In order to capture a piece, there must be exactly one piece (friendly or otherwise) between the cannon and the piece to be captured. The cannon then moves to that point and captures the piece. They may also move or capture diagonally along the diagonal lines in either palace, provided there is an intervening piece in the centre (i.e. it can only happen if the cannon is at a corner of the palace) They are powerful at the beginning of the game when "hurdles" are plentiful, but lose value rapidly with attrition.