Crazyhouse is a chess variant similar to bughouse chess, but with only two players. It effectively incorporates a rule in shogi, a Japanese chess-like game, where a player can introduce a captured piece back to the board as his own.
Rule difference and notation
A captured piece turns colour and goes to the capturing player's "reserve", or "pocket". At any time, instead of making a move on a piece on a board, a player can "drop" a piece in his reserve back to the board.
Promoted but captured pawns are dropped as pawns.
Pawns must not be dropped at the 1st or 8th ranks.
For recording games, the usual algebraic notation of chess is extended to allow specifying the drop of a piece. For example, "p@d5" means "pawn is placed on d5 from reserve".
Crazyhouse has been analysed much less than regular chess. Consequently, less is known about ideal variations; however, the general differences between chess and crazyhouse require different strategies. Pawns and knights increase in relative importance in crazyhouse chess, while rooks, queens, and bishops decrease in relative importance. If a king is put in check by any of the latter three pieces from two or more squares away, dropping a pawn next to the king becomes defensively useful. A knight, on the other hand, cannot be blocked by anything and its offensive value is more manifest. It is an effective way to maintain a strategic influence over a region.