Definitions

In these rules, "adjacent" always means "orthogonally adjacent".

A group is a stone along with all other like-colored stones one could reach from it through a series of steps between adjacent stones of its color. The size of a group is the number of stones in it.

Play

Black plays first, then turns alternate. On your turn, place a stone of your color on an empty square and move every stone adjacent to it directly away from it to the first empty square it encounters. If this causes a stone to reach the edge of the board without encountering an empty square, said stone is transferred to the last empty square in the opposite direction. If there is no such empty square, the stone is returned to its original square. If two stones land on the same square, the one of those two that had reached the edge of the board without encountering an empty square is returned to its original square.

The game ends when the board is full. Then, the players jointly remove pairs of opposite-colored groups of the same size from the board until there are no more such pairs to remove. The player with the biggest group left over wins. If the board is empty, whoever added the last stone to the board loses.

Pie rule

The pie rule is used in order to make the game fair. This means White will have the option, on their first turn only, to swap sides with their opponent instead of making a regular move.

Variants

Squared: A player's score is the sum of the squares of the sizes of their groups. The player with the higher score in the final position wins. Komi is used instead of the pie rule. Komi is the even number of points which is added to White's score at the end of the game as a compensation for playing second. Before the game starts, the first player chooses the value of komi, and then the second player chooses sides. Alternatively, experienced players may agree on a standard value for all games.