Recognizing an Eye
This page is intended for people who are beginning to learn the game of Go and prefer a non mathematical and visual way to recognize an eye. While the technique discussed here works in most cases, it is not a foolproof way of determining whether the shape is a real eye or a false eye.
More formal discussions can be carried on formal definitions of eye.
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1. The group must surround at least one empty point.
In each of the three cases above, the group surrounds an empty point a.
If the above two conditions are satisfied, then we definitely have a real eye.
In each of the three cases above, the point a is a real eye.
Note that in the third case, it is not neccessary for black to occupy the point at b.
Some people may misunderstand the meaning of the two conditions above, so here is an illustration of what is not an eye.
In the three cases above, there is no eye. In each case, condition 2 is satisfied but condition 1 is not, as the group did not surround any empty points.
Some people may prefer this alternative for condition 2.
At the corner, for a to be a real eye, the point marked x must be occupied by a black stone.
At the side, for a to be a real eye, both the points marked x must be occupied by black stones.
In the middle, for a to be a real eye, three out of four points marked x must be occupied by black stones.
Wrenn: I would suggest instead of using "'x' must be occupied by black stones", you use "'x' must not be able to be occupied by White stones". The difference may seems small, but it stops people from getting confused about eyes in real game, where they will not have to go back and fill in those 'x's as long as the opponent cannot play there
unkx80: You are right. Actually I mentioned it right at the bottom of this page.
tapir: Guo Juan calls the diagonals shoulders (as in shoulder hit) and then it is simple "one shoulder in the corner, two shoulders at the edge, three shoulders in the center". Instead of occupy use control and you have a mnemonic fitting into one sentence: "To make an eye you need to control one shoulder in the corner, two at the edge and three in the center."
Using the eye in the middle as an illustration, we say that this shape that looks like an eye is a false eye. Either version of condition 2 is violated: the stones are not having solid connection; and white managed to occupy the two points, so black only occupied two out of the four x points.
Here we assume that the two stones cannot be captured.
To illustrate this, we shall have white surround the three stones such that these stones are under atari.
If black saves the three stones, then condition 1 is violated, since is no longer an empty point. Thus, this is not an eye.
On the other hand, if we allow to capture the three black stones, then obviously there is no eye for black here.
A more accurate way of stating the alternative for condition 2 is to change
- ...the point(s) marked x must be occupied by black stone(s).
- ...the point(s) marked x cannot be occupied by white stone(s).
so that we recognize more possible kinds of real eyes, such as the example below.
The method for recognizing an eye as described on this page is also known as the 2/4 rule.