Crazyhouse (also known as drop chess, mad chess, reinforcement chess, turnabout chess and schizo-chess) is a chess variant in which captured enemy pieces can be reintroduced, or dropped, into the game as one's own. The drop rule is based on that of shogi. Crazyhouse is similar to bughouse chess; however, a game of Crazyhouse involves only two players.
The rules of chess apply except for the addition of drops, as explained below.
Unlike in shogi, dropping a pawn on acontaining another pawn of the same color and dropping a pawn to deliver checkmate are both permissible.
To extend the standard algebraic notation, a drop is notated by inserting an at sign between the piece type and the destination square. For example, N@d5 means "knight is dropped on d5."
There is no standard FEN specification for Crazyhouse. Lichess uses an extended version of FEN, adding a 9th rank as a reserve. Here is an example of Lichess's FEN implementation:
r2qk3/pp2bqR1/2p5/8/3Pn3/3BPpB1/PPPp1PPP/RK1R4/PNNNbpp b - - 89 45
In XBoard/Winboard's notation system, the reserve is given in square brackets following the board position:
r2qk3/pp2bqR1/2p5/8/3Pn3/3BPpB1/PPPp1PPP/RK1R4[PNNNbpp] b - - 89 45
In Chess.com's notation system, the reserve is located after the full-move number.
To keep track of which pieces are promoted, Lichess and XBoard/Winboard use "~" after the letter designation. Chess.com uses the coordinates of the pieces.[failed verification]
r2q1r1k/2p1ppb1/p2p2pp/3P1p2/B6B/2N2NPp/1PP2P1K/3Q3q w - - 0 26 NNBRpr h1
Crazyhouse has several related variants: