The right corner
Against good defence, king, bishop and knight can force mate only in a corner of the same colour as the bishop. Here the bishop moves on light squares, so the mating corners are a8 and h1. The defender's aim is to stay in or near a dark corner, h8 or a1.
In this position the black king is close to h8, the wrong corner. White has to drive it along the eighth rank to a8, the light corner on the same side.
Driving the king along the edge
The three pieces build a moving net. The bishop takes the light squares, the knight takes dark squares the bishop cannot reach, and the king stays close, usually a knight's move or two away from the enemy king.
Textbooks describe a set pattern for this drive, often called the W manoeuvre after the shape of the knight's route. The tablebase line in the demonstration takes its own route, but the principle is the same: every check and every quiet move takes squares away until the king is pushed into the a8 corner.
From this position the tablebase mates in 16 moves. From the worst starting positions the mate takes 33 moves with best defence, inside the 50-move rule but with little room to waste moves.
Typical mistakes
Random checks rarely help. A check that lets the king step towards the middle undoes several moves of work. Keep the net tight and use quiet moves.
Watch for stalemate near the corner, and keep the knight close. A knight on the far side of the board takes several moves to come back.
Remember
- Mate only in the bishop's corner
- Drive the king along the edge
- Bishop, knight and king work as a net
- Mind the 50-move rule