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The open three: the threat that wins most games

Most games of Mòpyon are not won by a clever final move. They are won earlier, by a threat the other player did not take seriously. The most important of those threats is the open three.

What is an open three?

An open three is three of your marks in a row with an empty crossing at both ends, and enough room beyond them to grow into five. It looks harmless, because three is still far from five. But if your opponent ignores it, you add a fourth mark and get an open four: four in a row with both ends free. An open four cannot be stopped, because your opponent can only block one end per turn.

An open three. The green dots are the two squares that block it; if O plays elsewhere, X makes an open four.

The open four that follows. O can block one end, X plays the other and wins.

The broken three

A three does not have to be solid. Three marks with one gap inside a group of four crossings, like X X _ X, are just as dangerous when both outer ends are open. Filling the gap gives an open four. Broken threes are harder to see, so experienced players love them. Train your eye to read every line of four or five crossings, not only marks that touch.

A broken three (X X _ X). The best block is usually the gap itself (green); blocking an end (red) leaves a dangerous shape.

A closed three is not urgent

If one end of the three is already blocked by your mark or by the edge of the board, it is a closed three. It can still become a four, but a four with only one open end can be blocked in one move. You do not have to answer a closed three right away, and it is often better to use the tempo to build your own attack.

How to use this in your games

After every move your opponent makes, check the four lines through it: is there now an open three, solid or broken?
If there is, answer it unless you can make a four yourself, which forces your opponent to answer you first.
When you attack, prefer moves that make an open three while also helping a second line. That is how double threats are born (see the guide on forks).

The puzzles on Mòpyon.net are built from real positions where one side has a forced win. Solving a few every day is the fastest way to start seeing threes automatically.