Connect6 game, introduced by Professor I-Chen Wu at Department of Computer Science and Information Engineering, National Chiao Tung University, is a fair and highly complex game.

Two players, Black and White, alternately place two stones, black and white respectively, on empty intersections of a Go-like board, except for that Black (the first player) places one stone only for the first move. The one who gets 6 consecutive stones first (horizontally, vertically or diagonally) of her own wins.


Game Rules
The rules of Connect6 are very simple and similar to the traditional Go-Moku game, as listed as follows.

Players and stones: There are two players. The first player, called Black here, holds a set of black stones, like Go or Go-Moku games. The second player, called White here, holds a set of white stones. 
Game boards: In theory, the size of game board can be infinite. However, practically, it is hard to support such a game board. Therefore, our proposal is as follows. For casual players, simply use 19x19 Go boards. For professional players, use 59x59 boards. The reason we suggest this size for professional players in the following two senses. Players can tile up 9 Go boards to play, if games are played without the help of computers. In addition, judging from some current games, such as Go and Chess, we roughly estimate at most hundreds of moves per game in the extreme cases. Hundreds of moves inside a 59x59 board for Connect6 seem reasonable. 
Game moves: Black plays first and puts only one black stone on one unoccupied intersection (or called grid). Subsequently, Black and White alternately put two of their own stones on two unoccupied grids. 
Game winning: The one who first gets six consecutive stones (horizontally, vertically or diagonally) of her/his own wins. 
Unlike Renju, we do not need to impose any extra rules on Connect6, since the rules of Connect6 appear to be fair enough as shown in the section, fairness.


Fairness
In principle, even some complex games, such as Go, Chinese Chess, Chess, are not claimed to be absolutely fair. Herik, Uiterwijk, and Rijswijck gave a definition of fairness (Herik, Uiterwijk, and Rijswijck, 2002) as follows: A game is considered a fair game if it is a draw and both players have a roughly equal probability on making a mistake. From this, it is argued that Connect6 is fair in the following senses:

One player always has one more stone than the other after making each move. 
For about one thousand opening templates, Professor Wu let the AI program written by the team led by Professor Wu play with itself, and the result showed that the game does not favor either one for these templates. Note that the AI program can beat most of casual players. 
The initial breakaway (means to play far away from the initial Black stone) does not apparently favor White. It was proved by Professor Wu. If the initial breakaway did not get penalty, the game would favor White for the following reason: Black must go back to defend the two white stones and then the situation is good for White since the game becomes to let White place two stones initially. 
But, surely, the fairness of Connect6 needs more time to be verified. It is wanted to have more evidences, either fair or unfair.


Complexity
If Connect6 uses an infinite board, both state-space and game-tree complexities are infinite too. So, we assume to use a Go board, instead. Both state-space and game-tree complexities for it are still much higher than those in Go-Moku and Renju, in the sense that each move places two stones that make the branch factor increase by a factor of the board size. Based on the standard in (Herik, Huntjens, and Rijswijck, 2002), the State space complexity of Connect(19,19,6,2,1) is 10172, almost the same as that in Go. If a larger board is used, the complexity is much higher.

Now, let us investigate the Game tree complexity. Assume that the averaged game length is still 30, the same as the estimation for Go-Moku (Allis 1994). Then, the number of grids chosen to put one stone is about 300, and the number of choices of one move is about . Thus, the game-tree complexity is about ~ 10140, much higher than that for Go-Moku. Also, if a larger board, like 59x59, is used, this complexity becomes much higher.


History
One day in Summer 2003, Professor I-Chen Wu came up with this game when playing with his daughter. After this, he started thinking the potential of the game, Connect6, to be a popular game, and researching on it. However, as a popular game, this game must be fair enough and highly complicated. His first plan is to have an AI program to play the game to see how fair and complex the game is. 
In Spring 2004, one of Master students, Dei-Yen Huang, of Professor Wu joined this project as his Master Thesis. In Q1 of 2005, they completed the first Connect6 AI program, which already can beat most of us. Then, we let our AI program fight with our AI program. And, so far, they still cannot determine who (Black or White) has the advantage. 
In 2005, they wrote a paper, presented in the 11th Advances in Computer Games Conference (ACG11), held in Taipei, Taiwan, 2005. 
In September 2005, ThinkNewIdea Limited built the first Connnect6 game server at http://www.cycgame.com.tw/connect6/. 
In September 20-21, 2005, the game was covered by many news media in Taiwan. 
In 2005, interested persons started discussing to organize a Connect6 association at http://www.connect6.org, and supporting a forum for discussion.