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Mate or Stalemate: The One-Square Difference

Mate and stalemate can differ by a single square. In mate the king is in check with no escape, and in stalemate the king is not in check but has no legal move.

The finished mate

The queen on b7 gives check, and the king has no escape square.

FEN: k7/1Q6/1K6/8/8/8/8/8 b - - 0 1

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

The worked example

White plays Qb7, and the king has no escape square.

FEN: k7/2Q5/1K6/8/8/8/8/8 w - - 0 1

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

The stalemate version

With the queen on c7 instead, the king is not in check and has no legal move, so the game is a draw.

FEN: k7/2Q5/1K6/8/8/8/8/8 b - - 0 1

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

Key points

  • The queen on b7 gives check, and the king on a8 has no escape square, so the position is checkmate.
  • The queen on c7 gives no check, and the king on a8 has no legal move, so the position is stalemate.
  • One square changes the result from a win to a draw, so always check whether the king is in check.
  • Count the king's moves first, then ask whether the king is in check.
  • If the king is not in check and has no move, the game is a draw at once.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Rust Playground

Exercise

Find the checkmate in this position.