Also known as the swap rule, the pie rule works to take the first player advantage away and equalize fairness for both players. Using the pie rule, the second player has the option of swapping colors after the first player makes the first move. The second player would then become the first player, as if that move was theirs and the game proceeds from that opening position with the roles now reversed. Alternatively, the second player may allow the opening move to stand, in which case they are retained as the second player. This encourages the first player to play a mediocre position of power for their first move.
This rule is particularly useful for the game of Hex and games related to or derived from it, including Y, Poly-Y, Star, Onyx, and Havannah. These games have been mathematically demonstrated to give strong favor for winning to certain opening positions, such as placement in the middle of the board in Hex. The name refers to the idea of one person cutting a pie in half, and the other person choosing which half to have.
Gomoku, a Five-in-a-Row game where players alternate placing stones, utilizes a slightly more elaborate varitaion of the pie rule called the Swap2 Rule. With the Swap2 rule, the first player places three stones (2 black and 1 white) on the board and then the second player has the option to play as Black or White or to then place 2 more stones (1 black and 1 white) and let the first player chose color.