- For the Territory Line on a larger board, see the section Larger Boards below.
When we speak of the third line or territory line, this is what we mean:
On a 19x19 goban, it is said that the third line is the line of territory. This means that a fair exchange can result into stones on the third line facing the side, opposing stones on the fourth line building influence.
It is also said that "the third line is the line of completion". This means that playing on the third line is usually an attempt to complete a secure shape, rather than develop a big framework.
When speaking of extensions or approach moves, the third line line will be referred to as low, whereas the fourth line is called high. The third line was used extensively in Edo period go. Gradually the fourth line was used more for extensions.
Territory line is (as of 2019-01-14) a term used but not explicitly defined by Platinum Dragon on the pages Why 19×19 and Large boards. It appears to be used for the equivalent of the third line on a standard 19×19 board, i.e. the line best suited to claiming territory rather than creating influence. The description on Why 19×19 is:
- A constant number when multiple to the side of a square board gives the “territory” line, and 5×5, 13×13, 19×19 are such special cases of them.
A more extensive development of this idea is at Platinum Dragon / Boards that have tengen as best starting move. There they define it to mean the line which covers more than half the board both when combined with the area inside and when combined with the area outside it.