Crazyhouse is a chess variant similar to bughouse chess, but with only two players. It effectively incorporates a rule from the gameshogi, in which a player can introduce a captured piece back to the chessboard as their own.
Rules
All the rules and conventions of standard chess apply, with the addition of drops, as explained below.
A captured piece reverses color and goes to the capturing player's reserve or pocket. At any time, instead of making a move with a piece on the board, a player can drop a piece from their reserve onto an empty square on the board. For example, a check that would result in checkmate in standard chess can be answered in Crazyhouse, if the defender can play a legal drop that blocks the check.
Drops resulting in immediate checkmate are permitted. Unlike in shogi, this includes pawn drops.
Pawns may not be dropped on the players' 1st or 8th.
Pawns that have been promoted and later captured are dropped as pawns.
Unlike shogi, having two or more pawns on a file, and checkmating with a dropped pawn, are both permissible. The physical problem of changing the color of a captured piece can be handled by:
Swapping the piece for a piece of the same denomination and the other color taken from a second chess set.
An extension to the standard chess notation is used to record drops. Drops are notated by the piece type, followed by an @ symbol, then the destination square. For example, N@d5 means "knight is dropped on d5 from reserve".
There is no standard FEN specification for Crazyhouse. However at Lichessextended version of FEN is in use. Here is Lichess's FEN implemention example.
r2qk3/pp2bqR1/2p5/8/3Pn3/3BPpB1/PPPp1PPP/RK1R4/PNNNbpp b - - 89 45
Lichess simply adds a 0th rank as a reserve. There are more than 8 pieces on the reserve, so the last section may have more than 8 characters. A different notation is used by Xboard/Winboard. The reserve is given in square brackets following the board position.
r2qk3/pp2bqR1/2p5/8/3Pn3/3BPpB1/PPPp1PPP/RK1R4 b - - 89 45
Chess.com uses another notation. The reserve is put after full-move number. To keep track of which pieces currently on the board are actually promoted pawns, Lichess and Xboard/Winboard use "~" after letter designation. However, Chess.com uses coordinates of promoted pawn to resolve it.
r2q1r1k/2p1ppb1/p2p2pp/3P1p2/B6B/2N2NPp/1PP2P1K/3Q3q w - - 0 26 NNBRpr h1
Variations
Minor variations of the rules have resulted in some variants.
Loop Chess: promoted pawns keep their rank when captured.
Chessgi : promoted pawns keep their rank when captured. Pawns may be dropped on the 1st rank.
