If you remember one strategic idea from Go, make it this: a group with two separate eyes is alive forever. It can never be captured, no matter how many stones surround it. Everything in life and death — what Go players call the fight for a group's survival — comes back to this rule. This article explains why it is true and how to build eyes in practice.
What an eye is
An eye is an empty point (or a small empty area) completely surrounded by your own stones, so that the opponent cannot safely play there. Because of the rule against suicide, an enemy stone placed inside your eye would have no liberties — unless that move captures your group.
Why one eye is not enough
Suppose your group encloses one eye. Your opponent can fill all your outside liberties, one by one. Eventually the only liberty left is the eye itself. At that moment, a stone placed inside the eye takes your last liberty — so it captures your group, and it is legal. One eye can always be taken away.
Why two eyes are enough
Now suppose your group encloses two separate eyes. Your opponent fills all outside liberties. Your group still has two liberties: the two eyes. A stone placed in either eye would have no liberties and would not take your last liberty, because the other eye remains. That is suicide, which is illegal. So your opponent can never play in either eye, and your group can never be captured.
Check it yourself: the black group's only liberties are a1 and c1. White cannot play at a1 (no liberties, no capture) and cannot play at c1 for the same reason. The group lives.
The eyes must be separate
Two empty points next to each other inside your group are not two eyes; they form one eye space of two points. The opponent can place a stone in one of them (it has a liberty in the other), then later fill the other, capturing everything. Eyes must be separated by your own stones to count as two.
Where eyes are easiest to make
- In the corner: two edges help enclose the space, so a small group lives easily.
- On the edge: one edge helps; a group needs a bit more room.
- In the centre: nothing helps; a group needs a large area to make two eyes, which is why centre groups usually survive by running and connecting rather than by making eyes.
This is one reason corners are valuable. See corners, sides and centre.
Making eyes in practice
- Claim space early. A group with room along the edge can make eyes; a group squeezed into a small area often cannot.
- Make eye shape, not just territory. Sometimes a single stone in the right place divides one large eye space into two eyes.
- Watch for false eyes. An "eye" whose surrounding stones can be put in atari may not be a real eye. See false eyes.
- Know the vital point. In many eye spaces, one point decides whether the group can make two eyes. Whoever plays it first decides life or death. See eye shapes and vital points.
Killing: the other side of the coin
To kill a group, you do the opposite: reduce its space and play on its vital point so it can make only one eye. You cannot kill a group that already has two real eyes, so the time to attack is before the second eye is formed.
A simple test during a game
For each of your groups, ask:
- Does it already have two separate eyes? If yes, it is safe.
- If not, does it have room to make two? If yes, make sure the opponent cannot take that room.
- If not, can it escape to the centre or connect to a living group? If not, it is in serious danger.
Asking these three questions about your weakest group once every few moves prevents most disasters.
Practise
Build groups on the edge and in the corner in games against the computer on the home page and see which live. On 9 × 9 the computer will quickly test whether your "eyes" are real. Save games against people on the online page by signing in on your account, and replay any group that died: count its eyes at the end. Related: suicide and illegal moves, corner life and death basics, how to study life and death.
A story every Go player knows
Almost every beginner has this experience: a group surrounds what looks like a big territory, the game seems won, and then the opponent plays a single stone inside, and a few moves later the whole group is captured. When the game is replayed, the reason is always the same: the "territory" was one large eye, not two. The opponent played on the point that stopped it being divided, and the group could never make a second eye. The lesson sticks after one or two such games — and from then on, you start checking your groups for two eyes long before the opponent can attack them.
Two eyes versus connection
A weak group has two ways to become safe: make two eyes, or connect to a group that is already alive. In the opening and early middle game, connecting is often easier than making eyes, especially for groups in the centre. As the board fills up, eye space becomes the main question. Keep both options in mind: when eye space is tight, look for a connection; when connection is blocked, look for eyes.
Eyes and the final count
On this site every stone on the board counts at the end, and empty points surrounded by one colour count for that colour. The eyes of a living group are part of its owner's area. So the effort you spend making two eyes is paid back twice: the group survives, and its eyes count as points.
Frequently asked questions
Can a group with two eyes ever be captured?
No, as long as both eyes are real and separate.
Why can the opponent play inside my single eye?
Because once your outside liberties are gone, the eye is your last liberty, and taking it captures your group.
Is a large eye space the same as two eyes?
No. A large space can often be divided into two eyes, but until it is, the opponent may be able to prevent that.
What is a false eye?
An eye whose surrounding stones can be captured or put in atari, so it does not function as a real eye.
Do I need two eyes if my group is connected to a living group?
No. Connecting to a group that already lives also keeps your stones safe.
