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Gomoku lessons · Basics · Step 3
Blocking an Open Three
A three with both ends empty is called an "open three." Leave it alone and it becomes an open four next move — and an open four can't be stopped. So you need to block an open three early. A "closed three," blocked on one side, isn't urgent.
Puzzle 1
White has an open three. Where should you block? (Black to move)
Show answer and explanation
The green circle marks the answer. Blocking the end right next to it leaves White with at most a closed four. Blocking one space further away can still let White make an open four, which is dangerous.
Puzzle 2
A broken three (○_○○) is also an open three. Where do you block? (Black to move)
Show answer and explanation
The green circle marks the answer. For a broken three, the gap in the middle is the best vital point. Blocking an end also counts as correct, but the middle is more reliable.
Puzzle 3
White has two threes. Which one is urgent? (Black to move)
Show answer and explanation
The green circle marks the answer. The bottom three is closed — Black has already blocked one side — so even if it becomes a four, you can still block it. The vertical three on the right is open, so you must block it now.