3-4 point high approach, inside contact, hanging connection
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to is a very common 3-4 point joseki. Subsequently, a White move at a or a Black move at b is very large. The reason is that the checking extension of b amplifies the severity of a Black invasion at c. However, it should be noted that even with White a, the invasion at c still exists, but much less severe.
Depending on the left side of the board, may choose to play at d or e. here settles the position in one move, while at d or e emphasizes an additional extension on the left side. playing the clamp at f is an old joseki.
Compared to the joseki when makes a solid connection (see 3-4 point high approach, inside contact, solid connection), deep invasion by Black is quite possible - but it has good prospects for future development.
There are other variants to the joseki, see the section on "Variants" below.
The remainder of this page discusses the invasion when White did not defend his three-point extension.
Because the threatens to connect at a, the standard reply is at . Many of the sequences after are well-established enough to be called side joseki. can be played at b for territory, or c for outside influence. at d leads to a ko that is heavy for both players, so the timing for this move is important.
A discussion of why is better than a can be found here.
here goes for territory, so White will get the outside influence. The traditional reply here is . Here, is a tesuji which creates a weakness on White's exterior, so the loss-making exchange of with is necessary for preventing a Black cut at a.
This is one of the side josekis after the invasion.
A more recent variation by White is to play the diagonal move at , followed by a thick move at .
Black also has the option of attaching at and starting a ko. This is the only sequence that actually severes the connection of the stone, and usually is the only sequence available when there is no stone (or even when there is a White at a). The resulting ko can be heavy for both players, because the winner of this ko usually gets something that looks like a ponnuki.
When Black has just taken back the ko, and both players have no more suitable ko-threats, White may consider a move at b. This usually forces Black c, then White can look forward to taking the ko at d and then ending it with a ponnuki at e. Of course, Black gets to play two moves elsewhere.
is sometimes seen at b instead.
In recent years, the direct pincer at is also common. In professional games, one of the most common reply is , and typically Black will tenuki.
may also be played at a, but because the hanging connection is looser than the solid connection, typically Black will plat at b instead of c, and a fight will erupt. See 3-4 point high approach, inside contact, hanging connection, pincer fight for more.
I encountered this move 6 in a game today. It's new to me, does anyone know if white has a standard followup for attacking later? Uberdude: This move was popular around 2015-2016. It aims to get a slightly bigger territory, but leaves some thinness for white to exploit later. That would be too easy if white made the solid instead of hanging connection, but now it's quite balanced. e/g/h are some possible follow ups for white. See pro game pattern search http://ps.waltheri.net/124442 for some examples.