What are the differences? And do these differences influence strategy?
Although there are some minor differences between rule sets used in different countries, most notably in Chinese and Japanese scoring rules, these differences do not seriously affect the tactics and strategy of the game.
There are two basic scoring systems used to determine the winner at the end of a game; they almost always give the same result. Territory scoring counts the number of empty points your stones surround, together with the number of stones you captured. While it originated in China, today it is commonly associated with Japan and Korea. Area scoring counts the number of points your stones occupy and surround. It is associated with contemporary Chinese play and was probably established there during the Ming Dynasty in the 15th or 16th century.
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In most games (as already noted) the two rulesets produce the same strategy and outcome.
Differences between the current Japanese and Chinese rules:
Japanese counts enclosed territory (of groups able to form two eyes) minus prisoners (stones that the opponent was able to capture). Chinese counts all area: every intersection is attributed to whichever side’s stones enclose or occupy it; unfillable dame (i.e. intersections left vacant between opposing groups of stones, after captures are removed from the board) are split 50/50.
Chinese report on how far either player is from a draw (of 19×19÷2 points each), whereas Japanese report on how far apart the two player’s scores are (a margin which should differ by a factor of two).
Groups of stones that live in seki (e.g. that border unfillable dame) sometimes still enclose some space (e.g. one eye). This is not counted as territory by Japanese, but does count in Chinese.
Super-ko (i.e. cycles which exactly recreate a previous board position) nullifies the game for Japanese; in Chinese it is in principle an illegal move but can also be judged a draw, void, restart, etc.
After the game concludes, life or death (i.e. assumed captures) is resolved in Japanese by virtual play, under a modified rule that retaking a ko requires an extra move. (This seems intended as an alternative to super-ko for circumventing unremovable ko threats.)
As compensation for the first move advantage, Japanese gives white 6.5 points (komi) and Chinese transfers 3¾ from black to white (i.e. 7.5 komi).
Japanese rules are understood to have more ambiguity and/or inconsistency, related to the complexity of life and death.
- Only Japanese requires keeping track of captured stones (prisoners). In longer games this can use more playing stones.
- In Japanese, the game can finish before dame are filled.
- In Japanese, captures inside of seki groups are played out (because prisoners are counted despite there being no territory counted).
- Unnecessary defensive moves are more costly in Japanese rules (subtracting a point in addition to the wasted turn itself).
- Assumed captures are left on the board in Japanese rules, whereas under Chinese rules any "life or death" puzzles can ultimately be played out (simplifying the board) without loss of points.
Affecting high-level play:
- If the game runs out of dame and meaningful moves after an odd number of turns, then white gets an extra point in Japanese as compared to Chinese rules. (Some other rulesets negate this difference between territory and area counting, e.g. by insisting both sides make an equal number of moves.)
- During the opening, aggressiveness/defensiveness relates to the amount of komi.
- The urgency/worth of resolving the status of an unsettled (but potentially seki) group can be slightly different (since some seki have different value between Japanese and Chinese rules).
Note that in close games between players who are strong enough to accurately predict score, a small difference in points may cause a completely different choice of strategy for a particular board position.
With no komi, this 9x9 position is B+1.5 in Chinese rules and W+8 in Japanese rules.
Note that black can connect/fill the false eyes, and thereby threaten to encroach at the bottom left corner, forcing white to play in the top right corner. This exchange leads to a gain for black in Japanese rules (reducing the margin to W+7) but does not alter the score in Chinese rules.