Territory
To make territory is the ultimate goal in Go. In order to distinguish territory from a sphere of influence we can describe territory as:
A part of the board that is surrounded by stones belonging to a living group, and in which the opponent cannot make a living group (presuming the player holding the territory answers correctly).
This black group delimits two areas, in which he will easily make an eye.
It is impossible for White to create a living group with two eyes in the area. Black makes 21 points of territory.
The exchange up to is a common continuation of the 3-3 point invasion. The exchange of White a to Black d is more of an endgame sequence.
The circle-marked points constitute an area controlled by White, where Black can't make a living group (if White answers correctly). We can say that this 3-3 invasion makes roughly 10 points of territory.
ChihChan: I have a question here.
You consider White a to Black d is more of endgame sequence. Then White get 10 points of territory. However, it is also possible for Black to play a or c first. Then, White will get fewer points of territory. So, during the middle of the game, different calculations will have different results that may be favorable for both sides. So, are there any criteria for deciding this kind of situation?
Shouichi: Compare advantage to disadvantage. CostBenefitAnalysis or common sense can dictate the answer.
Sebastian: I think what Shouichi means is that, instead of playing a or c, both Black and White can better play at some other part of the board first. At some point of the game there will be no bigger move than a anymore. Then one of the players goes there and gets the remaining few points. You ask: "Why is it called a 10 point move, if it can be less?" The answer has to do with sente, which is beyond the introductory level. Simplified, we might say, it is like counting accounts receivable as assets in a balance sheet. Even though we know that the company may likely never receive the full amount, the accountant counts it as 100% because it simplifies things. -- 2003-09-12
Bruce: The amount of territory depends on how the game proceeds afterwards, which will depend on the surrounding position. For example, Black a is a severe threat against White, and if White answers at c Black has successfully reduced White's territory. But is some cases White might answer at e instead, and depending on the position further to the left Black may not like that result, in which case he might leave the position as is and let White play a.
Bill: The above diagram mixes estimated territory with the concept of territory. That can be confusing. (Besides, the estimate is questionable.)
But the concept of territory is broader than the definition at the top of the page. For instance, we talk about taking away territory or living inside the opponent's territory or playing inside the opponent's territory (usually with the result of taking some or all of it away). So the diagram showing estimated territory is not so far off.
See Also: