Life and Death in Go: Why Groups Need Two Eyes

In Go, some groups can be captured and some never can, no matter what the opponent does. Knowing the difference is called life and death, and it's the most important skill after the rules themselves. The good news: it all comes from one idea.

What an eye is

An eye is an empty point (or small empty area) completely surrounded by one player's stones. Your opponent can only fill it if the move captures something — otherwise it's suicide and not allowed.

Why one eye is not enough

If your group has only one eye, your opponent can fill all its outside liberties first. At that point, the last liberty left is the eye itself, and playing inside it is no longer suicide — it captures your whole group.

Why two eyes mean life

With two separate eyes, your opponent can never fill both. Playing in either eye is suicide, because the other eye is still a liberty. The group can't be captured, ever. This is what Go players mean when they say a group is alive.

False eyes

Not every surrounded point is a real eye. If the stones around an "eye" can be separated and captured, the eye disappears with them. This is called a false eye. A common case: the surrounding stones are connected only diagonally, and the opponent controls the diagonal point. When you count eyes, make sure the walls around each eye are solidly connected.

Vital points

When a group has an enclosed empty space but no eyes yet, there is often one vital point: the move that decides everything. If the owner plays it, the space splits into two eyes and the group lives. If the opponent plays it first, the space can only ever form one eye and the group dies. In life-and-death problems, look for this point first.

Basic shapes to remember

Eye spaces are named after the number of empty points inside the group and their shape:

• Straight three and bent three — three points in a row or an L. The middle point is vital: whoever plays there first decides life or death.
• Straight four and bent four (in most positions) — alive. There are two ways to make two eyes, so if the opponent takes one, the owner takes the other.
• Square four — dead. The shape can never form two separate eyes.
• Pyramid four (a T shape) — the centre point is vital.

You don't need to memorize every shape at once. Learn the three-point and four-point shapes and you'll recognize most beginner situations.

Seki: when nobody can attack

Occasionally, two groups share liberties in such a way that whoever plays inside first gets captured. Neither player attacks, and both groups stay on the board. This is called seki, or mutual life. It's rare in beginner games, but it's good to know it exists.

How this changes the way you play

• When your group is surrounded, look for space to make two eyes before anything else.
• When attacking, don't try to surround from the outside only — take away eye space by playing at the vital point.
• A big group with no eyes is weaker than a small group with two.

Try it yourself

Black to play. Can the black group in the corner live?

Bigger eye spaces

As a rule of thumb, the more room a group has to make eyes, the easier it is to live. A group with an enclosed space of five or six points, in a sensible shape, is usually alive even if the opponent plays first inside. The danger zone is the small spaces — three and four points — where a single move at the vital point decides everything. That's why strong players try to make their groups' eye space bigger early, by extending along the edge, rather than waiting until the group is already surrounded.

Practice

The fastest way to improve is to solve short life-and-death problems: a group, a few stones around it, and the question "black to live" or "black to kill". Five problems a day will change your games within a week.