Rules and Area and Territory Scoring

Table of contents Using simple ko (or ban lifting passes) under area scoring (or areafied encore) Using superko (or no ban lifting passes) under territory scoring Using territory scoring without excluding sekis Using komi 7 under area scoring - issues with B's surplus stone

Area and territory scoring have some natural differences. Minor things like valuable dame or black's extra area point if he gets a surplus move. But despite those the two scoring methods are fundamentally still the same game, with nearly identical gameplay and behavior. This principle also has implications about the "correct" or universal rules of the game, in a reasonably objective way.

Below are some rules design problems and mistakes - rule components and combinations that would artificially CREATE a significant divergence and behavior difference between the area and territory game. Note that here territory scoring refers to the common type with L/D and hypothetical play, but for completeness, the hybrid type (a territory phase followed by an areafied encore using pass stones) can also be considered.

Using simple ko (or ban lifting passes) under area scoring (or areafied encore)

This would allow prisoner-unbalanced area repetition thus draw or no result in common positions, which are plain sekis under territory scoring with its natural immunity to such.

 

One example is the non-ko line of sending three returning one.

For area scoring to overcome this, one possibility is some kind of prisoner-aware long cycle rule (like Chinese ko which prevents unbalanced repetition), or something like the Spight stop rule. Another potential alternative would be a complete superko rule without ban lifting passes - but this in itself would create a difference, see below.

Using superko (or no ban lifting passes) under territory scoring

This would lead to ko play pathology with territory scoring, diverging from both area scoring and common sense in ordinary positions.

Under area scoring (or areafied encore), even if the rules don't have ban lifting passes, the players usually have equivalent substitute - plenty of 0 pt ban lifting moves, plays inside territory. So ko play can still follow a natural path, recapture of open kos remains possible, and a lack of passes could only matter on very crowded and small boards - not in daily practice.

Under territory scoring, however, only dame could substitute, which are not always available (or enough). The players often have no means to change the board without losing points. So without complete and ban lifting passes, ko play could change drastically from both area scoring and normal go logic, creating permanently un-recapturable kos - even on 19x19 board, and even in common positions like a mannenko, or kos like this:

no komi no prisoners  

Lacking threats, normally B can only connect and let W do the same and win by his territory advantage. The outer ko could only give a temporary gain for B. This assumes W can eventually retake the outer ko should B make a futile attempt to take it. Which is true under area scoring (or areafied encore) - regardless of passes. Common (L/D based) territory scoring rulesets work similarly - but need ban lifting passes. (And possibly resumptions - note that if B tries the ko line, W needs to actually recapture in the game for the prisoner, it is not enough to declare B ko stone dead.)

Using territory scoring without excluding sekis

This would make territory scoring unstable, and unable to score some multi-state seki positions that are no problem for area scoring. Additionally, dead stone removal from sekis could change the cost of some plays into them (depending on how L/D is defined).

Correct scoring needs a final position with an unambiguous score, and sekis are a threat to that. They can take various forms with special properties, and some - like a double ko seki - have multiple stable (possibly scorable) states, between which it can be pushed around. Which is still ok as long as these states all have the same score - but this is where territory scoring start to have problems.

Under area scoring there is no scorewise difference between filling an intersection (like a ko mouth) or leaving it open and surrounded by live stones. Thus multi-state sekis usually still have - or can be stabilized to - an unequivocal area score. Under territory scoring, however, filling worth less than surrounding, which reduces possible stabilizing moves as they may lose points. Thus - if territory scoring would include sekis - some positions would have a stable area score, but undefined (oscillating) territory score.

 

Mannenko is probably the most common example where this could break territory scoring, with its one-sided ko and alive ko stone - see here.

The standard solution - excluding sekis - can be restated as territory is only granted in completely controlled (independently alive, or freely-pass-aliveable) regions. This can be formalized without even defining sekis. Another possible alternative is an areafied encore, which can score sekis similarly to area scoring.

Using komi 7 under area scoring - issues with B's surplus stone

Less significant than earlier points, and is not about creating a new difference but about an existing one that is usually fixed with the standard 7.5 area komi. The root problem is B plays one more stone than W in half of the games (since he plays first, and has chance to play last as well). This surplus stone has has no direct effect on territory scores, but is an automatic extra point in area. This introduces a problematic element into area scores, with various consequences - and various ways in which area rules try to correct it.

Territory komi is 6.5, and increasing area komi by the average value of the extra stone (0.5) to 7 is a possible approach. This would balance winning chances, but besides allowing ties, it has some problems. The surplus stone is not random. Due to how parities work out on an odd board, B will (normally, without sekis) get his extra stone specifically when the territory score is even.

board (odd) = territories (odd or even, after prisoner backfill) + stones played (odd if B had extra)

This leads to rounding both B+6 and B+7 (territory) outcomes on the board to B+7 area. The same happens with B+4 and B+5 and so on - creating superposed score pairs rounding to equal area score. But their difference remains visible even in area terms, as the first case becomes B+7 with B getting the last move (so B one more stone than W) while the second case is B+7 with W last move (so both sides equal moves).

Achieving the same area score with one surplus stone is not really equal performance as without it. This same problem manifests as allowing one-point mistakes within a score pair without consequences - like FreeTeireIfEvenDame (B may have free teire on odd, W on even territory parity). Hence some rulesets exclude the surplus stone from area scoring, with some first pass or last move compensation. The Taiwan rule or Ikeda's rule, WMSG rules or button go all let area scoring distinguish the two cases.

But the simplest fix is the standard 7.5 komi (or 5.5 as in the past, ODD+0.5 generally). It does not distinguish B+6 and B+7 territory, but make both lead to the same area winner, with or without rounding. So while it cannot produce the same winner as territory scoring with territory komi (6.5), it can produce the same winner as territory scoring would with this komi (7.5). Thus the extra stone (or free teire) - when present - can at most affect winning margin, not win/lose/draw.

In contrast, komi 7 would allow the surplus stone to actually change the result. While this would indeed make a difference to territory scoring, whether this is ok is a debated question. One practical effect would be extending the tie outcome from a single score/performance to the entire [B+6,B+7] pair (thus twice as wide and frequent).