Tandem-84

Tandem-84 was inspired by Alice Chess. Normal FIDE rules apply except where noted.

Setup

Figure 1. The opening layout in Tandem-84. In addition to 12 pawns and the familiar pieces, each player has a chancellor (C), a marshall (M), and two ghosts (G).

_ _ _ _ _ _    _ _ _ _ _ _
|g|b|m|c|n|r|  |r|n|k|q|b|g| 7
|p|p|p|p|p|p|  |p|p|p|p|p|p| 6
|_|_|_|_|_|_|  |_|_|_|_|_|_| 5
|_|_|_|_|_|_|  |_|_|_|_|_|_| 4
|_|_|_|_|_|_|  |_|_|_|_|_|_| 3
|P|P|P|P|P|P|  |P|P|P|P|P|P| 2
|G|B|Q|K|N|R|  |R|N|C|M|B|G| 1
a b c d e f    g h i j k l

The player with the white pieces begins by making a single move on either board. Thereafter, each player takes two moves per turn. The two moves made by a player in a single turn must be moves of pieces whose moves begin on different boards. The same piece cannot be moved twice in a single turn, even if its move is the first of the player's two moves and takes it from one board to the other.

When a player's king is in check, either move in a turn (or a combination of the two moves) may be used to remove the check. It is not required that the first move in a given turn be the one that removes the check.

A player who can make only one legal move is not required to make two moves in the same turn, but a player who can make two legal moves must do so. "Passing" one of your two moves is not allowed. (If the first legal move renders a second move illegal, leaving the player with no legal second move, a second move is not required. In this situation, the player is not required to make a different first move in order to have a legal second move.)

Drop (Vertical) Moves

Pieces move and capture in the expected manner within either board. Instead of making a normal move within a board, however, a piece can "drop" vertically from one board to the corresponding square on the other board and then (in the same turn) make its normal move starting on the square to which it has dropped, as shown in Figure 2. A drop move is optional, and is constrained by the following rules:

(a) The square onto which the piece drops (which corresponds, on the opposite board, to the square on which it begins its move) must be vacant, except in the case of a drop/swap with another friendly piece (see below).

(b) The king can only be checked by pieces on the same board as the king. This rule is logically equivalent to a rule stating that potential drop moves do not check the enemy king. (Just to be clear, an actual drop move can be used to move a piece into a position where it checks the enemy king.) Any other enemy piece can be captured in the move that begins with a drop.

(d) After dropping onto the other board, a piece must proceed with its normal move. A piece can't drop from one board to a vacant square on the other board without otherwise moving (but see the drop/swap rule, below).

(e) The king cannot make a drop move to get out of check. The king can, however, make a drop move in the same turn in which a check is removed, if the check is first removed by another piece.

Figure 2. The white bishop (c2) can move normally on the left board, or drop onto i2 and proceed diagonally to j3 and so on (the squares indicated with dots), but it can't stop on i2.

_ _ _ _ _ _    _ _ _ _ _ _
|_|_|_|_|_|_|  |_|_|_|_|_|_| 7
|_|_|_|_|_|_|  |_|_|_|_|_|_| 6
|_|_|_|_|_|.|  |_|_|_|_|_|.| 5
|.|_|_|_|.|_|  |.|_|_|_|.|_| 4
|_|.|_|.|_|_|  |_|.|_|.|_|_| 3
|_|_|B|_|_|_|  |_|_|_|_|_|_| 2
|_|.|_|.|_|_|  |_|.|_|.|_|_| 1
 a b c d e f    g h i j k l

Figure 3. It might appear that white is checkmated by the two black knights, as it's illegal to drop the king to a1 to remove the check. However, white is saved by the dual drop move P-b4/Rxh3. Dropping the pawn to the left board clears the way for the rook, which first drops to h1 and then moves across the now vacant h2 to make the capture.

 _ _ _ _ _ _    _ _ _ _ _ _
|_|_|_|_|_|_|  |_|_|_|k|_|_| 7
|_|_|_|_|_|_|  |_|_|_|_|_|_| 6
|_|_|_|_|_|_|  |_|_|_|_|_|_| 5
|_|_|_|_|_|_|  |_|_|_|_|_|_| 4
|_|_|_|_|_|_|  |_|n|n|_|_|_| 3
|_|_|_|_|_|_|  |_|P|_|_|_|_| 2
|_|R|_|_|_|_|  |K|_|_|_|_|_| 1
 a b c d e f    g h i j k l

Drop/Swap

When a player has two pieces on corresponding squares on the two boards, the two pieces can switch places. A drop/swap counts as both of the player's moves in a given turn: neither of the pieces in a drop/swap can move elsewhere until the following turn. (White cannot make a drop/swap on the first turn, as this turn includes only one move.) Two identical pieces can't make a drop/swap, as this would allow the player, in effect, to pass.

Other Moves

The chancellor and all move as they do in many variants: Just as the queen is a combination bishop/rook, the chancellor is a combination knight/bishop and the marshall a combination knight/rook.

Pawns have an initial two-square move, and are promoted to any other piece on reaching the last rank. En passant capture works normally: If a pawn drops to the other board and then makes a two-square advance, it can be captured en passant on the square across which it passes. A pawn can also capture another pawn en passant by dropping to the other board.

A pawn that has made a drop/swap from its initial position onto a square in the second rank of the other board is not considered to have moved; it is still entitled to an initial two-square advance in a later turn.

Pawns can capture with drop moves.

Ghost

When moving within a single board, the ghost moves and captures like a rook. When dropping onto the other board at the beginning of its move, it moves and captures like a bishop.

In addition to the drop/swap, which is available to all pieces, the ghost can swap squares with any friendly piece by moving onto the square occupied by the friendly piece, at which point the other piece is removed to the square where the ghost started its move. This swap counts as a single move within the turn, and the ghost is the piece that is deemed to have moved.

A ghost can't swap places with the other friendly ghost, as that would be equivalent to passing the move. A ghost can swap places with its king in order to get the king out of check. A ghost can't swap places with a pawn if the swap would put the pawn on the player's first rank. If a ghost on the last rank swaps places with a pawn, the pawn is promoted immediately.

A pawn that is swapped back to the second rank by a ghost regains the power to make a 2-square advance. (This rule insures that legal moves can be deduced from the board position itself, without the need to refer to the game record.)

Castling

The king can castle in either of two ways. On its home board it can move one square to the right; the rook then moves past it (to d1 for white, as shown in Figure 4). Alternatively, the king can drop-castle onto the other board, moving two squares to the left; again, the rook moves past it. The usual restrictions on castling apply.

Drop-castling is considered a move by the king, on the board where the king originates. It is illegal to move the castling rook again as the second move in a turn that includes a drop-castle. If the king is in check at the beginning of a turn, it is legal to make one move that eliminates the check and then make a castling move as the second move in the same turn.

Figure 4. The white king has castled on its home board. The black king has drop-castled.

_ _ _ _ _ _    _ _ _ _ _ _
|g|b|_|r|k|_|  |r|n|_|q|b|g| 7
|p|p|_|p|p|p|  |p|p|p|p|p|p| 6
|_|c|m|n|_|_|  |_|_|_|_|_|_| 5
|_|_|p|_|_|_|  |_|_|_|_|_|_| 4
|_|_|_|N|_|_|  |_|_|_|_|_|_| 3
|P|P|P|P|P|P|  |P|P|P|P|P|P| 2
|G|B|Q|R|K|_|  |R|N|C|M|B|G| 1
 a b c d e f    g h i j k l

Written by Jim Aikin.