Taiwan rules
The "Taiwan rule", is a--now seemingly uncommon--practice of removing the last played stone if it was played by black. (Applicable to area scoring rules prior to scoring a completed game.) This preserves an equal number of moves between players, having an intuitive, if modest mitigating effect of the advantage of black playing first (pre komi). This rule is not a feature of ING rules that are today associated with Taiwan (see discussion below).
jann: The above formulation of the rule seems a bit doubtful. If the last three moves were B dame, W dame, followed by B teire, then removing this last B stone would have no effect on the area score. Thus it would not neutralize the fact that B had an extra play and stone, would not align area to territory, and would even let this case remain a FreeTeireIfEvenDame - as well as give different score for the same position if B played his teire first and his dame last.
Instead of removing a stone, both Ing and Ikeda seemed to cite this rule (which was apparently commonly used in a time) as directly altering B/W area points if B played last. This essentially means adjusting komi, so a possible simplification is that B pays an extra point of komi if he played more stones than W. This rule has a similar effect to button go (and may be safer).
(Ikeda is a bit ambiguous here as he talks about the last competitive move, but his example about FreeTeireIfEvenDame - especially when modified from 2 to 0 dame - seems to indicate he considered teire a competitive move as well. This also seems logical for the above reasons on identical positions and move ordering.)
This rule and its compensation point also relates to handicap compensation points that B pays under area scoring for his initial stones (which are similarly unmatched by W).
Historical Precedent:
Chen Zuyuan, in his article The History of Go Rules (2011) makes the argument that an equal number of moves by black and white was an original prerequisite of play that has almost entirely been discarded over time. He uses his interpretation of ancient go texts and early preserved game records to support this claim.
Modern Manifestations in rule sets:
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Ikeda Area Rules III, Inspired by the "Taiwan Rule"
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World Mind Sports Rules
- Button go
A non-example--or rather a reverse example--is the AGA rule requiring an even number of moves/passes, with a cost of one prisoner per pass. Ironically, the purpose of this rule is to reconcile territory counting mechanics with area scoring that doesn't mandate equal number of moves, penalizing white in territory for passing first, to arrive at an identical score.
Discussion
Charles Matthews: I presume that reference to Taiwan rules can be taken to be the SST (Ing) rules. But since this area is a minefield ... perhaps someone can explain why I'm wrong.
Bill: No, that reference is to the Taiwan rules as they existed before they adopted Ing rules. Ikeda refers to them in On the Rules of Go, Section 3.6.
Ikeda writes:
Instead of talking about filling the last neutral point, we can say with slightly greater generality that half a point is deducted from Black's score and added to White's score whenever Black makes the last competitive move. This rule, known as the Taiwan rule, is an extremely interesting modification of area rules. In formulating this rule, the key problem is how to define the last competitive move. A concise but complete definition would be highly significant; it would put the finishing touch on area rules.