The 3-3 Invasion in Go: How It Works and How to Answer It

The 3-3 invasion is a move played underneath an opponent’s 4-4 stone, in the very corner of the board, and it is the reason a 4-4 stone never fully owns the corner. A single stone on the 4-4 point is too high to stop somebody living below it, so the invader drops in at the 3-3 point, takes the corner, and hands the defender a solid wall facing outward in exchange. Both sides get something, which is why the exchange is normal rather than an emergency.

A corner of a Go board with a black stone on the 4-4 point and the 3-3 point underneath it marked as the invasion point.
Black owns the 4-4 point. The marked 3-3 point underneath is always available to White.

If the 4-4 point and the rest of the first few moves are new to you, the Go opening covers them first.

The basic sequence

This is the plain version, and it is not the only one. Strong players have dozens of variations, and the modern ones can run twenty moves deep. What follows is the sequence you can play at any level without getting into trouble.

White invades at the 3-3 point. Black blocks on one side, choosing which direction White is allowed to run.

White invades at the 3-3 point and black blocks on the top side, forcing white to expand down the left side instead.12
1: the invasion. 2: Black blocks on the top side, so White has to go down the left.

White crawls out along the side that is still open, and Black blocks again.

White crawls down the left side to the 3-4 point and black blocks underneath, keeping white low.1234
3 and 4: White stretches down the side, Black keeps it low.

White plays the diagonal turn (a hane, a move that bends around the end of the opponent’s stones) to make a little more room, Black bends back, and White connects up and is alive in the corner.

The completed 3-3 invasion sequence: white ends with a living group in the corner and black ends with an outward-facing wall.1234567
5, 6 and 7 finish it. White is alive in the corner. Black has a wall.

Move 7 is not optional. Without it White’s stone at b5 is cut off from the rest and dies, so White has to join the two up. Once that is done, White’s group has the whole corner behind it, which is more than enough space for two eyes. Life and death in Go explains why that amount of room is safe.

Black’s wall is strong but not perfectly solid: the point directly below Black’s stone at c5 is a cutting point, and in a real game Black either patches it later or checks that the cut cannot work. That is normal, and it is why the invader often gets to play first somewhere else afterwards.

The same finished position with white's corner territory shaded, showing roughly ten points for white against black's outward-facing wall.
What each side got: about ten points of corner for White, and a wall pointing along the top side for Black.

Which side should you block?

This is the only real decision in the sequence, and it is worth more than knowing the moves. Block on the side you care about, so that your wall ends up facing your own stones and the open board rather than facing your opponent’s strength.

In the diagrams above, Black blocked on the top side, so White was pushed down the left and Black’s wall points along the top. Blocking the other way gives the mirror image.

The same invasion answered by blocking on the left side instead, which gives black a wall facing down the left side.12
Blocking on the other side sends White along the top and leaves Black facing down the left side instead.

The test: after the sequence, look at where your wall points. If there is open board or a stone of yours in that direction, the wall is worth a lot. If your opponent already has a strong group there, the wall does almost nothing and you have handed over the corner for very little. When both sides look equal, block on the wider one.

What to do with the wall afterwards

A wall is not territory, and trying to turn it directly into territory is the most common way to waste one. Stones packed against a wall gain very little, because the wall was already doing that work. What a wall is good for is fighting: any stone your opponent plays near it is in trouble, so you can attack it hard, chase it toward your wall, and take profit somewhere else while it runs.

The practical version is to extend away from the wall, not along it. Play at a comfortable distance, in the direction the wall faces, and leave room for your opponent to do something you can attack. If the wall faces the top side and you have nothing else there, the natural follow-up is a stone roughly five or six lines away along that side, which turns the whole stretch into an area your opponent cannot easily invade.

Why players invade so early now

For most of the game’s history, invading at 3-3 in the opening was considered premature. The corner is only worth ten points or so, and giving your opponent a wall that early was thought to be too generous. That changed around 2016, when the strongest computer programs started playing the early 3-3 invasion constantly and human professionals followed. The reasoning is that a wall built early in an otherwise empty board is worth less than it looks, because there is still plenty of room for the other player to work around it.

You do not need to have an opinion about this. It matters to you only because you will see the move played early in real games, and it is not a mistake when it happens.

When not to invade

Skip the 3-3 invasion when the wall your opponent gets would be pointed at something you care about. If they already have stones running down that side, the wall connects to them and turns a loose framework into solid territory, and the ten points you took in the corner do not come close to paying for it.

The other case is when you already have weak stones nearby. The wall will be aimed straight at them, and your opponent gets to attack them while making territory. A quieter reduction from outside is usually better, which is the same judgement call covered in Go strategy for beginners.

The 3-3 point on a 9x9 board

On a 9x9 board the 3-3 points are the corner star points, and playing one is an ordinary opening move rather than an invasion. There is no 4-4 stone to invade underneath, because a 4-4 point on this board is only one line from the centre. Corner play on 9x9 is about taking the corner outright and then fighting, which 9x9 Go strategy goes through properly. If you want to see it happen rather than read about it, play a 9x9 game against the computer and take a 3-3 point with your first move.

For the wider question of which standard corner sequences are worth learning at all, joseki for beginners is the next page to read.

Frequently asked questions

Can you always invade at 3-3?

Under a lone 4-4 stone, yes. The invader can always live in the corner, because a stone on the fourth line cannot cover the 3-3 point underneath it. Once the defender has added a second stone in the corner, usually an enclosure, the 3-3 invasion no longer works and you have to find another way in.

Which way should I block the 3-3 invasion?

Block on the side where your wall will do the most work: the wider side, or the side where you already have stones. Your opponent will take the corner either way, so the whole question is where your compensation ends up pointing. Blocking toward your opponent’s strength is the common mistake.

Is the 3-3 invasion good for the invader?

It is an even exchange rather than a win for either side. The invader gets around ten points of secure corner and a group that will not die. The defender gets a wall with no weaknesses, which is worth a lot if the rest of the board suits it and very little if it does not.

Try it on the board →