Liberty (tactical sense)

Difficulty: Intermediate Keywords: Tactics, Go term

The second sense of liberty, important for tactics, is a play required to capture a single stone or chain of stones.^[1] This is the sense you require if you want to say that the number of liberties measures time to capture a group. (See liberty for a comparison of the two meanings.)

There are stones that cannot be captured, e.g. stones that are part of groups with two eyes. No number of moves is enough to capture them, they are alive. There are also stones in atari, with no way of saving them. They can be captured with one move. In between there are stones with multiple liberties which may be involved in races to capture.

The easiest liberties to see and count are liberties in the first sense, or dame. The opponent must make at least as many moves to capture a stone or chain as it has dame.

Each marked chain obviously has three liberties. It will take Black 3 moves to capture the white+circle White chain. and likewise, it will take White 3 moves to capture the black+circle Black chain. The first player wins this race.

There also occur chains situated in a more complicated way: there may be moves to play, which are necessary in practical terms to capture them, on points that are not adjacent to them. Also see approach moves, approach liberty and secondary liberties.

While both marked chains ( black+circle and white+circle ) may appear to have only 2 liberties, in the sense of moves to capture them they each really have 3. Black must first connect at the marked point on the edge of the board, while White must take off black+square at a. These plays are required preliminaries, before filling a dame in the race to capture. Filling the dame before making the protective play first is a blunder (auto-atari).

Here again, the first player wins the race.

The assessment of the necessary moves can get very complicated.

For instance, a one-eyed group is often less close to capture than you'd think.

Here is an example where a big eye has more liberties than are apparent.

Black appears to have only 4 liberties, but actually has 5, the same number as White. This is also a first player win - details in the footnote^[2].

The number of moves to capture is normally one more than the number of moves to put into atari, but even then there are comments to make:

That isn't true if by putting the opponent into atari you put yourself into atari;

The player with the move can capture the other's stones.

You may never be able safely to play atari: seki happens;

In this seki, or standoff, the opposing chains share two dame, but there are no moves to capture. Whoever approaches puts himself in atari.

Sometimes the only way to play atari is a ko capture.

Black to play can take the ko at a, threatening to capture White's stones. White to play can make an approach move, say at b, to convert to a regular ko. Such a ko is called an approach ko.

Black plainly has 3 moves to capture, but how do we count White's?

[1]

This sense may be extended to discussion of groups of several chains, to help answer the question 'how many plays does it take to capture that group?' This is actually a considerably more advanced question, which cannot so simply be answered by inspection. See Secondary liberty / Discussion.

-- Charles Matthews

[2]