What is the 3-3 invasion in Go?

It's a move onto the 3-3 point beneath an opponent's 4-4 star-point stone, aiming to live in the corner. The invader gets corner territory; the defender gets an outward-facing wall of influence.

Is the 3-3 invasion good or bad for me?

It's a fair trade whose value depends on the whole board. Invade when you need the sure corner points and can use — or neutralize — the wall your opponent gets in return.

Why did AlphaGo change the 3-3 invasion?

AlphaGo demonstrated that invading the 3-3 point very early in the game is often stronger than waiting, which shifted professional opening theory toward much earlier 3-3 invasions.

The 3-3 Invasion (San-San)

The 3-3 invasion — san-san in Japanese — is a move that dives beneath a stone on the 4-4 star point to grab the corner underneath. It trades a small, near-certain corner territory for the invader in exchange for a strong outside wall for the defender, and it's one of the most common exchanges in modern Go.

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How the 3-3 invasion works

When your opponent has a single stone on the 4-4 point, the 3-3 point below it is still open underneath. Playing there lets you burrow into the corner. The defender blocks on one side, you extend, they block again, and you settle into a small living group in the corner — while they build a wall facing the outside.

What each side gets

The invader: a small but nearly certain corner territory and a living group.
The defender: a solid outside wall — thickness and influence facing the rest of the board.
The trade is considered fair; which side benefits more depends on the whole-board position.

Why AlphaGo changed how it's used

The 3-3 invasion is old, but for years pros usually saved it for later in the game. AlphaGo showed that invading the 3-3 very early is often excellent, and professional openings shifted almost overnight to match. It's a striking example of AI reshaping human theory — and a reminder that a wall is only as valuable as the board around it.

Frequently asked questions

What is the 3-3 invasion in Go?: It's a move onto the 3-3 point beneath an opponent's 4-4 star-point stone, aiming to live in the corner. The invader gets corner territory; the defender gets an outward-facing wall of influence.
Is the 3-3 invasion good or bad for me?: It's a fair trade whose value depends on the whole board. Invade when you need the sure corner points and can use — or neutralize — the wall your opponent gets in return.
Why did AlphaGo change the 3-3 invasion?: AlphaGo demonstrated that invading the 3-3 point very early in the game is often stronger than waiting, which shifted professional opening theory toward much earlier 3-3 invasions.

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