Wall, king, corner
The mate, move by move
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Win
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Step through the moves with the arrows, or click a move.
Lesson 3 of 4
King and Two Bishops Against King
Two bishops side by side make a wall the enemy king cannot cross. Move the wall towards a corner, bring your king, and mate in the corner, without stalemate and without leaving a bishop hanging.
King and two bishops against king is a win, but only in a corner or on a square next to it: the bishops cannot force mate in the middle of an edge. With the bishops' side to move, the mate takes at most nineteen moves in almost every position.
The key pattern is the wall: two bishops on neighbouring squares of the same rank or file cover two neighbouring diagonals each, and the enemy king cannot cross them. Move the wall step by step towards a corner, use your king to take away the squares the bishops cannot reach, and finish with checks in the corner.
Every result in this lesson comes from a tablebase. You get three conditions, each with the move that keeps the win and one that throws it away, the mate move by move, and four positions to solve.
The idea
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The wall
After 5.Be5 the bishops stand side by side on d5 and e5. Together they cover the diagonals in front of Black's king, which cannot cross the wall.
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The wall moves
After 7.Be4 the wall has moved down to e4 and e5, and White's king is close enough to help.
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hThe mate
In the corner, one bishop checks along the long diagonal, the other covers g1, and White's king covers g2 and h2.
Before you play it: the checklist
Each condition with two positions: one where it holds and the move works, one where it is missing and the same move fails. The number is Stockfish's evaluation after the move, from the attacker's side.
1.Watch your king moves too
With the enemy king hemmed in, even a king move can take away its last square.
It works
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8.Ke3Win
8.Ke3! comes closer and leaves Black a move. It is the fastest mate.
It fails
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8.Ke1Draw
8.Ke1? takes away d1 and d2 without check: stalemate.
The engine prefers Ke3 (Win).
2.Keep the bishops protected
Near the corner the enemy king is close to the bishops. Check that it cannot take one.
It works
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16.Be3+Win
16.Be3+! drives the king into the corner, and the bishop is safe.
It fails
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16.Bf1Draw
16.Bf1? puts the bishop where the king can take it, and it is a draw.
The engine prefers Be3+ (Win).
3.Mate, not stalemate
In the corner, many bishop moves take the last square away without check.
It works
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h17.Be4#Win
17.Be4#.
It fails
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h17.Bf2Draw
17.Bf2? covers g1 without check: stalemate.
The engine prefers Be4# (Win).
Move by move
Click a move or use the arrows. Under each note you'll find the defender's other choices and how to answer them. The number under the board is Stockfish's evaluation from the attacker's side.
cefYour turn
Each position has one best move. Find it, then check the solution.
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Exercise 1
White to move. Find the fastest mate.
Show solution
5.Be5
5.Be5! puts the bishops side by side. Every other move is slower.
King and two bishops against king.
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Exercise 2
White to move. Black's king is next to the corner.
Show solution
14.Kg3
14.Kg3! takes away g2 and h2. Every other move is slower.
King and two bishops against king.
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Exercise 3
White to move. Find the fastest mate.
Show solution
15.Bd3+
15.Bd3+! starts the final checks. Careful: 15.Be1? throws the win away.
King and two bishops against king.
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hExercise 4
White to move and mate in one. Most bishop moves are stalemate.
Show solution
17.Be4#
17.Be4#.
King and two bishops against king.
From history
Only in the corner
Wikipedia's article on checkmate gives three mating positions with two bishops: in the corner, on a square next to the corner, and on the middle of an edge. Only the first two can be forced. With the bishops' side to move, mate can be forced in at most nineteen moves, except in a few very rare positions.
Take these into your games
Build the wall
Two bishops side by side cover a barrier of diagonals the enemy king cannot cross.
Move the wall
Shift the bishops step by step, keeping them side by side, to shrink the king's space.
Head for a corner
The mate can only be forced in a corner or next to it.
King helps
Your king takes away the squares the bishops cannot cover.
Count the squares
Before every quiet move, check that the enemy king can still move and cannot take a bishop.