Connect Four on 6 high, 7 wide grid, there are 4,531,985,219,092 positions for all game boards populated with 0 to 42 pieces. The game was first solved by James Dow Allen (October 1, 1988), and independently by Victor Allis (October 16, 1988). Allis describes a knowledge-based approach, with nine strategies, as a solution for Connect Four. Allen also describes winning strategies in his analysis of the game. At the time of the initial solutions for Connect Four, brute-force analysis was not deemed feasible given the game's complexity and the computer technology available at the time. Connect Four has since been solved with
Connect Four research, rather than with physical game boards and gameplay by persons. The most commonly used Connect Four board size is 7 columns × 6 rows. Size variations include 5×4, 6×5, 8×7, 9×7, 10×7, 8×8, Infinite Connect-Four, and Cylinder-Infinite Connect-Four. Several versions of Hasbro's Connect Four physical gameboard make it easy to remove game pieces from the bottom one at a time. Along with traditional gameplay, this feature allows for variations of the game. Some earlier game versions also included specially marked discs, and cardboard column extenders, for additional variations to the game. "Pop Out" starts the same as traditional gameplay,