Gomoku Open Four: The Winning Threat That Can't Be Stopped

What Is an Open Four in Gomoku?

An Open Four is the most decisive winning pattern in Gomoku. It describes a straight line of exactly four consecutive stones — all the same color — where both ends of the line are completely empty.

Here's why it's a guaranteed win: your opponent can only place one stone per turn. That means they can block only one end of your Open Four. On your very next turn, you place your fifth stone at the unblocked end and claim five-in-a-row. Game over.

In competitive Gomoku, experienced players don't wait for their opponent to miss a five-in-a-row. They build Open Fours on purpose — creating unavoidable checkmates far in advance.

The key difference between beginner and intermediate Gomoku is understanding the Open Four. Once you grasp it, you stop reacting and start building.

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Open Four vs. Closed Four: Know the Difference

Not all "four in a row" patterns are equal. Here's the distinction that separates winning threats from dead ends:

Open Four — Four stones in a row. Both ends are empty. This is an instant win: the opponent cannot stop you.

Closed Four (Half-Open Four) — Four stones in a row. One end is blocked (by an opponent stone or the board edge). This is still dangerous — your opponent must respond — but it's not an automatic win because they only need to block the single open end.

Dead Four — Four stones in a row. Both ends are blocked. This pattern is completely useless and scores no threat value whatsoever.

When you're scanning the board, learn to evaluate every four-stone line for its open-end count. Only the fully open version wins the game for you outright.

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Why You Can't Build an Open Four Directly

Here's the uncomfortable truth for beginners: you almost never build an Open Four in a straight line from scratch.

The moment you place your third stone and have an "Open Three" (three in a row, both ends open), a competent opponent will immediately block one end. Your Open Three becomes a Closed Four — blocked on one side, and no longer a guaranteed win. You've wasted moves.

This is why Gomoku mastery isn't about building long lines. It's about forcing your opponent into impossible choices.

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The Fork: How to Force an Open Four

A Fork is the fundamental weapon of Gomoku strategy. A fork is any single move that simultaneously creates two threats at once. Because your opponent only has one turn to respond, they can only block one threat — leaving the other wide open.

The Double Three Fork

The most beginner-accessible fork is the Double Three. You position your stones in a cross or "V" shape so that placing one stone at the intersection instantly creates two separate Open Threes at the same time.

Your opponent now faces an impossible situation. They block one of your Open Threes. You develop the surviving Open Three into an Open Four on your very next move. They can't stop both.

This sequence — Open Three → Fork → Open Four → Five-in-a-Row — is the foundation of virtually every high-level Gomoku win.

The Double Four Fork

A more advanced fork creates two separate Fours simultaneously. This is an immediate, no-delay checkmate: your opponent must block a Four right now or lose. But two Fours can't both be blocked. In standard (non-Renju) Gomoku, this is a legal winning pattern for both players.

Note: In Renju rules (official tournament Gomoku), the Black player is actually forbidden from creating Double Threes and Double Fours. These rules exist specifically because the fork attacks are so powerful they would make Black nearly unbeatable as the first mover. If you're playing casual or free-play Gomoku, these restrictions don't apply.

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The Broken Three: Your Hidden Weapon

Advanced players rarely build clean, unbroken lines of three — that's too obvious. Instead, they build what's called a Broken Three (also called a "skip three" or "gap three").

A Broken Three looks like: Stone — Stone — Empty — Stone

To the untrained eye, this looks like a scattered, harmless group. But placing a single stone in that gap instantly converts it into a solid, threatening block of four — often an Open Four.

The strategic power of the Broken Three is deception. Because the pattern doesn't look like an immediate threat, opponents may prioritize blocking elsewhere. By the time they recognize the danger, it's too late to stop both ends.

How to use Broken Threes in practice:

• Build multiple Broken Three patterns crisscrossing the center of the 15×15 board.
• Each one can be activated with a single fill move.
• When you activate one, it simultaneously threatens a second Broken Three elsewhere — creating a fork.

This is how high-level Gomoku games create what looks like a sudden explosion of threats: the "minefield" was laid quietly, five or six moves earlier.

