8. On Which Side to Block
The Two-Step Hane Tesuji

by Richard Bozulich

With the advent of go-playing AI programs, invading at the 3–3 point early in the game against a 4–4-point stone ('direct 3–3', as it has come to be known), has become quite common nowadays. In the past, such invasions were usually made in the middle game. In either case, the problem arises on which side to block. The usually strategy followed by the defender is to let the invader live in the corner but build a thick wall on the outside.

The invasion presented here is not considered to be an example of a direct-3–3 joseki, as Black has made an enclosure with the marked stones. However, the solution is similiar to one of them.

Black to play
Problem. Black to play
In this four-stone handicap game, White has invaded at 1. On which side should Black block, at 'a' or at 'b'?