This variant was invented in 1988 by Ed Pegg Jr. Postal tournaments in this game are held in NOST.
Each turn, a player can make one of two types of moves: a `normal move', i.e, a move that is also allowed in orthodox chess (with the exception of taking en-passant), or a castling move.
A castling move involves two pieces: one piece of the player, and one piece that may be of the player or of his opponent. These pieces must be on the same horizontal, vertical, or diagonal line, with no other pieces in between, and at least two squares apart. The first piece (of the player making the move) moves two squares in the direction of the other piece, and the other piece is then placed on the square that the first piece passed over.
Taking en-passant is not allowed. Pawns that went back to the second row by castling can also make a double step when leaving the second row again.
For example, in the starting setup, white can make a castling move by moving his pawn on e2 to e4, and the black pawn from e7 to e3. Another castling move from the starting setup is for instance to move the pawn on g2 to e4, and the black pawn on b7 goes to f3. After 1. d2-d4, d7-d5, white can move his queen to d3 and his pawn from d4 back to d2.