The Story
Where it came from, and why it turned out this way.
DesignI-Chen Wu · 2005 · National Chiao Tung University, Taiwan
Designed to answer a known problem
Most games acquire their rule controversies after the fact; Connect6 is the reverse. Once freestyle Gomoku was proved a first-player win in 1993, "can a line-forming game be both simple and fair?" became an explicit open question. In 2005 Professor I-Chen Wu of National Chiao Tung University in Taiwan published his answer as a paper in the ICGA Journal. This is a game that walked out of an academic publication.
Two stones is a calculated number
The heart of Connect6 is not the six — it is the two stones per turn. The balancing logic fits in one sentence: Black places a single stone on the opening turn and both sides place two thereafter, so at any moment the difference in stones played is never more than one. The first-move advantage is compressed to its theoretical minimum, permanently worth half a turn. The same idea generalizes to any k-in-a-row, studied as the Connect(m, n, k, p, q) family.
Why it needs no forbidden moves
Renju weakens Black with prohibitions, and pays for it in complexity: you have to know what an open three is, what counts as a double four, how deep the recursive check goes. Connect6 takes the other road. If the asymmetry is about who places first, compensate with the number of placements rather than with bans. The result is perfectly symmetric rules — double threes, double fours, and overlines are all legal, and both colors have identical rights. Simplicity was itself a design goal.
The rhythm is completely rewritten
Two stones per turn brings more than fairness; it brings a different arithmetic of attack and defense. In Gomoku you ask whether a threat exists. In Connect6 you count how many, because your opponent can seal two of them per turn. Creating two six-making points only forces a standoff; three is a kill. That is exactly why strong Gomoku players often lose their first Connect6 games without understanding what went wrong.
- Connect6 was quickly adopted by the International Computer Games Association as an official Computer Olympiad event, and program-versus-program play was its main early competitive arena.
- I-Chen Wu also works on computer Go; the CGI program developed by his group has placed well in international competition.
- Connect6 has still not been proved a first-player win, a second-player win, or a draw — the state space on a 19-line board dwarfs Gomoku's, which is precisely why the game retains its competitive value.
Complete Rules
Start with what a turn allows and how the game is won.
- Board
- 19 × 19 intersections
- Opening turn
- Black places only 1 stone
- Winning conditionas soon as six or more consecutive stones of one color appear on a horizontal, vertical, or diagonal line, that player wins. Note the "or more" — an overline of seven or eight wins just the same, so there is no Gomoku-style penalty for overshooting.
- How a turn worksBlack moves first and places exactly one stone on the opening turn — this single-stone opening is what balances the first-move advantage. After that, each player places two stones on two distinct empty intersections per turn, and the turn passes only after both are down.
- Victory is checked after each individual placement, not at the end of the turn. If the first stone of a two-stone turn completes six in a row, the game ends immediately and the second stone is never placed.
- There are no forbidden moves whatsoever. Double threes, double fours, and overlines are all legal, and the rules are completely symmetric between Black and White. This is the fundamental difference from Renju, and it means there is no forbidden-shape table to memorize.
- Your two stones may go on any two empty intersections. They need not be adjacent, on the same line, or related in any way. But you may not pass, and you may not end a turn after placing only one stone. Stones are permanent once placed and can never move or be removed.
- If all 361 intersections are filled with no line of six, the game is a draw. A final turn can therefore end after a single placement, when only one empty intersection remained.
- Coordinates follow Go notation: columns A through T skipping I, rows 19 down to 1. Undo rewinds to the start of your previous complete turn, taking both stones back together.
Your First Game
Use this order of thought to get through a complete game quickly.
- Replace your Gomoku instincts entirely. The unit of counting here is not "a threat" but "how many threats can I create this turn versus how many can my opponent block." Since the opponent blocks two points per turn, winning requires creating three or more independent six-making points in a single turn.
- Start each turn by scanning your opponent's shapes and counting every point where one more stone makes six. When there are three or more such points, your two defensive stones are already insufficient — which means you should have acted a turn earlier. Real defense is preventing that shape from growing at all.
- Memorize the classic losing shape: five of your opponent's stones in a row with both ends open. Both six-making points must be blocked, which consumes exactly your two stones for the turn, leaving you with no attacking move at all and permanently on the back foot. Start worrying as soon as your opponent reaches four in a row.
- Keep your own two stones in the same theater, working with your existing stones to build a new threat. Two stones dropped in opposite corners of the board amount to a wasted turn — the power of the double placement comes from coordination, not from the count.
- After placing your first stone, look at the whole board again before choosing the second. The first stone may have changed which second point is best — for example, the block you just made may have opened a new line for you — or it may already have won the game.
- In the opening, Black has just one stone while White gets two on the very first turn, so White actually holds a slight initiative early. Black should stay flexible near the center rather than rushing to expand, while White can use two stones to build a threatening shape immediately.
Practical Strategy
Move from knowing the rules to playing with a plan.
- Initiative comes from the threat count differential: only when the six-making points you create this turn exceed the two your opponent can block do you truly hold the initiative. Two threats merely force an even trade; three is a kill.
- The most common rhythm is one defensive stone plus one attacking stone: block your opponent's most urgent threat with one, and keep growing your own shape with the other. A player who sustains that rhythm gradually steers the position toward a shape the opponent cannot fully answer.
- When defending, look first for the intersection that cuts two of your opponent's lines at once. Solving two threats with one stone frees the other for counterplay, and such double-purpose blocking points are worth exactly as much as attacking moves.
- Because there are no forbidden moves, shapes that grow in several directions at once are extremely strong. Place stones where multiple potential lines cross so that a single stone participates in more threats — and on a board this large, the center holds far more crossing points than the edges.
- When calculating a kill, treat the turn's two stones as one combined move rather than two independent half-moves. Many winning sequences run "the first stone forces a specific reply, and the second seizes the key point that reply vacated" — split them apart and you will never find the line.
- Because winning lines are longer here, thickness is worth more than it is in Gomoku. A mutually supporting cluster can grow in several directions, while an isolated four dies the moment both ends are covered. Prefer to build solidly and slowly.
Winning and Special Rules
Details that are easy to miss but can change the result.
- Overlines win. There is no Exact-Five-style rule where overshooting scores nothing — six, seven, or eight in a row all end the game the same way.
- If the first stone of a turn already makes six, the second stone is never placed and the game ends at once. This matters in calculation: you do not need to guarantee a good square for the second stone in a winning line.
- This page fixes sequential, immediate settlement as an explicit product rule. The original literature defines a turn as a pair of stones but does not prescribe whether the pair is revealed simultaneously or settled in sequence; this page settles each stone immediately, under ruleset ID connect6-standard-v1.
- Master hints label the suggested placements 1 and 2, and the AI searches both stones of a turn as one combined move before playing them in sequence — so the two suggested points are a package, and playing only one of them weakens the idea.
Common Mistakes
The traps new players fall into most often.
- Applying Gomoku habits and answering only the single most obvious threat, then placing the second stone casually — and losing to two coordinated stones on your opponent's next turn.
- Spending both stones on attack while missing a point where your opponent completes six this turn. Missing a defense costs far more here than in Gomoku, because the opponent advances two steps per turn.
- Failing to re-examine the board after the first placement, so the second stone lands on a point that is already irrelevant — a line the first stone already sealed, or a game that is already won.
- Spreading the two stones to opposite ends of the board for "balanced development," so neither develops into a real shape, effectively handing the opponent a full turn of tempo.
- Assuming your four in a row is strong. In Connect6 a four is still two stones short of six, while your opponent needs only one stone to seal an end — an open five is the real winning shape.