After an initial move from the back rank, a Hawk or Elephant piece can be placed into the vacated square.
Seirawan Chess, also known as S-chess or Sharper chess, plays on a regular board but introduces two new pieces, the Hawk and the Elephant, which are are optionally placed once a home rank piece makes an initial movement.
This variant is played from the standard starting position of Standard Chess.
The Hawk and Elephant do not start on the board intially, but are introduced once a piece moves from the home rank. The Hawk or Elephant can then be optionally added to the board in the vacant square.
Only one Hawk and one Elephant can be added through this manner. The Hawk and Elephant cannot be placed to block check.
If the Hawk or Elephant are not introduced before all the home rank pieces have moved, then the opportunity to do so has passed.
The Hawk moves like a Bishop and Knight combined
The Elephant moves like a Rook and Knight combined.
On reaching the 8th rank, a pawn can promote to a Queen, Rook, Bishop, Knight, Hawk or Elephant.
Castling is the same as Standard Chess, but allows the Hawk or Elephant - if not already used - to be placed into either of the vacated squares.
En passant rules remains the same as Standard Chess.
Check and Checkmate rules remain the same as Standard Chess.
Stalemate conditions remain the same as Standard Chess.
Draw conditions remain the same as Standard Chess.
Winning conditions remain the same as Standard Chess.