The rules of Reversi

In brief

You place a disc that brackets your opponent's discs, and they flip. A player who cannot move passes. When neither player can move, the player with more discs wins.

Black
White
possible move
disc just placed or square highlighted
disc that was just flipped

The board and the starting position

The board is 8 squares by 8, so 64 squares in all. At the start, 4 discs sit in the center: two white (d4 and e5) on one diagonal, two black (e4 and d5) on the other. Black always moves first.

Start: Black's 4 possible moves are marked with a green dot.

In the app, the "Who starts" setting in the New game window decides your color: "You" (you play Black), "CPU" (the computer does) or "Random".

The legal move

You place a disc of your color on an empty square. The move is legal only if it brackets at least one opposing disc: an unbroken line of opposing discs, horizontal, vertical or diagonal, closed at the other end by one of your own discs. A move that flips nothing is refused.

Flipping discs

All bracketed discs change color, in all 8 directions at once. Only discs bracketed by the placed disc are flipped: a flipped disc does not trigger further flips. In the annotated game, the move g8 flips 12 discs at once.

After g8: the placed disc is outlined in gold, and the flipped discs carry a dot.

Passing

A player with no legal move passes their turn, and the opponent plays again. You cannot pass by choice. In the app, the turn passes automatically. In the annotated game, Black passes after move 56.

The end and the count

The game ends when the board is full, or earlier when neither player can move. The discs of each color are counted; the larger number wins, and a tie is possible. The app does not award the empty squares to the winner: the shortest game ends 13 to 0 with 51 empty squares.