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Hostage chess is a chess variant invented by John Leslie in 1997. It uses a single chess set. Captured pieces are not eliminated from the game but can reenter active play through drops, similar to shogi, but, the piece a player may drop is one of his own pieces previously captured by the opponent. In exchange, the player returns a previously captured enemy piece which the opponent may drop on a future turn. This is the characteristic feature of the game.
Hostage chess follows all the standard rules of chess excepting how captured men are treated. Each player owns reserved spaces off the chessboard: a prison to the player's right, and an airfield to the player's left. There should be a clear boundary between each prison and the other player's airfield. Captured men are not removed from the game but are held in the capturer's prison. Instead of making a normal move, a player can perform a hostage exchange to "rescue" a man held prisoner by the opponent and drop the freed man back into play on the board on an open square. The man exchanged for the dropped man is transferred from the player's prison to the opponent's airfield. On any turn, instead of making a normal move, a player can drop a man from his airfield into active play on the board.
A hostage exchange is performed by transferring a man from one's prison to the opponent's airfield, then selecting and releasing a man from the opponent's prison and immediately dropping it onto an empty square on the board. The drop completes the turn. The man transferred must be of equal or greater value than the man released from prison and dropped. The relative piece values are: Q > R > B = N > P. (So, any man can be exchanged to free a pawn; whereas only a queen can be exchanged to free a queen.) A hostage exchange cannot be refused by the opponent.
A drop can occur as part of a hostage exchange, or directly from a player's airfield. The square dropped to must be unoccupied. Additional drop rules:
* A pawn may not be dropped on the 1st or 8th ranks. A pawn dropped on the player's 2nd rank inherits the normal two-step move option. A dropped pawn may not be captured en passant immediately after the drop.
* A rook dropped on a rook starting square can be used in castling.
* A bishop can be dropped on the same color square as a friendly bishop already on the board.
A pawn can promote only to a Q, R, B, or N which is in the opponent's prison. The promoting player selects which piece to release and promote to. The pawn is transferred to the opponent's prison. If the promoted piece is later captured, it keeps its type when entering prison.
If a pawn is on a player's 7th rank with no available piece to promote to,:
* The pawn cannot advance.
* If the opponent's king is diagonally in front of the pawn, the pawn does not give check; and
the player owning the king may not capture a Q, R, B, or N (since to do so would make the pawn eligible to promote, putting the player in self-check).
Standard notation is used, with extensions: see examples:
* (B-N)*c7 :: a bishop was exchanged to free a knight, and the knight was dropped on c7. Pawns are notated P in hostage exchanges, for example: (P-P)*g5.
* N*c7 :: a knight was dropped on c7 from the player's airfield.
* *g5 :: a pawn was dropped on g5 from the player's airfield.
It likely will get some new interest if Chess.com's list of chess variants included Hostage Chess (invented by John Leslie in 1997) :: for information see the Wikipedia page named "Hostage chess", and the links at the end of that page; and try a Google search for "Hostage chess" (in quotes).