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The Path to an Open Four: Step-by-Step

Here is the exact sequence that produces a guaranteed Open Four win:

Step 1 — Establish the center. Place your first stones near the center of the 15×15 board. Center positions have the most directions available (horizontal, vertical, and both diagonals) giving you maximum flexibility.

Step 2 — Build overlapping Open Threes. Don't build one long line. Build two or three Open Three patterns that share stones or intersect, creating a network of threats.

Step 3 — Lay Broken Threes as decoys. Intersperse Broken Three patterns between your solid threes. This forces your opponent to track multiple half-hidden threats simultaneously.

Step 4 — Find the fork move. Look for the single move that activates two threats at once. This is your "aha" move — the moment your opponent's position collapses.

Step 5 — Convert to Open Four. Once your opponent is forced to block one of your threats, develop the surviving Open Three into an Open Four. You win on the next move.

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Defending Against an Open Four

Recognizing the Open Four on your opponent's side of the board is just as critical as building your own.

The basic rule: Never let your opponent have an Open Three that you can't answer. An unchallenged Open Three becomes an Open Four on the next turn, and an Open Four is a game-ending loss for you.

Key defensive habits:

• Check every move: Before you play, scan the board. Does your opponent have any Open Threes? Do they have a move that would create two threats simultaneously?
• Block Open Threes early — at one end. You only need to close one side of an Open Three to downgrade it to a Closed Three, which is far less dangerous.
• Watch for Broken Threes. These are easy to miss. Look for any three stones of the same color in a line, even with a gap.
• Don't let the game become reactive. If you're only blocking, you're losing. Try to find moves that defend and simultaneously create your own threat.

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VCF and VCT: Advanced Open Four Sequences

Once you've mastered the basics, the next step is understanding forced win sequences.

VCF (Victory by Continuous Fours) — A sequence where you make one Four after another, each forcing your opponent to respond in exactly one way. VCF chains can be computed in advance: if your opponent must block every move you make, and each block leads to another forced Four, you will eventually create an Open Four that can't be stopped.

VCT (Victory by Continuous Threats) — Broader than VCF. You alternate between Open Threes and Fours, keeping your opponent perpetually on defense. VCT sequences are harder to spot but appear constantly in high-level play. Any move that forces a specific reply counts as part of a VCT.

If you have a VCF or VCT sequence available on the board, you have a won game — it's just a matter of executing it.

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Practice: Build Open Four Setups Against Our AI

Understanding the Open Four in theory is step one. Applying it under pressure against an opponent who actively tries to stop you — that's where real improvement happens.

Start by trying to build a Double Three fork in the first 10 moves. Notice how the AI responds. As you get comfortable, try laying Broken Threes before activating your fork. Work up to the harder AI difficulty settings once you can consistently create Open Four threats.

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Frequently Asked Questions

What is an Open Four in Gomoku?

An Open Four is four consecutive stones of the same color in a straight line, with both ends unblocked. It guarantees a win because the opponent can only block one end per turn.

How do you get an Open Four in Gomoku?

You rarely build an Open Four directly. Instead, you use a Fork — a move that creates two Open Threes simultaneously. Your opponent blocks one; you convert the other into an Open Four.

Is an Open Four always a winning move?

In standard Gomoku, yes — an Open Four is an automatic win. In Renju (official tournament rules), the same principle applies, but Black is forbidden from creating certain fork patterns that lead to it.

What's the difference between an Open Three and an Open Four?

An Open Three (three in a row, both ends empty) is a potential threat — your opponent should block it, but it doesn't win immediately. An Open Four (four in a row, both ends empty) is an immediate, unstoppable win.

What is a Broken Three in Gomoku?

A Broken Three is a pattern of three same-color stones in a line with a gap (e.g., Stone–Stone–Empty–Stone). It's a disguised threat: filling the gap creates an instant four-in-a-row.

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Gomoku is an architectural game. The five-in-a-row at the end is almost a formality — the real game is won in the setup, move by move, ten turns before the final stone is placed.