Settled region

A region of the board is settled if neither player can gain anything by playing there.

Examples
The simplest example of a settled region is an area of the board that is completely filled with stones. The following region is settled:



However, a region does not need to be completely filled with stones to be settled. The following region is also settled, and is in fact equivalent to the above one:

 To see why, note that, from Red's point of view, b5 is connected to the top edge and e3 is not. Nothing Red could do in the region could achieve a better outcome. Similarly, from Blue's point of view, e2 and c4 are connected to each other, and neither of these stones is connected to the left edge. Again, nothing Blue could do in the region could improve this outcome for Blue. Finally, the empty cells at the region's boundary, e1, d3, and a5, are such that their color does not matter, since each of them is already next to a red stone (or edge) and a blue stone (or edge). Consequently, no player can achieve anything that that player does not already have, and the region is settled.

Usage
Being able to recognize when a region is settled is a useful skill, because it allows the player to focus their attention elsewhere. A move in a settled region is always a wasted move, and therefore neither player needs to consider moving there. (If the whole board is settled and both players know it, the losing player may as well resign.)

A move in a settled region can sometimes be used as a time-tesuji.