Mate with knight, bishop and king against a lone king is always possible but requires precise play: in the worst case up to 33 moves. And the 50-move rule allows no more than 50. Even strong players sometimes fail to deliver it in tournament games.
The main rule
Mate can be delivered only in a corner of the same colour as the bishop's squares. A light-squared bishop — corners a8 and h1; a dark-squared bishop — a1 and h8.
The enemy king will of course head for the “wrong” corner. So the technique has two parts:
- Drive the king to the edge of the board.
- Chase it along the edge from the “wrong” corner to the “right” one and mate it there.
The mating position
The W-manoeuvre
To drive the king along the edge, the knight follows a path resembling the letter W, while the king and bishop “push” the enemy king and keep it from returning to the wrong corner. For example, with a light-squared bishop and the black king in the h8 corner, the knight successively occupies squares that cut off the king's way back, until it ends up near the a8 corner.
This method is hard to learn from a description — it is mastered at the board with a coach, going through the position move by move.
Is it worth learning
In practice this endgame is rare — roughly once in several thousand games. But studying it is useful: it is excellent training in piece coordination. Beginners should first master mate with the queen, with the rook and with two bishops.