Definitions

A group is a stone along with all stones one can reach from it through a series of steps onto orthogonally adjacent stones of its color. A group is dead if there is no free path along the lines of the board between that group and any other group of its color. The path does not need to be a straight line.

A territory is an empty point along with all points one can reach from it through a series of steps onto orthogonally adjacent empty points. You own a territory if all stones orthogonally adjacent to it are of your color.

An incursion is a placement by one player in a territory owned by the other player. An incursion is minimal if it causes exactly one enemy group to be moved to the prison.

Play

Black plays first, then turns alternate. On your turn, perform exactly one of these actions:

• Remove an enemy stone from the prison.
• Place a stone of your color on an empty point, then move all dead enemy groups to the prison. If it is your first turn of the game, perform this action twice.

At the end of your turn, there must be no dead friendly groups on the board. You cannot make a minimal incursion if your opponent just made a minimal incursion.

The last player to perform an action wins. If a stone placement recreates any previous board position with the same player to move, the game ends in a draw. This is possible in theory, but probably requires colluding players.

To make the game fair, before the game starts, the first player places a number of black stones in the prison, and then the second player chooses sides. This balancing method is called komi pie.

—description from the designer