Reversi is a strategy board game for two players, played on an 8x8 board.
Reversi starts with an empty board, and the first two moves by each player are in the four central squares of the board. The players place their disks alternately with their colour facing up and no captures are made.
Reversi game. Historical version.
Dark must place a piece with the dark side up on the board, in such a position that there exists at least one straight (horizontal, vertical, or diagonal) occupied line between the new piece and another dark piece, with one or more contiguous light pieces between them. In the below situation, dark has the following options indicated by grey coloured pieces.
After placing the piece, dark turns over (flips, captures) all light pieces lying on a straight line between the new piece and any anchoring dark pieces. All reversed pieces now show the dark side, and dark can use them in later moves unless light has reversed them back in the meantime. In other words, a valid move is one where at least one piece is reversed.
Now light plays. This player operates under the same rules, with the roles reversed: light lays down a light piece, causing a dark piece to flip.
Light takes the bottom left option and reverses one piece:
Players take alternate turns. If either player cannot make a move, the game ends. The game ends also happens when the grid has filled up. This means the game may end before the grid is completely filled. This possibility may occur because one player has no pieces remaining on the board in that player's colour.