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Underpromotion Exists (Why Not Always a Queen)

Underpromotion means promoting to a piece other than a queen. Underpromotion is rare, but it matters in some positions.

Knight promotion gives check

White pawn is on e7 and Black king is on f6. White promotes to a knight on e8, and the knight attacks f6, so Black is in check.

FEN: 8/4P3/5k2/8/8/8/8/K7 w - - 0 1

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

Queen promotion here

The same pawn promotes to a queen on e8. The queen does not attack f6, so Black is not in check after queen promotion.

FEN: 8/4P3/5k2/8/8/8/8/K7 w - - 0 1

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

Bishop promotion

The same pawn promotes to a bishop on e8. A bishop is weaker than a queen, but it can be the right piece when a queen would draw the game.

FEN: 8/4P3/5k2/8/8/8/8/K7 w - - 0 1

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

Key points

  • Underpromotion means promoting to a rook, bishop, or knight instead of a queen.
  • A knight moves in a different way from a queen. That different move can give check in a place a queen cannot reach in the same way.
  • A queen can sometimes leave the enemy king with no legal move. That result is stalemate, which is a draw.
  • A smaller piece can avoid that draw in some positions. The right piece is the piece that keeps the game going.
  • Always ask what each new piece would do before choosing. The strongest piece is not always the best choice.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Rust Playground

Exercise

White pawn is on e7 and Black king is on f6. Which move promotes to a knight and gives check?