Life and Death

Living group is an important concept in go. Living group is safe against any play by the opponent. At the end of the game, all group should be able to defend themselves.

Two eyes

A most common secure group is a group that has (or can always make) two eyes.

Diagram

This black group cannot be captured.

Diagram

Diagram

This black group has only one eye. White can capture the group.

Diagram

Now black has only one liberty, and white can capture black with next move.

One eye is not enought for living.

Making two eyes

Diagram

This black group can make eyes if its blacks turn to play. If white is to play, white can reduce the group to one eye.

If black can play first, black can make two eyes.

The only way to prevent making two eyes is to play white 1.

If black plays, black is left with just one liberty, and white can capture black.

Eventually black is reduced to one liberty, and white can capture black stones.

White cannot play to A, because white stone would not have liberties.
Also white cannot play to B, because again white stone would not have liberties.
So white cannot start killing at all.