(from http://www.gamerz.net/pbmserv/bughouse.html)

Bughouse is a chess game for four players, played on two boards. Actually, it is two chess games played simultaneously in teams. The black player on board 1 is partnered with the white player on board 2, and vice versa. To win, a team must win on either of their boards.
The most notable difference from normal chess is that pieces captured on one board are handed to the partner on the other board. He can drop these pieces on his board for one of his turns.
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Piece drops work pretty much like loop chess, except you get to drop the pieces your partner captures from his board, and you hand the pieces you capture over to him.