Chess Variants/Bughouse Chess

Introduction
Bughouse chess, also known as exchange chess or Siamese chess (not to be confused with Thai chess, makruk), is a variant similar to crazyhouse, but played with four players rather than two.

History
It is not known how bughouse chess developed. What is known is that the earliest references to a bughouse-like variant appeared in the 1930s, and bughouse chess itself first rose to prominence in the 1960s. Its drop mechanic was probably inspired by the drop rule in shogi, Japanese chess.

Due to the fast time controls used for the variant bughouse chess can appear chaotic to an observer. hence the name "bughouse", slang for a mental hospital.

Rules
Bughouse chess is played using two boards, two sets of pieces, and four players. The boards are set up so that the white army of one board is directly next to the black army of the opposite board, and vice versa.

The four players are grouped into two teams of two, with partners sitting next to each other. Team A controls the white army of board A and the black army of board B, and team B controls the black army of board A and the white army of board B.

Both boards play the same as standard chess, with one important addition: whenever a player captures an enemy piece, they hand that piece to their partner, who sets it just off their board as a reserve. On subsequent turns, instead of moving a piece a player may take a piece from their reserve and place it on any unoccupied square on the board. This is called a drop, and counts as that player's turn. A dropped piece immediately begins affecting the game (e.g. a drop can be used to deliver check).

Pawns may not be dropped on the first or eighth ranks, and if a promoted pawn is captured it reverts to being a pawn.

If a rook is dropped on the rooks' typical starting squares (a1 and h1 for White, a8 and h8 for Black), that rook is allowed to take part in castling provided the other conditions for the technique are met.

Pieces in reserve must be readily visible to all players.

Partners are allowed to communicate with each other (e.g. asking for a specific piece), but are not allowed to act on the other board.

As soon as one player is checkmated, resigns, makes an illegal move or runs out of time, the entire game ends and the team containing the losing player loses the overall game.

If two players on opposite teams run out of time or are checkmated simultaneously, the game ends in a draw.

Sub-variants

 * Three-board bughouse, as its name suggests, is played with six players (in two teams of three) and three boards. Upon capturing a piece a player may choose which of their partners to hand the piece to.