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Connect 4 is played on an empty 6x7 square board.
- DROP - Each player chooses a column, and then drops a stone on the lowest possible empty cell (the lowest row is the bottom row).
- GOAL - Wins the player that achieves a 4 in-a-row (orthogonal or diagonal)
Black wins in three moves, by playing at cell . White must defend at b4, then Black moves c4, forcing White into e7, and then c5 and wins by creating a double attack.
There are several websites that play Connect 4 online. There is a perfect player (first player wins) called Velena by Giuliano Bertoletti (also with source code). Also check the page of Victor Allis about Connect 4 and Gomoku.
Connectris is a variant where when the bottom row is filled, it disappears and all the other stones fall down. Two other variants are AntiStack (the first to make 4 in-a-row loses) and Line4Plus (upon each checker drop, all adjacent pieces are flipped. Red -> WhiteBlk -> Black -> WhiteRed -> Red. WhiteBlk and WhiteRed checkers are neutral).
In Beppi Menozzi's Centrifugal Connect4 (invented in 1994), played in a 8x8 board, the stones are dropped in an empty cell and then they fall down towards the closest edge, while stones put on the diagonal lines fall towards the closest corner.
In Forze 4x4, played on a 8x8 board (initially empty), each player drops a friendly soldier on an empty cell at any edge of the board, and the stone slides to the opposite side until it finds the edge of the board or an occupied cell. E.g., if White drops a stone at  it will slide to the right until it stops at . If White drops at  the stone will slide down to :
In Caramba each player has 16 stones plus 5 wild stones (that can be used by both players to make winning rows).
Quintego is a Connect-4 variant (with 5 columns): when a vertical column has six stones, the bottom stone is removed and returned to the player's reserve (and all the others on that column move down).