Go

Seki

Occasionally, adjacent opposing groups will become surrounded while neither group has two eyes. If one group has enough extra liberties, it may be able to capture the other. Otherwise, a seki, or dual life, is a likely result of the battle.

In the below picture, the White group in the lower left has two eyes, and so is safe. The two Black groups in the corners at the top have just one eye each (the corner points), but are safe because White cannot play on either point marked X without allowing Black to capture the White stones. Meanwhile, the five White stones on the right and the large Black group at the bottom are both eyeless, but both groups are safe because neither player can occupy either of the points marked Y without allowing the opponent to play on the other point Y and capture them. Since neither player can approach the other anywhere, the various groups with fewer than two eyes are alive in seki.

Ko

In the below picture, Black can capture one White stone by playing at X. The resulting position is shown. If White plays at Y, the former arrangement is restored. However, White may not make such a move without playing somewhere else first, thus giving Black a chance to play at Y and retain the captured point.

Suicide Moves

A stone may not be placed in such a way that it (or the group it's part of) has no liberties. (Chinese rules allow such "suicide" moves; the stone that was just played (or the group it's part of) is captured right away; occasionally, such a move can be useful as a ko threat.

This is illustrated by the below position, in which a White move at X would cause two White stones to be removed, but White would now threaten to play at X again, making Black's eye to the left of X a false eye.