free teire if even dame

The 'Free Teire if even Dame' phenomenon only affects Area Scoring rulesets. The phenomenon itself is best explained by quoting Ikeda Toshio whose explanation is most concise:

Quotation (they denotes common area scoring rulesets in below paragraph):

"...and they are logically complete, but they suffer from a reinforcement problem: when there are an even number of neutral points a player can reinforce an uncertain position blindly without losing anything, which makes the game less interesting. It was also stated that this problem can be solved by deducting half a point from Black's score and adding half a point to White's score when Black makes the last competitive move..." -- Ikeda Toshio, On the Rules of Go, Section 4.1

Thus we have the following opportunities for more interesting play:

• apply Ikeda Area Scoring III Rule 7 (see Ikeda's Rule)
• play Go using a Button (see Button Go)

This is a specific subcase of the general "free 1 pt territory mistake depending on territory parity" phenomenon, caused by the granularity of area scoring which groups territory scores into pairs that round to identical area scores. While the Taiwan rule is usually cited as the complete solution, the issue is normally handled with a simpler workaround. Area komi 7.5 itself is a sufficient mitigation, which ensures that the phenomenon can only affect the margin of victory, not the side who wins. See also RulesAndAreaAndTerritoryScoring#areakomi.

click here for the full example  

Diagram 3-7, Section 3.5 "The disadvantage of Area Rules", On the Rules of Go, Ikeda Toshio:

"...The problem arises in positions like Dia. 3-7. Only neutral points are left, the number of neutral points is even, the black group at the bottom has lost its outside liberties, and the safety of this group is uncertain. Suppose it is Black's turn. Even if Black plays 'a' to remove the danger, he can still get one of the two neutral points. Under area rules, that is, he can make a free reinforcing move at 'a.' He does not need to verify whether it is actually necessary..." -- Ikeda Toshio

See also:

• Granularity
• /Counter Argument

/discussion