Renju Forbidden Moves Explained: Double Three, Double Four & Overline

1. Why Forbidden Moves Exist

Renju's forbidden move rules were not invented arbitrarily — they exist to solve a fundamental fairness problem in five-in-a-row games. In standard Gomoku (freestyle rules), the player who goes first — Black — holds a mathematically proven forced win on a 15×15 board. With perfect play, Black can always force a victory regardless of White's responses. This makes competitive Gomoku at the highest level effectively a lottery: whoever wins the coin toss wins the game.

Renju's solution was elegant. Rather than changing who goes first or how the board is structured, it restricts what Black is allowed to do. By forbidding Black's most powerful double-threat patterns, the game becomes genuinely balanced. White, as compensation for going second, plays with complete freedom.

The three forbidden move types for Black are: double three (三三), double four (四四), and overline (長連). If Black places a stone that creates any of these patterns, the move is illegal — and in formal play, Black loses the game immediately.

2. The Three Types at a Glance

• Double Three (三三禁手): Black may not place a stone that simultaneously creates two or more open threes in different directions.
• Double Four (四四禁手): Black may not place a stone that simultaneously creates two or more fours (open or closed) in different directions.
• Overline (長連禁手): Black may not create a line of six or more Black stones in any direction.

All three rules are checked the instant Black places a stone. If the resulting position contains any of these patterns, the move is forbidden.

3. Forbidden Move 1: Double Three (三三)

The double three is the most frequently encountered forbidden move in casual and intermediate Renju play — and the most commonly misunderstood. Getting this rule right requires a precise understanding of what counts as an "open three."

3a. What Is an Open Three?

An open three (also called a live three or straight three) is a formation of three Black stones in a line that can be extended in one move to form an open four — four consecutive Black stones with both ends free. The key requirements are:

1. The formation contains exactly three Black stones (consecutive or with one gap).
2. Both ends of the resulting four-stone line are unblocked by a White stone or the board edge.
3. The extension to a four does not itself create a forbidden move for Black (advanced edge case).

The two most common open three shapes are the straight three (· ● ● ● ·) and the broken three (· ● · ● ● ·). Both count as open threes because each can be extended to an open four in one move.

3b. Double Three Examples

A double three occurs when placing a single Black stone simultaneously creates two or more open threes in different directions. Here is a clear example:

Example — Horizontal + Vertical Double Three:

Example — Diagonal + Horizontal Double Three:

3c. Common Misconceptions

"Any two threes at once is forbidden." — Almost right, but only open threes count. If one of the created threes is blocked on one end by a White stone or the board edge, it is a closed three and does not contribute to a double three violation.

"Broken threes always count." — Broken threes (with a gap, like ● · ● ●) count as open threes if they can be extended to an open four. In most rule sets, they do. When in doubt during casual play, use the straight three interpretation.

"Double threes are forbidden everywhere on the board." — There is an advanced edge case: if one of the potential threes, when extended to a four, would create another forbidden move for Black, then that three does not count. This nuance rarely appears in casual play but matters in competitive settings.

4. Forbidden Move 2: Double Four (四四)

The double four rule forbids Black from placing a stone that simultaneously creates two separate four-threats — each requiring one more stone to complete five in a row. While double threes involve the three-stone level, double fours are even more direct winning threats and cannot be permitted for Black.

4a. What Is a Four?

A four in Renju is any arrangement of four Black stones in a line where one more Black stone would complete five-in-a-row. Unlike the open three rule, a four does not need to be open on both sides — both open fours and closed fours count toward the double four rule.

• Open four (straight four): · ● ● ● ● · — four consecutive stones, both ends free. Opponent must block one end immediately or lose.
• Closed four: ○ ● ● ● ● · — one end blocked by White or board edge, one end open.
• Jump four: ● ● ● · ● — four stones with a single internal gap. Filling the gap completes five.

4b. Double Four Examples

In this position, Black has three stones going down column d and three going left in row 4. Playing at d4 creates a four in both directions at once — a forbidden double four.

