1.N@e7+ Kh8 2.Bxg7# (@ notation)

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.

Rules

The rules of chess apply except for the addition of drops, as explained below.

• A piece that is captured reverses color and goes to the capturing player's reserve, pocket or bank, where it is considered held or in hand. At any time, instead of making a move with a piece on the board, a player can drop one of their held pieces onto an empty square on the board.^[2]
• A pawn may not be dropped on the 1st or 8th ranks.^[3]
• A pawn that is dropped on its 2nd rank may use its two-square initial advance; a pawn that is dropped on any other rank cannot.
• When a piece that is promoted from a pawn is captured, it enters the opponent's reserve as a pawn.^[3]

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]

Notation

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]

FEN

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

Variations

Crazyhouse has several related variants:

• Loop Chess: A promoted piece keeps its rank when captured.^[6]
• Chessgi (also known as Mad Mate or Neo Chess): A promoted piece keeps its rank when captured. A pawn may be dropped on its 1st rank.^[7]