The classic 3-3 invasion is where one player takes the 3-3 point underneath their opponent's star-point stone, as in in the diagram below.
Invasions such as this represent an exchange of territory for outside strength. The important questions here about timing & direction seek to find the answer to the question of which is worth more, the territory white gains (along with potential black territory eliminated) or the outside force that Black develops.
This page looks at the ideas behind the invasion for both black and white, before a summary of the standard sequences that result.
In high handicap games, black often sees his 'territory' destroyed by a direct white invasion, such as at the 3-3 point. Nevertheless, Black should often welcome this development. Black shouldn't see this move as an attack on his territory; the 4-4 point is not meant to take the corner with a single move, but to make thickness and seek a balance between territory and influence. The white invasion stone is separated from the rest of the board by the black stone on the 4-4 point. By playing correctly Black will get a strong position and early in the game, the thickness black develops is more valuable than white's territory.
These two points outline the key strategic decisions that need to be made with an invasion at the 3-3 point. In summary, if White can invade and live, then white is going to gain some small territory in the corner and black is going to build a wall which is hopefully thick and which will outweigh the value of white's tiny life.
Some points for Black to consider:
White's perspective is the reverse of Black's. The key concern for white is the value of the thickness that Black will build in response to the invasion. This is a question of timing; invade too soon and Black's thickness will overwhelm the whole board, wait too long and the invasion will be prevented. One rule of thumb is:
This is all too much chat and too few stones on the board. Let's take a look at an example:
With white to play, Black has built what looks like a fairly thick group in the centre, at the expense of some corner territory. Black has a big moyo on the right, and whilst white has a moyo on the top and left, it's smaller than Black's so the time has probably come to do something about black's moyo.
An invasion around one of the points marked a would be likely to undergo a severe attack as black has ready made pincers.
When White invades, black has to decide on which side to block. Block on the wider side says the proverb, but which side is wider? There are two clues here. First, of the two marked Black stones, the one up the right hand side is one stone further from the corner than the one on the bottom. Second, the marked White stones will allow White to undercut black's position on the bottom, reducing any potential territory there severely. Therefore Black plays , to block the right hand (wider) side.
This is how the game continues. After Black blocks, White plays along the edge at , and whilst Black keeps the group low with , and . The sequnece ends with White bending around the bottom, and black making shape at . White takes Sente to play elsewhere.
As can be seen, white has taken some territory in the corner, but black managed to make thickness which is well placed with regard to the other black stones on the right. Shin used his sente to play at a, expanding white's moyo at the expense of Black's and went on to win by resignation
The way that the invasion was played out in the example above is the industry standard. That is to say, whilst it is by no means the only sequence, it is the simplest commonly occuring one, so it bears closer inspection.
To Black answers with , checking first that he is blocking on the wider side.
is the natural move to begin to build life for the corner group,
prevents white from making progress out into the centre. Note though that it leaves a cut behind at a. To Tenuki here would be a disaster for black.
White plays at to avoid Black playing at the same point, shutting him into the corner and black replies again with . This is the basic continuation, although the severe double hane of at 5 is playable.
: Usually crawling along the 2nd line is bad, but is important to prevent Black turning here in sente.
: again Black answers. Black has successfully kept White to the side, whilst developing strength in the middle.
: White resists the temptation to continue crawling at a. Every extra point white takes, is far less valuable than the outside strength that Black takes and so switches to the other side.
: Black again blocks white and then protects the shape with the hanging connection of .
White has Sente to play elsewhere.
Yesterday i was having a comversation with a beginner, and he said "stronger players' advice always seems to boil down the phrase 'it depends'." Well, of course subtly different situations sometimes need to be handled in completely different ways, but this is one of the beauties of Baduk. Whilst the sequence given above is a useful template consider, the suitability of the 3-3 invasion is highly dependent on the number and location of neighbouring stones. In certain situations the invasion may require a different sequence, or may result in ko.
Here is an example. The invasion behind black's large knight enclosure is still playable.
: white takes the atari,
: then plays the Hane.
, , , are all standard, but then White needs to go back to play , making the group alive.
: this protects the group by limiting the stone.
Thus White has invaded the corner successfully but because it was stronger than before, Black has made the corner smaller.
There are other pages con sidering the 3-3 invasion under different local conditions, for example:
3-3 Point - When Does It Work?
There is more detail at 4-4 point 3-3 invasion joseki and the following pages: