The pie rule, sometimes referred to as the swap rule, is a rule used to balance abstract strategy games where a first-move advantage has been demonstrated. After the first move is made in a game that uses the pie rule, the second player must select one of two options:
- Letting the move stand. The second player remains the second player and moves immediately.
- Switching places. The second player becomes the first-moving player, and the “new” second player then makes their “first” move. (I.e., the game proceeds from the opening move already made, with roles reversed.)
This rule acts as a normalization factor in games where there may be a first-move advantage.
In Tic-tac-toe Collection, the pie implemented by allowing the second player to replace the first player's move (this is equivalent to swapping pieces, but allows players to keep using their own icons).
In games with more than two players, in the first round of moves, any player may replace any other player's first move.