British Go Journal No. 0.  Spring 1967. Page 2.
Here we deal with safe extensions from a wall of stones. These are usually made along the third line, but on occasions may be along the fourth line. There is a general rule for these extensions that is vital to remember, it is that you can extend 1 point more than the number of effective stones in the wall, from the base, along the third line without being disconnected. This does not always work if there are stones very close to the single stone.
The number of effective stones is, in general, the number of stones in the wall above the second line, however there are a number of important exceptions, which will be dealt with.
Diagram 1 ||
Diagram 2 ||
Diagram 3 |
Thus from a Black stone at A you can safely extend to B (or C) without fear of being cut, for if White plays Dia 2 or Dia 3 Black remains connected.
Diagram 4 |
Similarly from a two stone wall, A, you can safely extend to B (or C), for if White plays 1 etc, black 4 captures the white stones.
For a three or more stone wall the maximum extension is not always the best, but the general rule still applies.
Here is a good example of this, two closely related joseki show the general principle:
Diagram 5 |
Black plays 2 avoiding complications in the corner. With 6 White has made a perfect extension from a two stone wall.
Diagram 6 |
However in Dia 6 White's extension is only three points because if White 8' at A, Black can play B threatening the disconnection at C, and if White now protects against this Black can gain a large potential along the left hand side for almost no cost.
Another good example are the two joseki:
Diagram 7 |
White 8 is the perfect extension. However if the situation in the south-east corner has the two marked stones in Dia 8 then the joseki White should play is:
Diagram 8 |
This play has white 10 again getting the three point extension, but also combining it with a pincer on the black stone preventing him from making a proper extension from it. Note also that the wall does not have to be vertical, but white 4 contributes to the strength of the wall.
A major exception arises from the position in Dia 9:
Diagram 9 |
Here Black can now make an extension as far as 1, depending upon the situation in the south-east corner, without great fear of invasion, for if White plays 2 Black can play as Dia 9 gaining the requisite wall for the extension and can capture white 2. Black will only extend this far if there is a pincer move to be made in the south-east corner, for if there is a strong position there White will probably be able to pull out his stone, 2, by connecting underneath the black 1, so in that case Black would not extend as far, perhaps only playing 1' at 2.
Furthermore, in this situation after Black 1, White will usually play 2 in Dia 10:
Diagram 10 |
White plays 2 in Dia 10, blocking the sequence of Dia 9 for black
and thereby threatening the invasion and cut at A, so black must protect
against this and the best play is 3. However if black has only extended
with 1' to A, as in Dia 11, this is not so important as there is no*
room for an invasion stone at B to manoeuvre in as shown in the
exchange in Dia 12.
* BGJ omits word "no"
Diagram 11 ||
Diagram 12 |
If White plays 7 then black can make the cut at 8.
The rule gives the maximum extensions possible, except in a very few
cases, so this gives* you the most economical use of your
stones, something that is very important in the opening stages of the
game when you are trying to map out the maximum area with the fewest
moves. Also at the same time you are trying to decrease your opponent's
area, thus you should try to make him under-extend if possible. So you
should try and obstruct his best extensions, and also if he has
obstructed your optimum extension you should not play to
over-concentrate your forces because this is just what he wants.
* BGJ omits word "gives"
However if he cannot make a good extension on the other side you can play there and attack him. One example of this is the position in Dia 13:
Diagram 13 |
Here White should play 1. If he plays A Black can play B, the
perfect extension from his corner enclosure*. Black now plays 2
and White plays 3, threatening the corner. The black position on the
right is not yet completely secure and may possibly be invaded later.
* BGJ had "his two stone wall"
Diagram 14 |
Another example is Dia 14 where Black should play 1 forcing white 2 and over-concentration. However if the white stone is at A, not , this sequence only makes the position perfect. In that case, direct invasion is impossible because of the weakness at B, so Black should protect the corner first by playing C or D and wait for the chance to invade at E.