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Two Knights Cannot Force Mate

Two knights and a king cannot force checkmate against a lone king. Mate is possible only when the defender helps, and correct defence draws.

A mate with help

The knight on b6 gives check, and the king has no escape square, but the defender had to step into this corner first.

FEN: k7/2K4p/1N6/1N6/8/8/8/8 b - - 1 1

a b c d e f g h
8 k . . . . . . . 8
7 . . K . . . . p 7
6 . N . . . . . . 6
5 . N . . . . . . 5
4 . . . . . . . . 4
3 . . . . . . . . 3
2 . . . . . . . . 2
1 . . . . . . . . 1
  a b c d e f g h

The worked example

White plays Nb6, and the position is checkmate, but only because the defender is already in the corner.

FEN: k7/2KN3p/8/1N6/8/8/8/8 w - - 0 1

a b c d e f g h
8 k . . . . . . . 8
7 . . K N . . . p 7
6 . . . . . . . . 6
5 . N . . . . . . 5
4 . . . . . . . . 4
3 . . . . . . . . 3
2 . . . . . . . . 2
1 . . . . . . . . 1
  a b c d e f g h

Correct defence draws

Black plays Kb8 here and keeps moving instead of stepping into the corner, so the two knights cannot force a mate.

FEN: k7/8/2K5/1N1N4/8/8/8/8 b - - 0 1

a b c d e f g h
8 k . . . . . . . 8
7 . . . . . . . . 7
6 . . K . . . . . 6
5 . N . N . . . . 5
4 . . . . . . . . 4
3 . . . . . . . . 3
2 . . . . . . . . 2
1 . . . . . . . . 1
  a b c d e f g h

Key points

  • Two knights can give checkmate when the defender steps into a corner, but the attacker cannot force that step.
  • The worked example shows a mate that needs the defender's help, because the defender chose a corner square.
  • Correct defence keeps the king moving and stays away from the corner, and the game is a draw.
  • Do not claim a forced mate with two knights, because the defender can always avoid the corner.
  • The honest result with best defence is a draw, even though a mate position exists.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Rust Playground

Exercise

Find the checkmate in this position.