Crazyhouse

Crazyhouse is a chess variant in which captured enemy pieces can be reintroduced, or dropped, into the game as one's own. It was derived as a two-player, single-board variant of bughouse chess. Its drop rule is reminiscent of shogi and the games are often compared, though there is no known evidence suggesting that shogi provided direct inspiration for the gameplay of bughouse or crazyhouse.

History
Though the four-player "bughouse" chess became prominent in western chess circles in the 1960s, the crazyhouse variant did not rise to prominence until the era of 1990s online chess servers, though it may be traced back further to the "Mad Mate" variant made in 1972 by Alex Randolph, a Bohemian-American game designer who moved to Japan and became an amateur dan-level Shogi player.

Rules
The rules of chess apply except for the addition of drops, as explained below. Unlike in shogi, dropping a pawn on a containing another pawn of the same color and dropping a pawn to deliver checkmate are both permissible.
 * 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.
 * A pawn may not be dropped on the 1st or 8th.
 * 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.

Notation
Crazyhouse's notation system is an extension of the standard algebraic notation. A drop is notated like a standard move, except an at sign is placed immediately before the destination square. For example, N@d5 means "knight is dropped on d5."

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: 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. r2q1r1k/2p1ppb1/p2p2pp/3P1p2/B6B/2N2NPp/1PP2P1K/3Q3q w - - 0 26 NNBRpr h1

Criticisms
GM Larry Kaufman wrote: "[Crazyhouse] is rather fun and interesting, but the games tend to be short, and it is almost certain that White has a forced win, although it would probably be too difficult to prove this and certainly too difficult to memorize all the possible variations."