Crazyhouse

Crazyhouse is a chess variant in which captured enemy pieces can be reintroduced, or dropped, into the game as one's own.

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.

A drop is notated like a standard move, except an at sign is placed immediately before the destination square.

Lichess uses an extended version of FEN, adding a 9th rank as a reserve.

[6][failed verification] 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.