Grand Chess was created by the Dutch game designer Christian Freeling and the details are presented on his web site along with more interesting games. This game is played on 10x10 chessboard and brings several unique features which make the strategies much more complex.
The following picture shows a start position:
As you can see, there are two new pieces which are not used in classic Chess:
Marshall (on F2 and F9) - has the combined moves of a Rook and a Knight.
Cardinal (on G2 and G9) - has the combined moves of a Bishop and a Knight.
The game follows the standard Chess rules with these exceptions:
Pawns standing on their start row (3 for white or 8 for black) can make an initial two-step move and the en passant capture can be applied on them.
Castling is not possible.
A pawn can be promoted when it reaches the 8th or 9th row (3rd or 2nd for a black). The promotion is optional on these rows.
A pawn must be promoted if it reaches the last row (as in standard Chess).
A pawn can be promoted only to a piece which was previously captured by the opponent. When the promotion happens, the corresponding piece will disappear from the captured pieces on the opponent's side.
If the previous point cannot be fulfilled, the pawn on the 9th (2nd for a black) cannot move to the last row. However, such a pawn can still give a check.