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CapturingRace

Referenced by
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Kikashi
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RelyingOnTrickPlays
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Semeai
  Difficulty: Beginner   Keywords: Tactics, Go term

A semeai is a 'capturing race'. I.e. a race where both parties are closed in and are reducing the others liberties. The first one to put the opponent into atari and capture him wins the race, and by doing so, makes life for his group.

[Diagram]
Diag.: Simple semeai

Here, both black and white need to avoid capture of their own groups. Since they both have the same number of liberties, whoever has sente will be able to win.

More accurately, according to Richard Hunter's theory on counting liberties, this is a fight between two groups who share less than two liberties. Since they have the same number of exclusive liberties, the side that starts the fight, will kill the other.

'a' is known as 'shared' liberty, since it is a liberty for both black and white.

When attacking your opponents group in a semeai, you should play the shared liberties last. Otherwise, you take away one of your own liberties as well as the opponents, and then give him sente afterwards.

In this diagram both groups have 5 liberties. If black plays first, he will capture the white group first. However, if Black played 'a' instead of 1 he would suddenly have one liberty less than white after white 2.

Correct play for black would be e.g. black 1. White should not respond with white 2, since he cannot win anyway. However, it does make a good ko threat.


[Diagram]
Diag.: Semeai with one eye

The presence of eyes in a semeai complicate matters somewhat. In this position, at first glance black seems to have more liberties than white (6 against 5). Nevertheless, black cannot win the semeai even if he plays first. Why this is the case, and how one should correct the count so as to count a semeai like this correctly can be read in EyesWinSemeais. Another complicating factor is that when there are big eyes (four spaces or more), the number of liberties also is higher than a naive count would suggest. See Four Is Five And Five Is Eight And Six Is Twelve or counting liberties for this effect.


[Diagram]
Diag.: Approach moves make a difference

Although you can easily count external liberties, sometimes it is necessary to play more than just on the liberties to capture the group.

In this example, both players have 3 liberties. However, even with sente, black cannot win the semeai. Black must play 3 before he can play atari at 'a', and by that time he is in atari himself. In practice this approach move has given white one extra liberty.



authors:

  • Morten Pahle (10 kyu)
  • Arno Hollosi (1 dan)
  • Andre Engels (2 dan)


This is a copy of the living page "Semeai" at Sensei's Library.
(C) the Authors, published under the OpenContent License V1.0.