The wall
One bishop can never mate, because it only ever reaches squares of one colour. Two bishops cover both colours, and when they stand side by side, as on c3 and d3, their diagonals join into a barrier the enemy king cannot cross.
Here the barrier runs right across the board, from a5 and a6 on the queenside to h7 and h8 on the kingside, and the black king is shut in above it.
The method
The bishops cannot do the job alone. Bring your king forward first, then step the bishops forward one diagonal at a time, keeping them close together, so the box shrinks while your king takes away the squares in front of the enemy king.
Drive the king to the edge and then into a corner. Any corner will do, because between them the bishops control squares of both colours. The final mate is given by one bishop while the other bishop and your king cover the escape squares.
From this position the tablebase mates in 13 moves against the best defence.
Typical mistakes
Stalemate is the main danger. A king in the corner often has only one or two squares left, and a careless waiting move can take the last one. Before every move, check that the king still has a legal move or is in check.
Keep the bishops out of reach of the enemy king unless your own king protects them. Losing a bishop leaves a draw.
Remember
- Bishops side by side make a wall
- Bring your king forward
- Shrink the box one diagonal at a time
- Watch for stalemate in the corner