| Position 1 | Position 2 | Position 3 |
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Let us look at the Position 1 in Fig 1-1. It is possible for black to cut at 'a' - and this will capture the white marked stone - no escape. The killing sequence is given in Position 2 and the resulting situation - in Position 3. This is important - since while in Position 1 the upper right can be considered to be white's territory, in Position 3 it is completely eaten away by black (the x's denote the killed white stones).
| Position 1 | Position 2 | Position 3 |
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Q: What should white do in Position 1 to defend?
A: He should play at 'a' or at 'b' to defend the cut.
A very common application of this sequence is depicted in Fig 1-2:
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Here we see that white has stacked out a territory on the top, white leaving black the area at the bottom. Both areas are too small for a successful invasion, so what is left to do is to end-of the edges and count the points. Black starts ending-of the edges by reducing white's area with '1', at which white blocks at '2'. Black connects at '3', and white concludes the sequence by defending the cut with '4'. After black connects at '5', white returns the favor by playing the same sequence on black with '6' to '9'. Game over - now the score needs to be counted (not counting the possible prisoners, black has 29 points at the bottom, and white has 28 points at the top, so black wins by 1 point on the board - not counting possible prisoners and komi).
Another common situation is given in Position 1 of Fig 2-1. Here the marked white stone is in danger of being killed by black if white does not defend it (the usual defense is at 'a', 'b', or 'c'). Once black is allowed to play at 'a', there is no escape for the white stone - as shown in Position 2. The resulting situation is shown in Position 3, with the x's denoting the killed white stones.
| Position 1 | Position 2 | Position 3 |
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Note:
In Position 2,
instead of playing at '5', black can also play at 'd', initiating a
sequence somewhat similar to Fig 1-1 - I leave it u p to
you to figure out the exact technique.
Continuing from the previous example, by re-iterating the stones a little, we get the situation shown in Position 1 of Fig 3-1. Pay special attention to the cut at 'a' and what happens if white does not defend there (the usual defense for white would be to play 'a' or 'b'). Let us assume that white did not defend, and that black cut at 'a', or at '1' in Position 2. This is the deadly cut on the second line! - there is no escape for white, as can be seen in this Position. Another common sequence arising from the cut on the second line is shown in Position 3 - this neatly ties up the ideas of Fig 1-1 and this of Fig 2-1.
| Position 1 | Position 2 | Position 3 |
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The reason why white has to play at '4' in Position 3 (as opposed to playing at '5') is tied up with the concept called snap-back which is one of the basic tesujis (=combinations, or skillful finesses) for killing stones - explanations of which may be found here.