If players wish to adjust a piece on the board, they must first say "J'adoube" (I adjust) or the equivalent. Apart from that, if a piece is touched it must be moved if possible. This is the 'touch and move' law. If no legal move is possible with the touched piece, the player must make a legal move with another piece. When a player's hand leaves a piece after moving it then the move is over and may not be changed (if the move was legal).