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:

  1. Letting the move stand. The second player remains the second player and moves immediately.
  2. Switching places. The second player becomes the first-moving player with the move already done by the opponent, and the opponent plays the first move of their new color.
A chess game using the pie rule, player 2's options in bold
Player 1 plays first move as White

Player 1

a8 white rook
b8 white knight
c8 white bishop
d8 white king
e8 white queen
f8 white bishop
h8 white rook
a7 white pawn
b7 white pawn
c7 white pawn
d7 white pawn
e7 white pawn
f7 white pawn
g7 white pawn
h7 white pawn
f6 white knight
a2 black pawn
b2 black pawn
c2 black pawn
d2 black pawn
e2 black pawn
f2 black pawn
g2 black pawn
h2 black pawn
a1 black rook
b1 black knight
c1 black bishop
d1 black king
e1 black queen
f1 black bishop
g1 black knight
h1 black rook

Player 2

 ↙ ↘ 
Player 2
lets move stand
Player 2
switches places

Player 1

a8 white rook
b8 white knight
c8 white bishop
d8 white king
e8 white queen
f8 white bishop
h8 white rook
a7 white pawn
b7 white pawn
c7 white pawn
d7 white pawn
e7 white pawn
f7 white pawn
g7 white pawn
h7 white pawn
f6 white knight
a2 black pawn
b2 black pawn
c2 black pawn
d2 black pawn
e2 black pawn
f2 black pawn
g2 black pawn
h2 black pawn
a1 black rook
b1 black knight
c1 black bishop
d1 black king
e1 black queen
f1 black bishop
g1 black knight
h1 black rook

Player 2

Player 1

a8 black rook
b8 black knight
c8 black bishop
d8 black queen
e8 black king
f8 black bishop
g8 black knight
h8 black rook
a7 black pawn
b7 black pawn
c7 black pawn
d7 black pawn
e7 black pawn
f7 black pawn
g7 black pawn
h7 black pawn
c3 white knight
a2 white pawn
b2 white pawn
c2 white pawn
d2 white pawn
e2 white pawn
f2 white pawn
g2 white pawn
h2 white pawn
a1 white rook
c1 white bishop
d1 white queen
e1 white king
f1 white bishop
g1 white knight
h1 white rook

Player 2

Player 2 to play
as Black, as before
Player 1 to play again,
now as Black

Depending on the game, there may be two ways to implement switching places.

  1. Switching colors means that the players exchange pieces. The player who made the first move becomes the second player and makes the second move on the board. This is demonstrated in the chess diagrams shown here.
  2. Switching the first piece can occur in games where the board starts empty and the first move consists of placing one piece. Suppose the colors are white versus black, and black places the first piece. This piece is replaced by a white piece in the corresponding location for white, and the black piece is returned to black's supply. In a game such as Hex or TwixT, the corresponding location is at a cell "reflected" across the nearest (or either) diagonal. In games such as Y, where the board is not directional, the white stone replaces the black stone in the same cell. Players keep their respective color pieces, and play continues with black making the next move. This is effectively the same as switching colors.

The use of pie rule was first reported in 1909 for a game in the Mancala family.[1] Among modern games, Hex uses this rule.[2] TwixT in tournament play uses a swap rule.[3] In Meridians, the first player places 2 stones on the board before the second player chooses the color. The rule can be applied to other games which are otherwise solved for one player, such as Gomoku or Tablut.[4]

The rule gets its name from the divide and choose method of ensuring fairness in when dividing a pie between two people: one person cuts the pie in half, then the other person chooses which half to eat. The person cutting the pie, knowing that the other person will choose the larger piece, will make as equal a division as possible.

This rule acts as a normalization factor in games where there may be a first-move advantage. In games that cannot end in a draw, such as Hex, the pie rule theoretically gives the second player a win (since one of the players must have a winning strategy after the first move, and the second player can choose to be this player), but the practical result is that the first player will choose a move neither too strong nor too weak, and the second player will have to decide whether switching places is worth the first-move advantage.

Use for determining komi in Go

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In Go, one player can choose the amount of komi. (These are the points given to the second player as compensation for not going first.) The other player then decides whether to accept that or switch colors with the other player. This leads players to choose fair komi amounts because if they choose a komi that is too advantageous, the other player can just choose to play White and take advantage of that high komi.[5]

References

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  1. ^ Parker, Henry (1909). Ancient Ceylon: An Account of the Aborigines and of Part of the Early Civilisation. London: Luzac & Co. pp. 601–602. LCCN 81-909073.
  2. ^ Browne, Cameron. Hex Strategy: Making the Right Connections. ISBN 1-56881-117-9.
  3. ^ Mind Sports Olympiad Twixt page. Archived 2018-07-14 at the Wayback Machine.
  4. ^ Schmittberger, R. Wayne (1992). New Rules for Classic Games. John Wiley & Sons Inc. pp. 25–27. ISBN 978-0471536215.
  5. ^ "Pie Rule at Sensei's Library". Impressum bzw. November 18, 2009. Retrieved June 10, 2022.