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.

[Diagram]

Example 1: 3 vs. 3

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

There are also chains where there may be moves which are necessary to capture them on points that are not adjacent to them. Such moves are called approach moves, or perhaps secondary liberties.

[Diagram]

Example 2: 3 vs 3

While each marked chain appears to have only 2 liberties (in the sense of moves to capture them), they each really have 3. Black must connect at the marked point on the edge of the board, while White must take off BS, before they fill a dame in the race to capture. If they filled the dame without making the protective play first, they would put their own stones in atari.

Here again, the first player wins.

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.

[Diagram]

Big eye: 5 vs. 5

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:

[Diagram]

Mutual atari

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

[Diagram]

Seki

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

[Diagram]

Approach ko

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]

[Diagram]

Black first

After B7 Black has 3 liberties to White's 2.

[Diagram]

Black first (ii)

Black just wins the race.

[Diagram]

White first

After B6 it is 3 vs. 3.

[Diagram]

White first (ii)

B4 creates another liberty,

[Diagram]

White first (iii)

but White fills it right away.


This is a copy of the living page "Liberty (tactical sense)" at Sensei's Library.
(OC) 2004 the Authors, published under the OpenContent License V1.0.
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