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 is based on that of shogi. 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.^[1]
• A pawn may not be dropped on the 1st or 8th ranks.^[2]
• 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.^[2]

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.^[2]

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."^[1]

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:^[3]

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.^{[4][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.^[5]
• 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.^[6]