Crazyhouse

Crazyhouse is a chess variant in which any piece captured from an opponent is not removed from the game but instead placed in the capturing player's reserve, to be dropped back onto any empty square as that player's own piece on a later turn, a mechanic borrowed from shogi. It developed as a two-player, single-board adaptation of bughouse chess, the team variant in which two boards share a common pool of captured pieces, and it rose to prominence through online chess servers in the 1990s; some historians trace its deeper roots to Mad Mate, a drop-based variant designed by Alex Randolph in 1972. Because dropped pieces can appear anywhere on the board, including squares that would be illegal for a normal move, Crazyhouse produces sudden mating attacks and material swings unlike anything possible in standard chess, and it has become one of the most widely played variants on major online chess platforms.