Alice Chess
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Alice Chess is a chess variant played using two chess boards rather than one. Its name is a reference to Lewis Carroll's book Alice's Adventures in Wonderland. Like a number of other variants, it was invented by V.R. Parton.
All pieces move in the same way as in standard chess, except that Alice Chess is normally played without the en passant rule (see rules of chess). At the start of the game, the pieces are set up in their normal position on one board ("board A"), with the second board ("board B") empty. Every time a move is played, the piece moved passes "through the looking glass" onto the other board; if the piece started on board A it goes to board B; if it started on board B it ends up on board A. For example, after the opening moves 1.Nf3 e6 (see algebraic notation), the position on the two boards is as follows:
Image:Alice chess example 1.png
Both the white knight and the black pawn have gone from board A to board B. If the game were to continue 2.Ne5 Bc5, the following position would arise:
Image:Alice chess example 2.png
The white knight has moved back onto board A, while Black has moved another piece, the bishop, to board B.
Pieces can only capture other pieces on the board which they begin their move on; so pieces on board A capture other pieces on board A, and pieces on board B capture other pieces on board B. After a capture has been made, however, the capturing piece is moved onto the other board, just as it would be after a non-capturing move. If, therefore, from the above position, White were to play 3.Nxf7, that would result in the following position:
Image:Alice chess example 3.png
The black pawn on f7 has been removed from the game, and the white knight has moved back onto board B.
For a piece to be able to move to a certain square, that square on the destination board must be empty. So in this last position, the black moves 3...Kxf7 and 3...Bxf2 are both illegal. Additionally, moves must be legal under normal chess rules on the board on which they are played; so in the last position, for example, Black cannot play 3...Qd4, because under the rules of chess, it cannot hop over the pawn on d7. However, 3...Bg1 is possible, despite the fact there is a white pawn on f2 on board A: the black bishop has started out on board B, the move Bc5-g1 is legal there, and the square g1 is empty on board A. This is all that matters. The position after this last move is as follows:
Image:Alice chess example 4.png
These rules can result in checkmate positions which are not obviously mate at all. The following position is a simple example:
Image:Alice chess checkmate.png
The king is checked by the rook on b1. Kf1 and Kh1 are not possible as these moves are not legal on the board on which they are played (they do not escape the check); the fact that the king would not be checked on those squares on board B is irrelevant. Kf2, Kg2 and Kh2 are also illegal, as when the king emerges on board B it is checked by the rook on a2. White is therefore checkmated.
A short game of Alice Chess (notated in algebraic notation) might run 1. e4 d5 2. Be2 dxe4 (on the second board) 3. Bb5 mate (the bishop moves back onto the first board). All the pieces which moved to the second board have either moved again and so returned to the first, or have been captured, so the final position in this game is as follows:
Image:Final position of short Alice chess game.png
At first glance it might seem that black can interpose a piece between white's bishop and his king to block the check (playing, for example, Bd7 or Nc6), but any piece so interposed immediately disappears onto the other board. Black can also not play Kd7 to move his king onto the other board, because this move is not legal on the board on which it is played (the king on d7 on the first board is still in check). Therefore, black is checkmated.