Extension From a Wall
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From a two-stone wall, extend three is a proverb.
It means that if Black has the wall of the two marked stones, the most appropriate extension is to Black 1 here on the third line (sometimes also to a on the fourth line). Here three refers to a three-space extension. As that page explains, such an extension from an unsupported stone is easy to invade. The meaning of the proverb is that with Black's extra stone in the wall, the three-space is ideal [1]. A two-space extension is in general too close - it can be criticised on general principles for Overconcentrated shape.
I've read someplace (haven't been able to find the source again) that there's a rule of thumb saying that from a wall, a safe extension is the number of spaces of the height of the wall, plus one. For example:
In this example, the rule of thumb would indicate that the stone marked with a square is a good extension. Has anyone heard of this rule of thumb before? Is it based on something definitive, or just someone's "good idea" with no basis? -- Remillard
The rule of thumb is based on the fact that with the supporting wall, an extension along the side will have a harder time being cut then one with just a single stone, therefore you can make a larger extension. This rule is time-tested, not just someone's good idea. Two things though. One, the extension should be closer to the edge of the board, on the third or fourth line at this stage of the game, and the size of an extension is measured in spaces between the stone and the wall, not just the wall; so the correct five-space extension would be at a. Also, there is also the caveat that past a three stone wall, the variations involved in the cutting become complex enough that this rule of thumb can no longer be followed.
One other thing to keep in mind is that for extending into the center, the rule is just one space per stone in the wall. Again, anything past three spaces here is risky. --BlueWyvern
There's an interesting quantitative point here: if the area of the framework defined by wall plus proper extension from it really goes up quadratically with the height of the wall, we have various possible conclusions, such as
- the influence of the extra stones in the wall is greater
or
- the framework is less securely held, but this is still correct because to get value for the wall you should fight
or
- come on, this must break down for walls of height ten and probably well before.
Actually there is no reason to think any of these statements is completely misleading - all perhaps aspects of the truth.
[1]
The only way Black can prevent to be separated is by granting White a powerful ponnuki on the third line.
With the marked stone, separation is not possible at all, neither gets White a good result. Of course, White stones in the area will influence the position. --Dieter
I am used to this sequence, which appears to be a mistake. I will be avoiding it in the future, thanks Dieter. --Jared
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