The double four rule applies regardless of whether the fours are open or closed. Both counts. Here is an example involving a jump four:

The key insight: for a double four, the single placed stone must become the fourth Black stone in two separate four-sequences simultaneously. If it becomes the fifth stone and wins, it is a win (unless another forbidden pattern is also created).

5. Forbidden Move 3: Overline (長連)

The overline rule is the simplest of the three forbidden moves: if Black places a stone that creates a line of six or more consecutive Black stones in any direction, the move is forbidden and Black loses immediately.

In standard Gomoku, six or more in a row is still counted as a win. In Renju, for Black, only exactly five wins. Six or more is a loss.

Overlines are relatively rare in experienced Renju games because strong players plan their line lengths carefully. However, they occur in two situations:

1. Accidental overline: A player builds a long formation without noticing the sixth stone is being placed. Beginners are most at risk here.
2. Forced overline: A skilled White player deliberately steers Black into positions where every natural attacking move creates a six-stone overline. This is a legitimate and effective White strategy.

6. White Has No Forbidden Moves

Worth emphasizing clearly: White is completely unrestricted. White can create double threes, double fours, overlines, or any other pattern at any time without penalty. If White creates six in a row, that is a win for White. If White creates a double three, play continues normally.

This asymmetry is intentional and central to Renju's design. White goes second, which is a disadvantage, and complete freedom of movement is White's compensation. Many of White's most powerful strategies revolve around this: creating the very patterns that Black cannot create, and deliberately maneuvering Black into positions where every strong attacking move is forbidden.

For how this asymmetry shapes strategy for both players, see our Renju Strategy Guide.

7. When a Forbidden Move Overrides a Win

One of the most surprising aspects of Renju's rules: if Black places a stone that simultaneously creates five-in-a-row AND a forbidden pattern, Black does not win — Black loses. The forbidden move always takes priority over the win.

This rule has significant strategic implications. White can maneuver Black into positions where Black's only winning moves are also forbidden — a type of zugzwang where every strong move is self-defeating. Black must constantly verify that any winning move does not also trigger a forbidden pattern.

8. How to Spot Forbidden Moves in a Real Game

Learning to quickly identify forbidden move violations — for your own moves and your opponent's — is a core Renju skill. Here are practical techniques:

Scan All Eight Directions Before Every Black Move

Before placing any Black stone, scan in all four axes (horizontal, vertical, diagonal ↗, diagonal ↘) from the target point. For each direction, count how many consecutive Black stones the placement would create — including the new stone. Ask:

• Does any direction reach three stones with both ends open? If two directions do → double three.
• Does any direction reach four stones (with at least one end free)? If two directions do → double four.
• Does any direction reach six or more stones? → Overline.

Watch for White's Forbidden-Move Traps

Experienced White players deliberately create board positions where Black's natural attacking moves are forbidden. Watch for formations where White has placed stones that would convert Black's most tempting responses into double threes or double fours. Recognizing these traps — and avoiding or defusing them — is an advanced skill.

Practice on the Computer

The fastest way to internalize forbidden move detection is repetition. Playing on our Renju vs Computer page lets you experiment with positions that approach forbidden territory and immediately see the consequences, without the pressure of a live opponent.

9. Quick Reference Table

Rule	Trigger Condition	Applies To	Result
Double Three (三三)	One Black move creates two or more open threes simultaneously	Black only	Black loses immediately
Double Four (四四)	One Black move creates two or more fours (open or closed) simultaneously	Black only	Black loses immediately
Overline (長連)	Black creates a line of 6+ stones in any direction	Black only	Black loses immediately
Five in a Row	Either player creates exactly 5 consecutive stones	Both players	That player wins
Forbidden + Win overlap	Black's move creates both a forbidden pattern and five-in-a-row	Black only	Forbidden move applies → Black loses
White overline (6+)	White creates 6 or more in a row	White only	White wins (no restriction for White)

Recommended Next Reading

• Complete Renju Rules Guide — Everything from setup to win conditions, including the swap2 opening protocol.
• Renju Strategy Guide — How to play Black and White effectively within the forbidden move constraints.
• Gomoku vs Renju: Key Differences — A side-by-side comparison of the two most popular five-in-a-row variants.
• Renju Hub — All Renju resources, play modes, and guides in one place.