GOMOKU Three shapes decide most beginner games. An open three is a warning. An open four is usually too late to stop with one stone. A broken four is an immediate win or an immediate block, because the empty gap completes exactly five.
Practice this opens the position on the play page with you as Black against the computer. The computer answers after your move. Diagrams are drawn for this page; row numbers and column letters match the 15 by 15 board. If you still have an unfinished online match in this browser, the site keeps that match instead of loading the lesson.
Open three
An open three is three stones in a row with empty intersections at both ends. Here White has G8, H8, and I8. F8 and J8 are empty. It is not a win on this turn. If White is allowed to play F8 or J8 next, that three becomes four with the other end still empty: an open four.
Threat: the next white stone on an end creates two places to win. Defense: play on one end now, before that happens. A stone in the middle of nowhere does not close an end.
Practice thisOpen four
An open four is four stones with both ends empty. White has G8, H8, I8, and J8. F8 and K8 are both winning squares. Black can play only one stone this turn.
Threat: White wins on the following turn no matter which end you take. Defense with a single stone does not exist in this position, unless it is your own winning move somewhere else. The real defense was to answer the open three.
Try it. Block one end. The computer plays the other end and wins.
Practice the open fourBroken four
A broken four, sometimes called a gap four, is four stones with one empty intersection inside the line. White has F8, G8, I8, and J8. H8 is the gap. Filling H8 makes F8 through J8, which is exactly five, not six.
Threat: if it is White’s turn, H8 wins immediately. Defense: if it is Black’s turn, H8 is the only block. Playing next to the shape, such as F7, leaves the gap empty.
What happens next: H8 stops the line. Any other move lets White play H8 and win.
Practice thisThe same three shapes work on columns and diagonals. A line of six is not a stronger version of these threats on this site. Only exactly five wins. That difference is covered on the exactly-five page.