The natural question is the following : can we design a game **very similar** to chess in which the human can beat
the computer ? One proposition is Arimaa. My proposition is
**Twilight Chess**.

The idea behind Twilight chess is twofold:

- Twilight Chess is really close from standard chess : actually all standard rules of classical chess apply, there is an addition of two new kind of moves. Hence, good chessplayers, to a large extent, may use their expertise in Twilight Chess.
- The number of legal moves of twiligh chess is many times the one of classical chess, thus making exploration much harder for computers. For human there is still this instinct of king-hunting that helps to "understand" a position.
- The extreme closeness of Twilight Chess and classical makes it almost trivial the transformation of a classical chess engine into a Twilight chess engine. It is interesting to study the relative impact, in terms of quality of play, of the new rules for both humans and computers. It could draw interesting discoveries in AI field.

- All classical laws of chess apply.
- Moving to the Twilight zone (Warp move) is a legal moves for all piece but for the King.
- Moving a piece from the Twilight zone to any free square of the chessboard (Drop move) is a legal chess move but for a pawn into the last rank (8th for white player, 1st for black player).
- Warp and Drop moves are considered as standard moves with relation to classical laws of chess.

- If I drop a Pawn on the first rank (as a white player), afterwards can I move it two squares forward ? The answer is no since it has already moved (to go into the twilight zone), and moving two squares forward only apply at Pawns that has not moved yet.
- If I warp and drop a Rook later to its original square can I castle on this side ? The answer is no because the Rook has moved.
- Mate, stalemate, 50 moves rule etc. behave accordingly.

Double click
on Winboard program which is already parameterised. In order to play
to twilight chess, one has to select **new variant** in the
**File** menu, and to select **Twilight Chess**.

If you want sources you can ask me to send you them by mail me at Frederic.Prost@imag.fr.

Games can be saved and loaded from file using PGN notation. It is extended in the following way:

- Nothing is changed for moves that are neither Warp nor Drop moves: ie see pgn definition
- Warp move : to move a Quee located on e2 to the twilight zone use
**"e2@Q"**N is for Knight, Q for queen, R for Rook, B for bishop, P for Pawn. - Drop move : to drop a Knight in e4 use
**"N@e4"**.

- Same as field 1 of classical FEN notation
- List of pieces outside the board. White in upper-case and black in lower-case letters. If all pieces are on the board then this field is empty.
- Same as field 2 of classical FEN notation
- Same as field 3 of classical FEN notation
- Same as field 4 of classical FEN notation
- Same as field 5 of classical FEN notation
- Same as field 6 of classical FEN notation
- Integer between 0 and 255. When written in binary this number indicates whether a pawn has the possibility to move two squares ahead (since a pawn on the second rank may have been warped and droped back and in that case it can no longer move two squares in a row). 1 means that the pawn can move two squares in a row, 0 that it cannot.
- Same field as 8th for black.

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