Armpit

The armpits of the side  

Charles This elegant, new go term^[1] is apparently required for discussion of current advanced opening theory.^[2]

To be more specific, something like this is said: the particular areas marked here with squares, on the lines 6 to 8 out from the corner, have a special role in relation with 4-4 point openings.

In relation with 4-4 and 10-4 points  

One can make preliminary sense of the left-hand 'armpit', in relation to the stone occupying the corner; and the 10-4 stone .

Plays by White in this marked area do not 'touch' either stone.

Corner area  

That means we are thinking of the corner area as bordered by these circle-marked stones.

Outlier area  

Looking at this area - outlier points - also clarifies the contemporary (post-Chinese opening) idea that it is valuable for Black to prevent White making a strong group here, in developing .

This all leads to the idea of looking at White's joseki options within the armpit.

Diagonal attachment variation  

For example, this one.

Here is perhaps better known, played at a. The play at emphasises making a base.

Characteristic is therefore a base made as a short extension. This isn't very efficient. It may lead though to immediate life, which is of value according to Black's framework prospects on the left side.

Side approach  

We can compare that with this . It creates a type of miai position (White expects next to have a choice of the circled points, or of the squared points.) White also may just jump out to a.

This direct entry into sanrensei was played in the shinfuseki period, but not so much since. White in pro games may play , but in a context with some other stones played outside before that.

Contact on top  

It is in rather a different setting that this currently seems to matter for aspiring theoreticians.

Here is a potentially difficult joseki play, in some variations, taking into account the stone. White may take a short base; or influence across the armpit, in both the tsukehiki and tsukenobi variations.