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 resembles that of shogi;[1] the two games are often compared though no evidence suggests the one developing from the other. 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 a file containing another pawn of the same color and dropping a pawn to deliver checkmate are both permissible.[3]
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."[2]
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:[4]
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.[5][failed verification]
r2q1r1k/2p1ppb1/p2p2pp/3P1p2/B6B/2N2NPp/1PP2P1K/3Q3q w - - 0 26 NNBRpr h1
Crazyhouse has several related variants: