A player can either move one piece twice (this is the source of the famous trap in the "balanced" version of the game where White opening with pawn-two appears to be trivially busted by an unprincipled defense with the pawn on an adjacent file) or move two different pieces on their turn.
Castling is considered a single move. When a player gives
check on the first move, they lose the second move of their turn. If a player is in check, they must move out of check on the first move of the turn. It is not allowed to move the
king into check on the first move of a turn and then move out of check on the second.
En passant capture is allowed even if the opponent moved the corresponding
pawn on the first move of their turn. Capturing
en passant, however, must be done on the first move of the turn. When
two pawns can be captured
en passant, both can be captured in the turn. To ensure
White's advantage of moving first is not excessive, usually a "balanced" version of the game is played. In the balanced version, White makes only
one move on the first turn. The moves are made in the following order:
White, Black, Black, White, White, Black, Black, and so on. This rule was introduced in 1963 by Robert Bruce and has gained wide acceptance since then. == See also ==