Life and Death
We know that when a group is completely surrounded and loses it’s liberties, it’s captured.
This may confuse beginners. If everything is surrounded by everything else, how do we know if it’s alive or dead.
Indeed, some groups can be completely surrounded while being invincible and uncapturable if they have the right shape.
This is known as two eyes. If a group has two eyes, it is absolutely invincible, whether surrounded or not.
For an example of two eyes, take a look at figure B. White’s group has 2 spaces (i.e. eyes) and 2 liberties.
If black tries to play in the left eye, it is immediately captured and the same for the right. Since it would take black playing two moves at the same time (which is impossible), black cannot reduce the liberties of this group to 0.
So let’s try to decipher which groups can live locally or will end up dying in figure A.
Top-Right Triangled Group
The top-right group is similar to the group in figure B: It has 2 eyes and – in the same way – cannot be killed.
Top-Left Triangled Group
White’s group is dead because it has only 1 eye and cannot ever make 2 separate eyes.
Black can kill the group with either of the sequences shown in fig. C.
In A2, black plays in the corner. White is now in atari and will be captured next turn if they can’t stop it. If white tries to take the black stone in A3, white is still in atari and black simply captures by playing in the corner as in A4.
It’s important to note that the A4 move is not a suicide move because after white’s stones are captured, black is left with some liberties.
The B sequence is another way to kill the group.
If white plays in the corner first to try and save it, it simply looks like A3 or B3 and black can resume killing it.
Because the white group is unconditionally dead, black doesn’t need to spend any more moves killing it. Black can ignore it and when the game ends black can simply take those dead stones off the board and count them as black’s prisoners.
Black’s group on the bottom right is clearly dead. It’s never going to have a chance to make 2 eyes, and white can put it in atari in just 1 move.
White will ignore this group and take the prisoners off the board when the game ends.
This one looks much more tricky. The more you play and improve in Go, the easier it will be for you to know whether a group is alive or not.
This white group is dead. Feel free to play it out on a board and see if white can make 2 eyes. See Tools for some Go programs.
Killing a Group (Basics)
So if a group can make two eyes, it’s alive, so what if we want to kill a group we’ve completely surrounded? Well we can’t let our opponent make 2 eyes with that group!
The picture above is from fig. A’s top-right corner.
We can see that if white plays first (slide 2), white is alive, but if it’s black’s turn to play, black can play at that same spot (slide 3) to prevent white from making two eyes. Now white’s group is absolutely dead.
A similar sequence to fig. C may unfold, but once it becomes absolutely dead, both players should ignore it as there’s nothing else that can be done there.