Impossibility of checkmate – if a position arises in which neither player could possibly give checkmate by a series of legal moves, the game is a draw. This is usually because there is insufficient material left, but it is possible in other positions too. Combinations with insufficient material to checkmate are:
king versus king
king and bishop versus king
king and knight versus king
king and bishop versus king and bishop with the bishops on the same colour. (Any number of additional bishops of either color on the same color of square due to underpromotion do not affect the situation.)
