There is no rule in go that says a group with two eyes is safe. It simply follows from the rules, and once you see why, the whole game reorganises around it.
Why two eyes work
An eye is an empty point surrounded entirely by one colour. To capture a group, the opponent must fill every liberty, including the eyes. Filling the first eye is legal only if it removes the group's last liberty — but with a second eye still open, the group still has a liberty, so the move is suicide and illegal.
One eye is not enough. A group with a single large empty space inside can usually be killed by placing a stone at the right point and dividing that space so it never becomes two.
False eyes
A point can look like an eye and not be one. If the stones surrounding it are not properly connected — typically when the diagonal points are controlled by the opponent — the surrounding stones can be captured, collapsing the eye.
The check: an eye is real if you control the diagonal points around it, or at least three of the four in the middle of the board and both of the two on the edge. If the opponent holds the key diagonal, the eye is false.
Eye space and the standard shapes
Most life-and-death questions come down to whether a surrounded group's internal empty space can be divided into two eyes. Some shapes work and some do not, and the common ones are worth memorising.
| Internal space | Status |
|---|---|
| Two separate single points | Alive unconditionally |
| Straight three | Alive if you play the centre point first; dead if the opponent does |
| Bent four in the corner | Dead — a well-known exception |
| Straight four | Alive |
| Square four | Dead |
| Five or more points in a row | Alive |
The general principle behind the table: the attacker plays at the vital point in the middle of the space, the defender plays there first to split it. Whoever moves first in a three- or four-point space usually decides the outcome.
The vital point
In almost every eye-space problem, both the killing move and the living move are the same point. If you can find where the opponent would play to kill, play there yourself.
Seki: living without two eyes
Occasionally two groups share liberties in a way that neither can fill without dying first. Both live, neither has two eyes, and the shared points count as territory for nobody. This is seki, and it is rare but legal — worth recognising so you do not waste moves trying to kill something that cannot be killed.
How to use this while playing
- For every group you own, ask whether it has two eyes, can make two eyes, or can connect to a group that does.
- If the answer is none of those, either run toward the centre or accept the loss and take points elsewhere.
- When attacking, do not try to capture immediately. Take away eye space first; the capture follows.
- Corners need the least eye space, edges more, the centre most. A group running to the centre is usually escaping rather than living.