stones at the end to allow counting by either method. Normally in the UK, it is the Japanese-style territory counting method that is used under AGA Rules. The result of the two methods is usually the same to within one point: the proof of this is left as an exercise for the reader.
Some reasons why the result may not be the same:
AGA Rules address point 2 by requiring White to play the last move of the game (or give an extra pass stone). Extra komi is given to help balance the effect of this.
AGA Rules address point 3 by subtracting a point per handicap stone from Black's score, apart from one which would be Black's first stone anyway, when using Chinese-style counting.
Note that under rules other than Japanese, filling a dame gains a point. So under Japanese rules, you should never make unnecessary defensive moves inside your own territory. Under other rules, you should only do so after all the dame have been filled, and it may be helpful to do so as it may make clear what is alive and dead.
SekiThe Japanese rules say that this position is seki, and no territory is counted in a seki. Other rules say that White counts the two points she has surrounded here. (The SST Rules used to count the 2-1 points pro rata, making this position worth one-and-a-third net to White, but this has been changed.)
Figure 1. A seki.
SuicideIn a position like this, White may have a ko threat. She can play between the two marked stones, suiciding three stones and threatening to kill the black group. But under Japanese, Chinese and AGA Rules, suicide is forbidden.
Here  are some much less likely positions in which suicide is a good move, if allowed.
Figure 2. A suicidal ko threat.
All rules agree in preventing two-cycles of repetition, in which one player moves, the other player moves, and the position is restored to what it was before. All those listed here do so by the familiar ko rule, which forbids a move that would restore the position to what it was just before one's opponent's last move.
Here is a page of some repeating Go positions .
(Tibetan rules, not treated here, prevent it by forbidding play on any point from which a stone has just been removed.)
But there are other, much less likely, ways in which a position can be repeated. These necessarily have a cycle length greater than two moves. One is triple ko. Others are cho-sei , and the molasses ko . Here are some more examples, and discussion, by Robert Jasiek  and by Harry Fearnley .
Under Japanese rules, if such a repetition occurs and neither side wants to give way, the game is a non-event, and the players will normally play again.
Under Chinese rules, repetitions are disallowed, but with certain of the commoner repeating positions (for instance triple ko), if neither side will give way, the game may be treated as a drawn game.
Under AGA, and New Zealand, rules, such repetition is explicitly forbidden by use of the Situational Superko rule (SSK). It is possible that Natural Situational Superko (NSSK) was intended for these rule sets, and so France have adopted AGA Rules with NSSK. Under SSK, a player may not make a move which brings about a position that has ocurred earlier with the same player to move. The three types of Superko are listed below.
The SST Rules attempt to restrict such repetition, and do so in a very complicated way, which involves a distinction between "fighting kos" and "disturbing kos", and using concepts that are not well understood.
But some ko threats cannot be eliminated.
The traditional Japanese rules state that the marked white group is dead regardless of the position on the rest of the board, and may be removed at the end of the game.
Other rules say that the position is to be played out. Normally the result will be the same as under the Japanese rules. But if Black has more uneliminable ko threats than White, the result will be different.
There is are more complicated positions which works the same way as "bent four".
If the marked white "bent four" group is allowed to live when White has an uneliminable ko threat, you might think that the marked white "moonlight life" group should be allowed to live if White has an infinite number of uneliminable ko threats (i.e. a double ko). However no ruleset allows the moonlight life group to live.
Figure 3. "Bent four in the corner".
It is possible that the correct komi is about 8, for good players. For poor players, it is less. In Japan, komi was once not used at all. For a long while it was 4½ points, but then it changed to 5½ points and a lot later to 6½; opinion is gradually moving towards a higher value.
With integral komi, jigo is possible. This may be thought undesirable (wanting most games to give a positive result and not wanting pre-arranged jigos) or desirable (more scope to tournament organisers in assigning prizes, belief that the correct result between two perfect players must be jigo).
AGA Rules give placement on the star points as the default, but permit tournament directors to specify free placement if they wish.
Other rulesets allow Black to place them anywhere.
Under SST and New Zealand rules, no such compensation is given.
Under Japanese rules, this is not an issue, as area is not counted.
The effect of this is that an n-stone handicap is n points larger under SST and NZ rules than under Japanese or Chinese rules.
Note that pass is always allowed and does not count as a play.
|PSK||Positional Superko||A play may not recreate a previous board position from the game.||This refers to the position just after the play and consequent removals.|
|SSK||Situational Superko||A play may not recreate a previous board position from the game, with the same player to move next.||Recreation refers to the position just after the play and consequent removals, and also takes account of who moves next.|
|NSSK||Natural Situational Superko||A player may not play to recreate a board position, if they played to create it previously.||This refers to the position just after the play and consequent removals.|
|ChSK||Chinese Superko||A play may not recreate a previous board position from the game by means of basic ko or sending-2-returning-1. If a cycle arises in another way and neither player varies from it, the referee may declare a draw or require a replay.||This, according to the sixth meeting of the International Go Rules Forum, is what the Chinese rules were intended to specify.|